Management and conservation of marine mussels
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.
The word "mussel" is most frequently used to mean the edible bivalves of the marine family Mytilidae, most of which live on exposed shores in the intertidal zone, attached by means of their strong byssal threads ("beard") to a firm substrate. A few species (in the genus Bathymodiolus) have colonized hydrothermal vents associated with deep ocean ridges.
In most marine mussels the shell is longer than it is wide, being wedge-shaped or asymmetrical. The external color of the shell is often dark blue, blackish, or brown, while the interior is silvery and somewhat nacreous.
- Kingdom Animalia -- Animal, animals, animaux
Phylum Mollusca -- molluscs, mollusks, mollusques, molusco
Class BivalviaLinnaeus, 1758 -- bivalve, bivalves, bivalves, clams, mexilhão, ostra, palourdes
Subclass PteriomorphiaBeurlen, 1944
Order MytiloidaFerussac, 1822
Family MytilidaeRafinesque, 1815
Genus MytilusLinnaeus, 1758
Species Mytilus edulisLinnaeus, 1758 -- blue mussel, edible blue mussel
The mussel's external shell is composed of two hinged halves or "valves". The valves are joined together on the outside by a ligament, and are closed when necessary by strong internal muscles. Mussel shells carry out a variety of functions, including support for soft tissues, protection from predators and protection against desiccation.
The shell is made of three layers. In the pearly mussels there is an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate, which is continuously secreted by the mantle; the prismatic layer, a middle layer of chalky white crystals of calcium carbonate in a protein matrix; and the periostracum, an outer pigmented layer resembling a skin. The periostracum is composed of a protein called conchin, and its function is to protect the prismatic layer from abrasion and dissolution by acids (especially important in freshwater forms where the decay of leaf materials produces acids).
Like most bivalves, mussels have a large organ called a foot. In marine mussels, the foot is smaller, tongue-like in shape, with a groove on the ventral surface which is continuous with the byssus pit. In this pit, a viscous secretion is exuded, entering the groove and hardening gradually upon contact with sea water. This forms extremely tough, strong, elastic, byssus threads that secure the mussel to its substrate. The byssus thread is also sometimes used by mussels as a defensive measure, to tether predatory molluscs, such as dog whelks, that invade mussel beds, immobilising them and thus starving them to death.
In cooking, the byssus of the mussel is known as the "beard" and is removed before the mussels are prepared.
Identifying features for common mussel
- Shell solid, equivalve, inequilateral and approximately triangular in outline, however, shell shape varies considerably with environmental conditions.
- Shell smooth with a sculpturing of concentric lines but no radiating ribs.
- The shell colour varies, usually purple or blue but sometimes brown and covered with a black outer layer
- Periostracum darker than shell, almost black, dark brown or olive in colour.
- Shell interior pearl-white with a purple or dark blue border or outer edge.
- Posterior adductor scar large but anterior adductor scar much reduced.
- Pallial line wide.
- Beaks are anterior, terminal, and rounded with straight umbones but not turned downward.
- Hinge lacking teeth or chondrophore but with 3-12 small crenulations below the umbones.
- Ligament runs from the beaks to the high point of the shell but is inconspicuous.
- Edge of mantle whitish yellow or brown.
- External ligament inconspicuous, running more than half the length of the shell from the umbones.
- Length varies, specimens usually ranging from 5 -10 cm although some populations never attain more than 2-3 cm, and the largest specimens may reach 15 -20 cm.
- Mytilus edulis may be confused with the Mediterranean mussel Mytilus galloprovincialis.
- the umbones turn down, giving the basal line of the shell a concave appearance;
- the valves are higher and less angular;
- the mantle edges are darker, becoming blue or purple, and
- Mytilus galloprovincialis tends to grow larger (Tebble, 1976).
Mytilus edulis and Mytilus galloprovincialis often occur in the same location in the northern range of Mytilus galloprovincialis. As they both show great variation in shell shape due to environmental conditions (Seed, 1968, 1992), they are often difficult to distinguish. In addition, they may hybridize. However, in Mytilus galloprovincialis:
No single morphological characteristic can be used to separate Mytilus species (Gosling, 1992c; Seed, 1992, 1995). Recent evidence suggests that there are only three lineages of the genus, Mytilus edulis, Mytilus galloprovincialis and Mytilus trossulus, although some authorities suggest that all of the smooth shelled mussels belong to the same species (for discussion see Seed, 1992).
Marine mussels are filter feeders; they feed on plankton and other microscopic sea creatures which are free-floating in seawater. A mussel draws water in through its incurrent siphon. The water is then brought into the branchial chamber by the actions of the cilia located on the gills for cilliary-mucus feeding. The wastewater exits through the excurrent siphon. The labial palps finally funnel the food into the mouth, where digestion begins.
Marine mussels are usually found clumping together on wave-washed rocks, each attached to the rock by its byssus. The clumping habit helps hold the mussels firm against the force of the waves. At low tide mussels in the middle of a clump will undergo less water loss because of water capture by the other mussels.
Marine mussels are gonochoristic, with separate male and female individuals. In most marine mussels, fertilization occurs externally or outside the body by spawning their free-floating gametes into the water column, with a larval stage or development is free-swimming and planktonic that drifts for three weeks to six months, before settling first on filamentous organisms such as seaweeds. After growing for a while, they detach and drift in the water on a long byssal thread; a mode of dispersal likened to that of young spiders floating through the air on a silk thread (2). After four weeks or so, the young mussel will have settled again, this time on a mussel bed (2). There, it is capable of moving slowly by means of attaching and detaching byssal threads to attain a better life position. Young mussels are thought to have evolved primary settlement on filamentous substrates in order to avoid having to compete with adult mussels (3).
The mussel is a filter-feeder; it filters bacteria, plankton, and detritus from the water (3). When large beds of this gregarious species form, individuals are bonded together with threads of byssus. Predation is the greatest cause of mortality; a range of predators take mussels, including dog-whelks (Nucella lapillus), crabs, sea urchins, star-fish, and birds such as the oystercatcher (Haematopus ostralegus) (3). Although mussels seem fairly defenceless, remarkably they are able to fend off marauding dog whelks and other predatory gastropods; a number of mussels work together to immobilise the predator with bysuss threads (3). Organisms that attach to mussels, such as seaweeds and barnacles, may increase the risk of the mussel becoming detached by wave action; however, mussels are able to sweep their foot over their shell, which may help to minimise the likelihood of such an organism becoming attached (3).
Mussels are host to the pea crab (Pinnotheres pisum), and a copepod (Mytilicola intestinalis), both of which are not parasites, as was once thought, but commensal organisms (they benefit from living with the mussel, but the mussel is not affected) (2). Furthermore, mussel beds provide habitats for a variety of marine life, and support higher levels of biodiversity than surrounding mudflats (3). The biodiversity of the bed increases with its size and age (3).
Extremely common around the coasts of Britain; very large commercial mussel beds occur in the Wash, Conway bay, Morecambe Bay, and estuaries of southwest England, west Scotland and west Wales (3). Elsewhere, it is found from the White Sea in northern Russia to southern France, and in the West Atlantic from Canada to North Carolina (3). It also occurs off Chile, the Falkland Isles, Argentina and the Kerguelen Isles (3).
Mytilus spp. are found throughout the world, namely in northern temperate latitudes, the Mediterranean Sea, the Pacific coast of North America, south-eastern and south-western coasts of South America, Australia, New Zealand, the Kerguelen Islands, and the Pacific coast of Asia (Gosling, 1992). Perna occurs extensively in tropical waters, for example in Yemen, Oman, India, Tahiti, Philippines, New Zealand, Venezuela, Prun Buri S, Thailand and Brunei (Gosling, 1992). The tropical mussel Mytella strigata is a lagoon species which is found from the Gulf of California (e.g. Guaymas) to the south of El Salvador and the Galapagos Islands in the Pacific, and from Venezuela to Argentina in the Atlantic (Keen, 1971).
Range of conditions
Full, variable. Mytilus edulis is tolerant of a wide range of salinity compared to other biogenic reef species and may penetrate quite far up estuaries. However, it may stop feeding during short-term exposure to low salinities (Almada-Villela 1984; Bohle 1972) and the most well-developed beds therefore usually occur low on the shore in the mid to lower reaches of estuaries. Almada-Villela (1984) reported greatly- reduced shell growth for a period of up to a month or so upon exposure to 16‰ compared to 26‰ or 32‰, while exposure to 22‰ caused only a small drop in growth rate. In the longer term (in the order of weeks) M. edulis adapts well to low salinities (Almada-Villela 1984; Bohle 1972) and hence can even grow as dwarf individuals in the inner Baltic where salinities can be as low as 4-5‰ (Kautsky 1982).
Sheltered, Very sheltered, Extremely sheltered
Mixed boulders, cobbles and pebbles on muddy sediment. In sheltered areas infaunal beds may occur on gravel or even quite sandy areas, although it is likely that some harder substratum embedded within the more sandy areas is required. Dense settlement also occurs on cockle shells in the Wash and Loughor Estuary where the byssus of the embedded mussels seem to serve a stabilising function. It has long been suggested that larval Mytilus will settle on most substrata provided they are firm and have a rough, discontinuous surface (Mass Geesteranus 1942). Settlement is in any case a two-stage process; initial settlement occurs primarily on filamentous substrata such as sublittoral hydroids and algae, with subsequent secondary dispersal and reattachment later in areas with adult beds.
Reef areas are normally found on the lower third of the intertidal, and in shallow subtidal, but can occur down to 10 m in some places such as the Wash and on Caernarfon Bar. Lower zonational limits for M. edulis are usually set by biological factors, normally predation by starfish, crabs and gastropods, and by physical factors. Sand burial has been shown to limit lower regions of M. edulis zonation patterns in New Hampshire, USA (Daly & Mathieson 1977). This is probably important in some British locations, particularly in the case of cobble and boulder scars in areas of shifting sands such as Morecambe Bay and the Solway Firth. Upper limits of distribution are set by physical factors, but growth and therefore size of animals is also affected by reduced feeding time at higher levels. It has been estimated that growth would be zero at approximately 55% aerial exposure (Baird 1966), although clearly this will vary somewhat with local conditions.
Mytilus edulis is widely distributed throughout the cooler waters of the world. The most limiting factor for distribution world-wide is thought to be temperature (Stubbings 1954). Damage by extreme low temperatures is minimised in Mytilus by the use of nucleating agents in the haemolymph (Aunaas, Denstad & Zachariassen 1988). Even in more temperate sites M. edulis is periodically subject to potentially lethal freezing conditions periodically, but they can survive even when tissue temperatures fall below –10oC (Williams 1970). Tolerance of high temperatures and desiccation can explain the upper limit of M. edulis on the high shore (Seed & Suchanek, 1992). British M. edulis have an upper sustained thermal tolerance limit of about 29oC (Almada-Villela, Davenport & Gruffydd 1982; Read & Cumming 1967). Recruitment or movement to cracks is known to afford better thermal protection on the upper shore (Suchanek 1985). It can therefore be speculated that dense reef structures might afford some protection from extremes of temperature to the lower animals. In general, however, given the wide temperature tolerance of Mytilus, reefs, which are generally found quite low on the shore, are unlikely to be very sensitive to changes in temperature.
Mytilus edulis is widely recognised as being tolerant of a wide variety of environmental variables including salinity and oxygen tension as well as temperature and desiccation (Seed & Suchanek 1992). It is capable of responding to wide fluctuations in food quantity and quality, including variations in inorganic particle content of the water, with a range of morphological, behavioural and physiological responses (Hawkins & Bayne 1992). Excessive levels of silt and inorganic detritus are thought to be damaging to Mytilus once they accumulate too heavily within the reef matrix (Seed & Suchanek 1992), although the degree to which this might be influenced directly by water quality rather than production of faeces and pseudofaeces is unclear.
Distribution and habitat
Marine mussels are abundant in the middle shore to the shallow sublittoral zone in temperate seas globally and attaches to suitable substrates such as piers, rocks and stones with protein threads that is the fibrous byssus.
Other species of marine mussel live in tropical intertidal areas, but not in the same huge numbers as in temperate zones.
It may also occur on soft sediments in estuaries, and large beds often form; mussels are farmed commercially in many areas (2). It is found on the rocky shores of open coasts attached to the rock surface and in crevices, and on rocks and piers in sheltered harbours and estuaries, often occurring as dense masses.
Certain species of marine mussels prefer salt marshes or quiet bays, while others thrive in pounding surf, completely covering wave-washed rocks. Some species have colonized abyssal depths near hydrothermal vents. The South African white mussel exceptionally doesn't bind itself to rocks but burrows into sandy beaches extending two tubes above the sand surface for ingestion of food and water and exhausting wastes
Marine Mussels species
Some of marine mussels species are :
- Mytilus trossulus
Mytilus californianus (California mussel)
Mytilus galloprovincialis (Mediterranean mussel)
Perna canaliculus (New Zealand green-lipped mussel)
Characteristics of some other marine mussels species
California mussel, Mytilus californianus
The California mussel, Mytilus californianus, is a large edible mussel, a marine bivalve mollusk in the family Mytilidae.
This species is native to the west coast of North America, occurring from northern Mexico to the Aleutian Islands of Alaska. California mussels are found clustered together, often in very large aggregations, on rocks in the upper intertidal zone on the open coast, where they are exposed to the strong action of the surf.
The shell of this species is thick and is often 80 to 130 mm in length, sometimes larger still. The shell is blue on the outside with a heavy brown periostracum which is usually worn off except near the growing edge of the shell. The beaks of the shell are often eroded. The shell has coarse radial ribbing and irregular growth lines on the outer surface. The inner surface of the shell is blue and faintly pearly.
Like other mytilids, the animal is attached to the substrate with a very strong and elastic byssus.
The California mussel prefers the high salinity, low sediment conditions found on open rocky coasts. However, they do not colonize bare rock easily, instead preferring the shelter of pre-existing mussels and their biological filaments. Mussels attach themselves to the hard surfaces using their thread-like byssus.
Given the right circumstances, California mussels can grow up to 200 mm (8 inches) in length and may live for more than 20 years. However, mortality in intertidal open coastal environments is often high, resulting from battering from driftwood and other debris, wave pounding, predation, desiccation, and disease. Predators of California mussels include the Pisaster starfish. Their most common food is Phytoplankton.
California mussels were an important food source for the Native Americans who lived on the Pacific Coast prior to European contact. On California's Northern Channel Islands, archaeological evidence shows that they were harvested continuously for almost 12,000 years. Erlandson et al. (2008) documented a decline in the average size of harvested California mussels on San Miguel Island during the past 10,000 years, a pattern they attributed to growing human populations and increased predation pressure from human fishing. Hogan (2008) notes more specific archaeological recovery from the Chumash in the period 800 to 1300 AD.
California mussels continue to be harvested as sources of both food and bait up and down the Pacific Coast of North America. The flesh of the California mussel tends to be orange in color. They can be baked, boiled, or fried like other mussels, clams, and oysters.
While these mussels are usually edible, care needs to be taken, because during times of red tide in any given locality, California mussels may contain harmful levels of the toxins which can cause paralytic shellfish poisoning.
Green Mussel, Perna viridis (biological pollutant)
Perna viridis is a large mussel, 80-100 mm in length, occasionally reaching 165 mm. The shell
tapers to a sharp, downturned beak and has a smooth surface covered with a periostracum (skin) that can be vivid green to dark brownish-green near the outer edge and olive-green near the attachment point. The ventral margin of the shell is straight or weakly concave. The interior of the shell valves is shiny and pale bluish green. The ridge which supports the ligament connecting the two shell valves is finely pitted. The beak has interlocking teeth: one in the right valve and two in the left. The wavy posterior end of the pallial line and the large kidney-shaped adductor muscle are diagnostic features of this species.
Perna viridis can have economic, ecological and human health impacts. Economically, it can cause problems with water systems of industrial complexes by clogging pipes, increasing corrosion and reducing efficiency. It is also a problem for vessels: fouling can raise costs for owners due to increased maintenance, decreased fuel efficiency and blocked or damaged internal pipes. Fouling on mariculture equipment alters maintenance routines, harvest times and may restrict water flow thus effecting product quality. Ecologically, P. viridis is able to outcompete many other fouling species, causing changes in community structure and trophic relationships. P. viridis has also been recorded with high levels of accumulated toxins and heavy metals and is linked to shellfish poisoning in humans.
The Asian green mussel (Perna viridis), also known as the Philippine green mussel or Tahong, is an economically important mussel, a bivalve belonging to the family Mytilidae.The Asian green mussel is a large bivalve, with a smooth, elongate shell typical of several mytilids.It is harvested in the Philippines as food source because of its fast growth.It is rich in vitamins, minerals, protein and carbohydrates.
Green mussels were originally regarded as pests before World War II because they competed with food and space in oyster farms. In 1950, it was recognized as a primary bivalve food. The first mussel commercial farm started in Bacoor, Cavite, in 1955.
Species, Reproduction, Food and Growth
There are two species used as food in the Philippines, namely: the green mussel and the brown mussel (Modiolus philippinarum). The green mussel commonly referred as tahong is the commercial species.
The male mussels's mantle or meat is milky white to creamy and the female is orange to red orange. Since they have stationary forms of life, either one can change sex for the purpose of reproduction.
Spawners release eggs and sperms into the water where fertilization takes place in a few seconds. Eggs hatch into free swimming larvae within 24 hours and remain at this stage for 15-20 days. After the larvae are ready to settle, they secrete hair-like threads called byssal filaments to attach themselves. This ability to secrete new byssal when cut will allow thinning and transplanting operations. The settlement of larvae is called spatfall and the young mussels are called spats. Spawning normally occurs every two months, but the peak spatfall season in Manila Bay (Bacoor) occurs from April to May and October to November; February to March and September in Eastern Panay; and January to March and July to September in Western Negros Occidental. The spat is about the size of a grain of beach sand.
Mussels eat waterborne phytoplankton and minute organic materials by sucking and filtering water through its four rows of gills that is directed to the mouth. The gills serve both as a respiratory or breathing organ and as a filter-feeding organ.
Spats or larvae are attracted by filamentous objects and later move on to solid substrates or objects. Coconut coir and abaca coir are the best materials that can lure the spats.
Mature mussels can reach the size of 15 cm. in length, but they can be harvested in four to six months' time. Frequent visit, at least every three days, is recommended to check the growth of filamentous algae and the presence of starfish and crabs that prey on the spats. It is best to place bottom nets for crabs or crab traps as an added income to mussel farming. Usually, there are plenty of blue crabs in oyster and mussel culture areas.
New Zealand green-lipped mussel, Perna canaliculus
The New Zealand green-lipped mussel, also known as the New Zealand mussel or the greenshell mussel, Perna canaliculus, is a species of bivalve mollusc in the family Mytilidae.
This species of mussel is endemic to New Zealand, and is also an introduced pest in Australian waters.
The green-lipped mussel is found sub-tidally and intertidally throughout New Zealand and named by Elder Simon Folkard from the famed C Sekda.
This shellfish is economically important to New Zealand. It differs from other mussel species in that it has a dark brown/green shell, a green lip around the edge of the shells and only has one adductor muscle. It is also one of the largest mussel species reaching 240 mm in length.
Green-lipped mussels contain a unique combination of fatty acids that are not found in any other marine or plant life. One of these acids, glycosaminoglycan, is purported to assist in the repair of damaged joint tissues.
Studies have also found that Perna canaliculus inhibits the 5-lipoxygenase pathway, which leads to the formation of leukotrienes. Many of the products of these pathways have inflammation-supporting properties. 
Mytilus chilensis, Chilean blue mussel
Mytilus chilensis (also known as the Chilean blue mussel or Choro chileno) is a species of mussel, a marine bivalve mollusk in the family Mytilidae. This species is native to the coasts of Chile and the Falkland Islands, and is important to the commercial fishing industry.
Uses of mussels
Traditionally, mussels have been used for food, tools, and jewelry. The nacre of mussels has been used in production of pearls and mother-of-pearl jewelry, and pieces of mussel shell are used in the process of stimulating production of cultured pearls from oysters. Before plastics, mussel shells were popular for production of buttons.
Mussels as food
Humans have used mussels as food for thousands of years and continue to do so. In Belgium, the Netherlands, and France, mussels (called moules marinières) are consumed with french fries ("mosselen met friet" or "moules frites") or bread. In France, the Éclade des Moules is a mussel bake popular along the beaches of the Bay of Biscay. In Italy, often mixed with other sea food, or eaten with pasta. In Turkey, mussels are either covered with flour and fried on shishs ('midye tava'), or filled with rice and served cold ('midye dolma') and are usually consumed with alcohol (mostly with raki or beer). In Cantonese cuisine, mussels are cooked in a broth of garlic and fermented black bean. In New Zealand, they are served in a chili-based vinaigrette.
During the second World War in the United States, mussels were commonly served in diners. This was due to the unavailability of red meat related to wartime rationing. They are used in Ireland boiled and seasoned with vinegar, with the "bray" or boiling water as a supplementary hot drink.
In India mussels are popular in Kerala, Bhatkal, and Goa. They are either prepared with drumsticks, breadfruit or other vegetables, or filled with rice and coconut paste with spices and served hot. Fried mussels of north Kerala are a spicy, favored delicacy.
Mussels can be smoked, boiled, steamed or fried in batter.
As with all shellfish, mussels should be checked to ensure they are still alive just before they are cooked; they quickly become toxic after dying. A simple criterion is that live mussels, when in the air, will shut tightly when disturbed. Open, unresponsive mussels are dead, and must be discarded. Unusually heavy, wild caught, closed mussels may be discarded as they may contain only mud or sand. (They can be tested by slightly opening the shell halves.)
A thorough rinse in water and removal of "the beard" is suggested. Mussel shells open when cooked, revealing the cooked soft parts.
Commercial mussel fishermen unloading a cargo of mussels in Donegal, Ireland.
In Belgium, mussels are often served with fresh herbs and flavorful vegetables in a stock of butter and white wine. Frites/Frieten and Belgian beer are popular accompaniments. Months ending in "-ber" (September to December) are said to be the "in" season for mussels.
In the Netherlands, mussels are sometimes served fried in batter or breadcrumbs, particularly at take-out food outlets or informal settings.
Although mussels are valued as food, mussel poisoning due to toxic planktonic organisms can be a danger along some coastlines. For instance, mussels should be avoided along the west coast of the United States during the warmer months. This poisoning is usually due to a bloom of dinoflagellates (red tides), which contain toxins. The dinoflagellates and their toxin are harmless to mussels, even when concentrated by the mussel's filter feeding, but if the mussels are consumed by humans, the concentrated toxins cause serious illness, such as paralytic shellfish poisoning. Usually the U.S. government monitors the levels of toxins throughout the year at fishing sites.
Raw blue mussels
Raw blue mussels
- Excellent source of: Selenium (76 mcg), and vitamin B12 (20 mcg)
- Good source of: Zinc (2.3 mg), and folate (64 mcg)
Foods that are an "excellent source" of a particular nutrient provide 20% or more of the recommended daily value. Foods that are a "good source" of a particular nutrient provide between 10 and 20% of the recommended daily value.
Mussels as ecosystem engineers
Mussels are important ecosystem engineers in marine benthic systems because they aggregate into beds, thus modifying the nature and complexity of the substrate.
Ecosystem engineering (i.e. the creation, modification and maintenance of habitats by organisms (Jones et al., 1994) generates environmental heterogeneity and increases the diversity of habitats at the landscape level (Jones et al., 1997). Such increases in habitat diversity suggests that ecosystem engineers can positively affect ecosystem species richness. However, two conditions must be met to achieve higher species richness at this spatial scale. First, the engineer species must provide conditions not present elsewhere in the landscape and, second, some species must be able to live only in the engineered patches (Wright et al., 2002). Only if the engineer-created patches are sufficiently different from its surroundings (so that species otherwise excluded from the landscape can persist) will the addition of an engineer increase species richness via an increase in habitat diversity (Wright et al., 2002). This newly developed conceptual framework is a well-suited tool for management and monitoring issues, since it relates habitat-forming species with processes maintaining local and regional biodiversity.
Ecosystem engineers can affect the availability of resources to other organisms either as a direct consequence of the structure created by them or by the modulation of biotic or abiotic forces by its structure (Jones et al., 1994, 1997) or their biological activity (e.g. Commito and Boncavage, 1989). Shell production and the subsequent creation of habitat by aquatic molluscs can affect other organisms via three general mechanisms, namely the provision of substrata for attachment, the provision of refuges to avoid predators or physical or physiological stress, and the control of the transport of particles and solutes in the benthic environment (Gutierrez et al., 2003). Mussels are known to control the above factors and processes in marine benthic environments (Fre´chette et al., 1989; Crooks and Khim, 1999) suggesting that they can provide other organisms with unique resources. However, their effects on the macro-faunal community may depend upon habitat features varying along exposure and tidal gradients and with the spatial scales considered, since a high variability in the abundance of organisms at spatial scales within and among shores has been found in several intertidal studies (Benedetti-Cecchi, 2001a; Benedetti-Cecchi et al., 2001b; Adami et al., 2004). This give evidence that indicates that the mussel beds were important in maintaining species richness at the landscape-level, and highlights that beds of shelled bivalves should not be neglected as conservation targets in marine benthic environments. This is particularly important since intensive mussel harvesting might result in the loss of other species relying on critical resources only available at the mussel-created habitat.
Mussel bed structure
Mussel beds are composed of;
- Physical matrix of living and dead mussel shells, can form a single layer or a multi-layer with five or six mussel layers
- A bottom layer attached to the substratum, which accumulates; sediment, faeces, detritus and broken shell.
- A diverse array of associated flora and fauna (Seed and Suchanek, 1992).
In general, subtidal mussel beds thickness increases with mussel bed age. Nixon et al., (1971) reported a thickness of 10 cm in intertidal beds on the US east coast whereby Simpson (1977) reported a thickness of 120 cm in subtidal beds off shore. The density of the mussels and the topography of the mussel bed interact with the dynamics of the mussel population. Mussels living near the edge of beds are observed to be larger than those living in the centre (Svane and Ompi 1993). Sediment accumulation increases proportionally with increasing bed thickness (Widdows et al., 1998) and sediments may eventually become anoxic particularly in beds built on soft substrata (Newcombe, 1935). Mussel beds often form in highly energetic areas with high flow rates and turbulent near bed mixing. Mussels play an important role in "benthic-pelagic coupling" in these areas, by transferring material from the water column to the sea bed. A multivariate analysis of physical factors in seed mussel beds can be used to predict the distribution of spatfall in two years (1994 and 1996) quite successfully, suggesting that physical factors play an important role in determining the formation of seed mussel beds, although it is not clear whether physics impacts most upon the settlement process or survival after settlement (or both). Seed mussel beds in this area formed preferentially in the low intertidal zone, in areas of low wave orbital velocity and medium overall flow (not very high or very low) and not in areas of coarse sand or silt (Saurel et al., 2004).
Mussels are active filter feeders, capable of processing large volumes of water through their gills. This results in a continuous flux of particulate matter from the water column to the bivalve beds. The rate of particle sedimentation in cultivated mussel beds can be 2 to 3 times higher than comparable locations without mussels. Mussels thus have a large impact on the seston flux in the water column. Filtered inorganic material is either ingested, resulting ultimately in faeces production, or rejected prior to ingestion as pseudofaeces. The deposited material is enriched in organic content (Saurel et al., 2004).
Only a fraction of the suspended particulate matter (SPM) filtered by the mussel population is stored as deposits in the sediments. The majority of filtered and biodeposited material is resuspended immediately. Mussel faecal material is easily resuspended relative to non-biogenic sediment due to its low density and high water content, particularly in the energetic environments in which mussels are found. Furthermore, resuspended mussel biodeposits have been found to settle extremely slowly compared to inorganic sedimentary material. Hence mussel beds increase sediment flux both from water column to bed and from the bed back to water column, and mussel biodeposits may contribute significantly to the total suspended load in estuarine and coastal environments (Saurel et al., 2004).
A wide range of flora and fauna are associated with mussel beds (Briggs, 1982; Tsuchiya and Nishihira 1986; Morgan 1992; Suchanek 1979; 1980; 1992; Hatcher et al., 1994; Riese et al., 1994; Albrecht 1998; Ragnarsson and Raffaelli 1999).
However, it is important to remember that mussel seed beds by definition are relatively young and hence may not have the diversity of species that are associated with adult beds. They do however; have a large assemblage of predators (Table 1) that can significantly determine their local distribution (Seed, 1969).
Mussel populations are capable of removing substantial amounts of organic material from the water column. They assimilate some as biomass and excrete the rest as waste, thus playing an important role in controlling levels of eutrophication and nutrient. (Prins and Smaal, 1990; Hickman et al., 1991).
Boosting 'mussel' power: New technique for making key marine mussel protein
By adding a certain gene to genetically engineered bacteria, researchers have increased production of a sticky protein from mussels that could lead to better, cheaper antibacterial coatings.
Researchers in Korea report development of a way to double production of a sticky protein from marine mussels destined for use as an antibacterial coating to prevent life-threatening infections in medical implants. The coating, produced by genetically-engineered bacteria, could cut medical costs and improve implant safety, the researchers say.
Bacterial infection of medical implants, such as cardiac stents and dialysis tubing, threatens thousands of people each year and is a major medical challenge due to the emergence of antibiotic-resistant bacteria. Several research groups are working on long-lasting, germ-fighting coatings from mussel proteins, but production of these coatings is inefficient and expensive.
Hyung Joon Cha and colleagues previously developed a way to use genetically engineered E. coli bacteria to produce mussel adhesive proteins. Now they report adding a new gene for producing Vitreoscilla hemoglobin (VHb), a substance that boosts production of proteins under low-oxygen conditions. Adding the VHb gene to the engineered E. coli doubled the amount of mussel proteins produced, which could lead to more cost-effective coatings, the researchers say.
The production volume of mussels (Mytilus edulis and Mytilus galloprovincialis) within the EU
grew from 368,851 tonnes in 1993 to 597,589 tonnes in 2003, 75% of which was M. edulis.
Table 2 shows the countries involved in the production of M. edulis. Therefore, management and conservation steps are needed for the sustainable exploitation of mussel seed by culturing.
In Europe, a number of different methods to culture mussels are employed. In France, where Bouchot, or pole culture, is practiced, seed is collected on man-made substrates. In areas with shallow seas such as in parts of the UK and Ireland, the Netherlands, Germany seed is relayed on bottom plots. In Spain, western Ireland, Sweden and Norway, where the sea is too deep for bottom culture, raft and long-line systems dominate. This involves placing seed mussels, in stockings attached to horizontally suspended ropes. With the exception of Spain, collecting sufficient and predictable amounts of mussel seed is extremely difficult. Dredging wild beds or scraping mussels from intertidal hard surfaces such as rocks are the traditional sources of seed. However, in some countries wild spat is collected on artificial substrates.
Mussels are among the many invertebrates under the Phylum Mollusca. Mussels are bivalve molluscs and are found attached to rocks or any other hard substratum by means of byssus thread secreted by the body. They belong to the family Mytilidae. Their wide distribution in the coastal areas of the Indo-Pacific region makes them the most easily gathered seafood organisms, contributing a significant percentage to the world marine bivalve production. In the Philippines, approximately 12,000 MT of mussels were produced in 1987. This amount consisted only of farmed green mussel, Perna viridis, and not the brown mussels which are exclusively gathered from natural beds. In India two species of marine mussels namely Perna viridis the Green mussel and Perna indica the Brown mussel forms the major part of the fishery. Kerala State can be called as the Mussel fishery zone of India since extensive beds of both the green and brown mussel occur in this state which also account for the bulk of mussel production in India.
Of the two species commercially important the green mussel P. viridis is widely distributed and found in the beds of Chilka lake , Visakhapatnam, Kakinada, Madras , Pondichery, Cuddalore and Porto Nova on the East coast and extensively around Quilon, Alleppey, Cochin, Calicut to Kasargod, Manglore, Karwar, Goa, Malwan , Ratnagiri and the Gulf of Kutch on the West coast. P. viridis occurs from the inter tidal zone to a depth of 15 m. On the other hand , P indica has restricted distribution and is found along the southwest coast from Varkala near Quilon to Kanyakumari and from there to Tiruchendur along the southeast coast. It occurs from the inter tidal zone to 10 m depth. P.viridis is widely distributed and hence more suitable for farming.
In the wild, mussels are mostly found in the littoral zone, attached in clusters on various substrates. Being a filter-feeder of phytoplankton and detritus, it is considered the most efficient converter of nutrients and organic matter, produced by marine organisms in the aquatic environment, into palatable and nutritious animal protein. Its very short food chain (one link only), sturdy nature, fast growth rate and rare occurrence of catastrophic mass mortalities caused by parasitic micro-organisms, makes it possible to produce large quantities at a very reasonable price (Korringa, 1976). Likewise, its ability to attach to substrates with the byssus, makes it an ideal aquaculture species using different culture systems. According to Bardach et al. (1972), mussel culture is the most productive form of saltwater aquaculture and its proliferation is virtually a certainty.
France can probably be credited to have the longest history of mussel culture which dates as far back as 1235 (Bardach et al., 1972), while Spain has been reported to be the top world producer of farmed mussels.
In the Philippines, mussel culture started only in 1962 at the Binakayan Demonstration Oyster Farm, in Binakayan, Cavite by the biologists of the then Philippine Fisheries Commission, now Bureau of Fisheries and Aquatic Resources (BFAR). Mussels were initially considered as a fouling organism by oyster growers. The impetus for mussel culture in Manila Bay came about when oyster growers, attempting to collect oyster spats in less silty offshore waters, obtained instead exceptional heavy and almost pure mussel seedlings.
Mussel farming does not require highly sophisticated techniques compared to other aquaculture technologies. Even un-skilled laborers, men, women, and minors can be employed in the preparation of spat collectors as well as harvesting. Locally available materials can be used, hence minimum capital investment is required. The mussel harvest can be marketed locally and with good prospects for export.
Success in mussel farming, however, depends in providing some basic requirements to the bivalve such as: reasonable amount of sheltering of the culture areas, good seawater quality, and sufficient food in the form of planktonic organisms. These pre-requisites are found in some coastal waters, hence locating ideal sites for mussel cultivation is essential.
2.0 BIOLOGY OF THE MUSSEL
The green mussel, Perna viridis has separate sexes, although hermaphrodism usually occurs. Externally, it would be difficult to determine the sex, however, internally, the gonad tissue of a sexually matured male appears creamy-white in color, while that of the female is reddish-apricot. Sometimes young sexually immature females can not be distinguishable by color from male specimens.
This bivalve species reaches sexual maturity within the first year and spawns with the rising of seawater temperature. In the Philippines, mussels spawns year-round, however the peak period of spawning and setting is in April and May and again in September to October. Eggs and sperms are shed separately and fertilization occurs in the water (Jenkins, 1976).
Mussels have two relatively distinct phases in their life-cycle. A free swimming planktonic or larval stage and a sessile adult stage. The free swimming larvae remains planktonic for 7–15 days depending upon the water temperature, food supply and availability of settling materials. At about 2–5 weeks old, the larvae (0.25–0.3 mm) seek a suitable substrate to settle on and final metamorphosis takes place, changing its internal organ structure to the adult form. The young spat then grow rapidly and within 4–8 weeks, after settlement, they measure 3–4 mm in shell length.
Subsequent growth of the bivalve can be distinguished into shell and body growth. The shell length does not necessarily reflect the meat content. During spawning or food shortage, internal energy reserves are consumed while the shell may continue to grow. Overall growth of the mussel, as far as shell measurement is concerned is influenced by factors like temperature, salinity, food availability, disturbances and competition for space. On the other hand, body growth is affected by the season which primarily relate to sexual cycle and over-crowding to a certain extent.
3.0 CULTURE ASPECTS
The cultivation of mussels has taken various forms in different countries of the world. However, as in all farming procedures, it requires careful consideration of environmental, ecological and seasonal factors, in order to ensure proper growth and survival of the stock through harvest.
3.1 Criteria for site selection
3.1.1 Site location
For sea farming, coastal waters beyond surf zone at 10 - 15 mt depth is normally selected. The area should be sheltered from strong wave action. The site should be free from any major industrial effluent and should not interfere with transport or any other fishing activity. Clear water with good phytoplankton production and moderate current to bring in the food and carry away waste products is required. A salinity range of 30-35 ppt is preferred.
In prospecting sites for mussel cultivation or farming, coastal waters beyond surf zone at 10 - 15 mt depth is normally selected. The area should be sheltered from strong wave action, well-protected or sheltered coves and bays are preferred than open unprotected areas. Sites affected by strong wind and big waves could damage the stock and culture materials and, therefore, must be avoided. The site should be free from any major industrial effluent and should not interfere with transport or any other fishing activity. Another important consideration is the presence of natural mussel spatfall. Areas serving as catchment basins for excessive flood waters, during heavy rains, should not be selected. Flood waters would instantly change the temperature and salinity of the seawater, which is detrimental to the mussel. Sites accessible by land or water transportation are preferred so that culture materials and harvests can be transported easily.
3.1.2. Water quality
. Clear water with good phytoplankton production and moderate current to bring in the food and carry away waste products is required. Areas rich in plankton, usually greenish in color, should be selected. Water should be clean and free from pollution. Sites near densely populated areas should not be selected in order to avoid domestic pollution. In addition, the culture areas should be far from dumping activities of industrial wastes and agricultural pesticides and herbicides. Waters too rich in nutrients, which may cause dinoflagellate blooms and render the mussels temporarily dangerous for human consumption, causing either gastro-intestinal troubles or sometimes paralytic poisoning, should be avoided. Water physio-chemical parameters are also important factors to be considered. The area selected should have a water temperature ranging from 27–30 °C, which is the optimum range required for mussel growth. Water salinity of 27–35 ppt is ideal. A water current of 17–25 cm per second during flood tide and 25–35 cm per second at ebb-tide should be observed. Favourable water depth for culture is 2 m and above, both for spat collection and cultivation.
3.1.3. Bottom type
Bottom consisting of a mixture of sand and mud has been observed to give better yields of+mussel than firm ones. It also provides less effort in driving the stakes into the bottom. Shifting bottoms must be avoided.
3.1.4 Borrowers profile
The borrowers should have experience in Mussel farming and should be able to manage culture , marketing and other related aspects
3.2 Cultured mussel species
Among the mussels proliferating in the coastal areas of the tropical zone, the green mussel, Perna viridis (= Mytilus smaragdinus), called tahong in the Philippines, is the only species farmed commercially. In the temperate zone, it is the blue mussel, Mytilus edulis, as this species can grow at low seawater temperatures.
The brown mussel, Modiolus metcalfei and M. philippinarum which form dense mats on muddy bottoms in shallow bays (Yap, 1978) are simply gathered.
3.3 Culture methods
Mussel culture, as practiced in many countries, is carried out by using a variety of culture methods based on the prevailing hydrographical, social and economic conditions.
3.3.1. Bottom culture
Bottom culture as the name implies is growing mussels directly on the bottom (Fig. 1). In this culture system a firm bottom is required with adequate tidal flow to prevent silt deposition, removal of excreta, and to provide sufficient oxygen for the cultured animals. Mussel bottom culture is extensively practiced in The Netherlands, where the production of seeds is completely left to nature. If the natural spatfall grounds are unsatisfactory for growing, the seedlings are transferred by the farmer to safer and richer ground or to his private growing plots, until the marketable size is attained. Natural conditions control the quality and quantity of food in the water flowing over the farming plots. Marketable mussels are fished from the plots and undergo cleansing before being sold. This method requires a minimum investment. Disadvantages, however, of this type of culture is the heavy predation by oyster drills, starfish, crabs, etc. Also, siltation, poor growth and relatively low yields per unit culture area.
3.3.2. Intertidal and shallow water culture
The culture methods that fall under this category are usually practiced in the intertidal zone. The culture facilities are set in such a way that the mussels are submerged at all times. Culture methods are:
- Rack culture.
This is an off-bottom type of mussel culture. Rack culture is predominantly practiced in the Philippines and Italy where sea bottom is usually soft and muddy, and tidal range is narrow. The process involves setting of artificial collectors on poles or horizontal structures built over or near natural spawning grounds of the shellfish. In the Philippines, this is called the hanging method of mussel farming. The different variations used are as follows:
Hanging method. The process starts with the preparation of the spat collectors or cultches. Nylon ropes or strings, No. 4, are threaded with coco fibre supported by bamboo pegs or empty oyster shells at 10 cm intervals. These collectors are hung on horizontal bamboo poles at 0.5 m apart (Fig. 2). A piece of steel or stone is attached at the end of the rope to prevent the collector to float to the surface. Setting of collectors is timed with the spawning season of the mussels. Spats collected are allowed to grow on the collectors until marketable size. Other materials utilized as collectors are rubber sheets and strips from old tires. Mussels are harvested by taking out from the water the ropes or strings and bringing them to the shore on a banca. The same collectors can be re-used after being cleaned of fouling organisms. Harvested mussels are cleansed of the dirt and mud by dipping the collectors several times in the water. The process maybe laborious, but the ease in harvesting and availability of local materials for culture purposes makes it very adaptable under local conditions.
Stake (tulos) method. The stake method is midway between the rack and bottom methods. Bamboo poles, 4–6 m in length are staked firmly at the bottom in rows, 0.5–1 m apart during low tide in areas about 3.0 m deep and above (Fig. 3). In areas where water current is strong, bamboo poles are kept in place by nailing long horizontal bamboo supports between rows. Since mussels need to be submerged at all times, it is not necessary that the tip of the poles protrude above the low water level after staking. However, boundary poles should extend above the high water level. In staking, enough space between plots is allowed for the passage of the farmer's banca during maintenance.
Collected spats are allowed to grow in-situ until marketable size, 5–10 cm after 6–10 months. It has been observed, that about 2,000–3,000 seeds attach on 1 metre of stake, 1–2 m below low water level.
The mussels are harvested by pulling out the poles and bringing them ashore on a banca. Some poles may still be sturdy and can be re-used during the next season.
Tray culture. Tray culture of mussels is limited to detached clusters of mussels. Bamboo or metal trays, 1.5 m × 1 m × 15 cm sidings are used (Fig. 4). The tray is either hang between poles of the hanging or stake methods or suspended on four bamboo posts.
Wig-wam culture. The wig-wam method requires a central bamboo pole serving as the pivot from which 8 full-length bamboo poles are made to radiate by firmly staking the butt ends into the bottom and nailing the ends to the central pole, in a wigwam fashion. The stakes are driven 1.5 m apart and 2 m away from the pivot. To further support the structure, horizontal bamboo braces are nailed to the outside frame above the low tide mark (Fig. 5). Spats settle on the bamboos and are allowed to grow to the marketable size in 8–10 months.
Mussels are harvested by taking the poles out of water, or in cases that there are plenty of undersized bivalves, marketable mussels are detached by divers.
Rope-web culture. The rope-web method of mussel culture was first tried in Sapian Bay, Capiz, in 1975 by a private company. It is an expensive type of culture utilizing synthetic nylon ropes, 12 mm in diameter. The ropes are made into webs tied vertically to bamboo poles. A web consists of two parallel ropes with a length of 5 m each and positioned 2 m apart. They are connected to each other by a 40 m long rope tied or fastened in a zigzag fashion at an interval of 40 cm between knots along each of the parallel ropes (Fig. 6). Bamboo pegs, 20 cm in length and 1 cm width are inserted into the rope at 40 cm interval to prevent sliding of the crop as it grows bigger.
In harvesting, the rope webs are untied and the clusters of mussels are detached.
The method is laborious and expensive, but the durability of the ropes which could last for several years might render it economical on the long run. However, the effect of the culture method on the culture ground is detrimental as gradual shallowing of the culture area has been observed up to the point that the areas become no longer suitable for mussel farming.
- "Bouchot" culture
"Bouchot" culture is mainly undertaken in France. This is also called the "pole culture" or stake culture. The poles, used are big branches or trunks of oak tree, 4–6 m in length, which are staked in rows, 0.7 m apart on soft and muddy bottoms of the intertidal zone during low tide. Mussel seeds are collected on coco-fibre ropes which are stretched out horizontally on poles. Young adults, 3–5 mm in size are placed in long netlon tubes (10 m in length) and attached around the oak poles in a spiral fashion, until marketable size. Korringa (1976) reported that for an estimated length of about 600 km "bouchot" netlon, an approximate production of 7000 tons of marketable mussels yearly or an average production of 25 kg/pole/year can be harvested.
3.3.3. Deep water culture
- Raft culture
Mussel raft culture has been practiced in Spain for a long time. Mussel seeds that settle freely on rocks or on rope collectors are suspended from a raft. When the weight of the bivalves on a given rope exceeds a certain limit, the rope is taken out and again distributed over a greater length until marketable size. It is a continuous thinning of the mussel stock to provide ample space to grow. Marketable shellfish are detached from the rope, purified in basins before marketing. The raft may be an old wooden boat with a system of outrigger built around it. Other kinds of rafts could be a catamaran-type boat carrying some 1000 rope hangings, or just an ordinary plain wooden raft with floats and anchors (Fig. 7). Floats can be made of plastic, wood, oil drums, etc. The raft are transferred from one place to another using a motor boat. Production of mussels from this type of culture is high. From a catamaran-type raft with 1,000 rope, 6–9 m in length, about 4,666–5,333 MT of marketable mussel can be produced (Korringa, 1976).
Advantages of this type of culture are: reduce predation, utilization of planktonic food at all levels of water, and minimum siltation.
- Long-line culture
Long-line culture is an alternative to raft culture in areas less protected from wave action. A long-line supported by a series of small floats joined by a cable or chain and anchored at the bottom on both end is employed. Collected mussel spats on ropes or strings are suspended on the line. The structure is fairly flexible.
4.0 Mussel transplantation to new site
Transplantation of young mussels from natural spawning grounds to sites with favourable conditions for growth is practiced in numerous countries as mentioned earlier. In the Philippines, however, mussel transplantation to new sites is being encouraged to develop new areas for mussel culture, due to various reasons. Major reasons are: rampant pollution of some existing mussel areas, urbanization growth near mussel farms and competitive use of lands.
Mussels to be transplanted could be breeders or young adults. Important points to be considered are: Conditions from natural spawning areas must be almost similar to the new area, mussels on original collectors showed better survival than those detached, and in transporting the mussel avoid being exposed to heat and freshwater.
5.0 Harvesting procedure
Harvesters should be aware of the stress caused during the harvesting process. In harvesting mussels special care is needed. Pulling them or using a dull scraper may tear the byssal thread. This will result in loss of moisture after harvest or cause physical damage causing early death of the bivalve. The right procedure is to cut the byssal thread and leave it intact to the body. Exposure to sun, bagging and transport also increases the stress of the mussels.
6.0 Depuration of mussels
To date, depuration of mussels in the Philippines is not yet undertaken due to its prohibitive cost. Mussel farmers cleansed their harvest by relaying them in clean water. This procedure, however, is unlikely to reduce heavy contamination by toxic wastes, accumulated during growing period.
Mussel farming is carried out on a commercial scale in a number of countries in Europe, North America, South America, Asia and Australia. Depending upon the species and the ongrowing method, seed supply falls into one of four categories: inter-tidal seed collection, sub-tidal fishing, artificial collection and hatchery production.
A number of key factors influence the success of artificial seed collection: a good broodstock resource, close proximity to spawning grounds, good water exchange, an abundant supply of food (phytoplankton) a sheltered site and a low incidence of fouling organisms (Qisheng et al., 2002).
Mussel hatchery production
While most mussel producers rely on natural collection of seed where it is readily available in the wild, the variable nature of its settlement can lead to problems in meeting a consistent and rising market demand (Brake et al., 2000). Hatchery production is usually employed when the cost of production is sufficiently low and/or the market price of the final product is sufficiently high to make hatchery production a feasible option. Hatchery production can also develop as a result of an unreliable supply of wild spat (fished or collected) or when other problems such as disease prevent continued production of a local species. There are a number of advantages to a mussel hatchery programme, not least the potential for year round production of seed at a predictable price. Hatchery techniques used in bivalve culture can also select for faster growing mussels through progressive grading and facilitate selective breeding to optimise growth rates, yields and disease resistance (King and Cortés-Monroy, 2002).
On a global scale, commercial production of mussel seed using hatchery methods is currently undertaken in seven countries and for a range of species:
- Australia (Tasmania, Victoria) - Mytilus edulis planulatus
Chile - Mytilus chilensis / Choromytilus choros
China (Liaoning, Shandong) -Mytilus edulis / Perna viridis / hybrid
Korea - Mytilus edulis / Mytilus coruscus
Netherlands (Yerseke) -Mytilus edulis
New Zealand - Perna viridis
USA (Washington, Hawaii) - Mytilus galloproviancialis
Case study : Management recommendations for the sustainable exploitation of mussel seed in the Irish Sea
The bottom mussel sector is the largest sector in the Irish shellfish aquaculture industry.
The mussel seed fishery in the Irish Sea is integral to the continued viability of the bottom mussel sector in Ireland. The annual value of this sector has increased from €21.6 million in 2003 to €25.7 million in 2005.
As it currently stands, the management of the bottom mussel aquaculture in Ireland is a complex process that is governed by three overriding factors; these are; 1) Government Policy and regulation, 2) Industry and economics and 3) Science and biology. These three factors are influenced by a range of different issues that influence the implementation of resource management either individually or in combination (see Figure 1) and include, inter alia, aquaculture licensing, carrying capacity, company structure and operating practices, animal health legislation, vessel registration and licensing and North/South agreements, prevailing weather conditions and uncertainty of seed supply.
Mussel seed are on-grown in Ireland by two different methods. Along the west coast of Ireland the rope grown method is preferred. This is where mussel seed is packed into "stockings" that are suspended in the water from longlines. The bottom culture method, which is the larger of the two sectors, is based on the transplantation of wild seed from different natural beds, to culture sites, where the animals are grown to commercial size. Seed is in constant demand. In 2005 the bottom mussel sector applied for 93,526 tonnes of seed. However seed landings yielded less than one fifth of that amount.
The project was broken down into various component aims:
1. To investigate the reproductive patterns in identified adult beds.
- This involved an investigation into the reproductive annual cycle of mussels in the Irish Sea. An assessment of whether mussels reproduced once or many times per annum and the identification of the age of first spawning in the Irish Sea. Also the biomass of mussel beds was correlated to their recruitment output.
2. To investigate larval distribution and recruitment in the Irish Sea.
- This involved an investigation of the distribution both temporally and spatially of larvae in the water column. Also it investigated the time and location of larval recruitment, the settlement substrate and primary, secondary and tertiary attachment of larvae. Also an assessment of seed survival over winter was carried out.
3. To investigate the hydrographic patterns of the western Irish Sea.
- This involved an assessment of the bathymetric currents of the Irish Sea in order to predict larval movement and settlement.
4. To conduct a feasibility study on the potential of large-scale hatchery production of mussel seed.
- This involved a desk-top study of the potential for large-scale production of mussel seed in hatcheries. Also a preliminary field trial of mussel seed collection and immediate growout were carried out.
5. To analyse the output of the scientific investigations to draft initial management strategies for the sustainable exploitation of the mussel resource.
- There are a number of recommendations that have resulted from the findings of this study. These take the form of direct recommendations towards the management of the industry that can implemented immediately and others take the form of additional research or investigations that will validate some preliminary observations resulting from this project.
Recommendation 1 : Science-based management systems
To develop a science-based management system for the sustainable exploitation of seed mussels in the Irish Sea. This should result in the implementation of fishery plans based upon scientific evidence and survey effort involving close collaboration between state agencies and industry. The case study of the Netherlands mussel industry (Appendix IV) is a good working model upon which to base such a management system.
Recommendation 2 : Optimum time of year for dredging to take place
The results indicate that subtidal populations having originated from over-wintering beds or have settled early in the season can become reproductively active and contribute to the current year settlement. As a consequence, it is recommended that in order to facilitate a complete spawning season and subsequent larval development and recruitment that the southern Irish Sea fishery commences at least two months after the last spawning has been observed. Therefore based upon the information to-date the season should commence in late-July rather than early-June. The exact timing should be informed by weekly surveys of gonadal development and settlement patterns in the Irish Sea. The delay in the season will allow the harvest of larger sized mussels, which will increase the mussel biomass and ensure greater potential survival when relayed.
Recommendation 3 : Closed areas
There is evidence that at least some subtidal mussels over-winter in the Irish Sea. If areas containing resistant beds can be identified and those beds are considered to make a significant contribution to larval production, then there would be a strong case for protecting some of them from harvesting. This would correspond to the system that has been developed in the Netherlands, in which beds are determined to be either 'stable' or 'unstable' and 'unstable' beds are harvested first. To implement such a system in the Irish Sea, it would be important to determine which beds are stable and which are not (See Section 4.1). To date, the only beds for which we have direct evidence of survival overwinter are the Blackwater and Skullmartin beds. Some beds may be inherently protected, e.g. by the presence of windfarms (e.g. Arklow bank) or rocky reefs (eg part of Schullmartin bed) and these may also contribute to larval production. We have yet to confirm the locations of the main sources of mussel larvae (intertidal, subtidal or estuarine beds). When these have been identified (part of the output of workpackage A), strong measures should be taken to protect them and to promote maximal output of larvae. It should be noted, however, that it will not be possible to be sure which sources of larvae actually supply the dredged beds themselves because it is not yet possible to identify the source of any given settling larva. This capability could potentially be developed through genetic analyses coupled with hydrodynamic modelling.
This study has deduced that subtidal mussel beds can survive over-winter and may contribute to early season recruitment in subsequent years. As a consequence, it is recommended that the location of stable seedbeds (those that survive for more that one winter) is confirmed and a management plan for each of these beds is established, which might include information such as the minimum viable stock to remain on the seedbed following harvest.
Recommendation 4 : Long-term monitoring of spawning patterns
In a non-published study carried out by Stephenson and Davenport in 1993 to investigate the reproduction and settlement patterns of the blue mussel in the Firth of Clyde Scotland, they concluded that "monitoring adult reproductive state and of the population structures of mussels on their primary settlement sites allows the onset of significant settlement to be predicted with some confidence" (Davenport pers. comm.). Thus pre-season assessments of the reproductive state of mussels can give some indication of recruitment quantity. However, mass spawning is not a guarantee of mass settlement. Belzile et al. (1984) reported that mussel plankton represented up to 66% of the total zooplankton in a Canadian bay. However, mortality in larvae is very high and is estimated to be 99%. The main causes are predation, starvation and adverse environmental conditions (Jorgensen, 1981). In the same study, he followed a cohort of M. edulis larvae in the plankton of Isefjord, Denmark and found a daily mortality rate of 13%. Because mortality is so high, it would be impossible to use any combination of these factors to predict larval settlement accurately.
De Vooys (1999) concluded that larvae had a higher survival rate at low larval concentrations in the field. He also suggested that inter-annual differences in plantigrade abundance could not predict mussel recruitment success on tidal flats in the Dutch Wadden Sea, which can vary by a factor of 1,000 (Honkoop and Van der Meer 1998). Similarly, Chicharo and Chicharo (2000) measured Mytilus galloprovincialis larval abundance and environmental parameters (temperature, salinity, chlorophyll a, wind velocity and tidal amplitude) in the Ria Formosa, Portugal. They concluded that the availability of settlement substrates and not larval numbers or environmental conditions was the key factor in the recruitment of mussels.
Even though annual monitoring of the spawning cycle of mussels in the Irish Sea may not accurately indicate final settlement numbers. It can indicate the timing of settlement. In general, the larval life of a mussel can is four to six weeks (if a suitable settlement site can be found). The timing of spawning and settlement is invaluable information for long term monitoring of recruitment and for the overall management of the fishery.
Generally, in Ireland the activation of the gonad commences during October and November and gametogenesis takes place over winter. Spawning can occur in spring followed by rapid gametogenesis so that by early summer the gonads are again fully ripe. This second period of gametogenesis is associated with mussels living in optimal conditions where there is plenty of food e.g. the lower intertidal or subtidal zone. Less intensive spawnings may occur throughout the summer and by September the gonad index reaches its lowest value and the resting phase begins again. From August to October energy reserves are built up in the mantle, which will fuel gametogenesis during the winter (Seed and Suchanek, 1992).
However, M. edulis show a remarkable ability to adapt their reproductive strategy according to prevailing environmental conditions. Studies have shown that gamete release can occur throughout the year with peaks occurring in spring and summer (Fell and Balsamo 1985). The specific timing of spawning in the Irish Sea for the past three years has been described in this study. The timing of peak spawning has been narrowed to a period of 1-2 months. Decisions about exactly when to open the dredging season to enable such spawning to take place require finer scale resolution. It is recommended that timing of spawning is measured on a weekly basis during the months of May–August for 2-3 years.
Recommendation 5 : Early recruitment and stock assessment – annual seed survey
Seed mussels are a natural resource that requires proper management and exploitation in order to maximise the potential return. Due to increased pressures on this resource in recent years a formal set of guidelines are required. The policy document is the result of consultation with the bottom grown mussel industry and government bodies both North and South and is subject to review from time to time.