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Group size and color: Implications on fish schooling behavior

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Group size and color: Implications on fish schooling behavior of promelas, innesi, sphenops and reticulata.


Fish use schooling as a coping mechanism for survival. The purpose of this study was to examine if fish preferred larger or smaller schools and if when the school sizes are the same size, will fish have a preference for the color red or blue. It was hypothesized that the fish would prefer large schools over small schools and the color blue over red. Both experiments were done inside of a tank and it was observed which side of a drawn line the fish was on at given times. The results of the first experiment showed a slight preference for small groups and were insignificant. The results of the second experiment were significant and showed a preference for blue over red. The hypothesis for fish having a preference for large groups over small groups was not supported by the results. The hypothesis for fish to prefer blue over red was supported by the results. The results of the first experiment could have happened because due to small group size, the fish were maybe choosing a school based on familiarity rather than size. The results for the second experiment could have occurred due to surface dwelling fish being sensitive to the colors blue and green and not the color red.


Animals have evolutionary forms groups for better survival. The types of group can vary from a family with parents to unrelated individuals (Alexander 1974 1974). The main benefits of forming groups is that there less risks spreading diseases since animals are not reproducing with animals outside of the group. Animals also tend to use groups to protect themselves from predators and gain more food because groups split the food (Alexander 1974). Animals that utilize groups effectively are fish. Fish have been well known to know perform schooling behavior for survival. In a recent study, it was found that fish will exploit food resources more effectively and efficiently when hunting in schools (Reuter et al 2015). Fish also form groups because evolutionarily groups benefit members is that there is less competition for group members when breeding (Alexander 1974). Fish also form schools because a predators are less likely to hunt an entire school of fish versus a whole group of fish (Scott and Sloman 2004). A school of fish also make it more visually confusing for a predator to spot a single fish (Scott and Sloman 2004).

Fish have three motor patterns within groups. There can be a tendency for a fish to initiate the other direction of swimming of another fish (Hemmings 1966). Fish also have a tendency return to another fish if they are not following and there also is a tendency for fish to follow fish swimming away from them (Hemmings 1966). Despite these motor patterns, there are typically not a difference in social status in groups (Pavlov and Kasumyan 2000). The fish only count the leader as the swimming at the center of the school, which for a half a second or less because the front fish will quickly move to either the rear or the middle of the school (Pavlov and Kasumyan 2000). It has been shown however, that fish in different positions in the school will respond differently to stimuli. In Abramas brama, the center of the school was less sensitive to sensitive stimulus compared to the sides, which was more than 5 times as likely to response to external stimulus (Pavlov and Kasumyan 2000). The purpose of this study was to see if fish schooling behavior is affected by group size and if fish are more drawn to the color red or blue when the school size is the same. The Hypothesis was that fish will prefer larger group sizes and prefer the color blue over the color red.


Both experiments utilized a 15-gallon tank with air stones on either end of the tank. The tank was divided into three by plexiglass dividers so there was a middle, right and left section of the tank. Both width sides and one length side of the tank were covered in Kraft paper to filter out distractions for the fish. A dip net was used to transport the fish to the tank, and there a bucket of chemicals to place the dipnet into for sterilization. A stopwatch, in this case an iPhone, was used to account for time. A dry erase maker was used so a line could be drawn halfway down the middle section of the tank, so it could be determined which side the fish was nearer. A difference with the second experiment what the plexiglass dividers had acetate colored paper in-between them and, with one divider being red and one divider being blue. The second experiment also had 3 fish on both sides of the tank.

To conduct the first experiment with 5 trials, the fish, Rosy Red Minnows, were placed in the left section of the tank and 2 fish were placed on the right side of the tank. A single fish, from a large tank of fish of the same species, was placed in the middle of tank using the dip net and was given three minutes of acclamation time. Then every 30 seconds for 15 minutes total using a stopwatch, it was recorded which side of the line the fish was on, so it could be later determined which group the fish preferred. The recording was done by checking a box in a two-column table if the fish was on the group with 5 fish or 2 fish. After the 15 minutes, the tested fish was placed in a holding tank to be later placed back in its original tank. This procedure was repeated for 5 separate fishes.

The grand total of the fish's observed position was then calculated with a total of 880 data points for the first experiment. Then the chi square test of goodness-of-fit with the Yates correction was done to determine if the results were significant. The second experiment had the same procedure, but both groups were 3 fish instead of 5 and 2 and the tank had a blue and red divider so the fish were either go to the blue or red side of the tank. The second experiment had the same procedure of checking boxes from a two-column table, but the two categories to check for were blue or red side. The second experiment was also done for 10 minutes instead of 15 minutes with a grand total of 95 trials done for the second experiment.


The results of the first experiment were insignificant and showed that the fish slightly preferred small schools because they were drawn to small schools with a grand total of 445 times and large schools 435 times (Figure 1) (chi square goodness of fit test, x2 = .092, df=1, p=0.762). In experiment two, the responses were significantly different and the fish were drawn to the blue side 88 times and the red side 7 times (Figure 2) (chi square goodness of fit test, x2 = 67.368-16, df=1, p=2.25-16).

Figure 1. The grand total number of the times the fish in the middle section of the tank swam to a large group with 5 fish versus a small group with 2 fish. (chi square goodness of fit test, x2 = .092, df=1, p=0.762)

Figure 2- The total number of times the fish in the middle section swam to the blue or red divider. (chi square goodness of fit test, x2 = 67.368-16, df=1, p=2.25-16)


The hypothesis that fish prefer larger schools was not supported because the results indicating that fish preferred small groups were insignificant, and so it cannot be concluded that this relationship occurs in nature. The school size results are insignificant, but other researchers have found that fish would prefer larger groups over small groups. The fish could have preferred the smaller group over the larger group because of the size of the fish in the group. Keenleyside (1955) showed that small fish would prefer a group of larger fish over a group of small fish. Perhaps in the school size experiment the fish in the smaller groups were larger than the fish in the large groups, therefore skewing the data to favor small groups slightly over the large groups. Another reason the small groups could have been favored in the large schools was that in another study it was shown that fish preferred familiar fish, fish they are cohesive with in antipredator behavior, less as group size increased (Griffiths and Magurran 1997). The fish might have chosen groups based on familiarity and not group size because the group sizes were small.

The hypothesis that fish prefer blue over red when choosing a school of the sizes was supported by the results and the results were significant (Pope and Fry 1997) showed that the color blue is absorbed more profoundly than red, which means the color blue would be found at deeper depths in water. Rosy Red Minnows would equally see red and blue because they live in shallow water such as rivers, ponds, and temporary pools. The question now is why the fish would preference the color blue over red. A possible explanation is that guppies, surface dwelling fish, are sensitive to blue and green colors and not red colors (Levine and MacNichol Jr 1982). Rosy Red Minnows are also surface dwelling; therefore, they might also not be sensitive to the color red. The minnows could just see a blue divider and a colorless divider; therefore, they preferred the blue divider because it was the familiar color. This would mean that the fish has the color based on familiarity and not preference.

The purpose of the experiment was to test the effect of school size on schooling behavior in fish, and if fish had a preference for color when the opposing school sizes were the same. The results of the first experiment for group size are insignificant but could possibly be explained by familiarity instead of group size. The results of the second experiment for color preference are significant but could be more likely due to familiarity than preference for a color. The results might reflect that fish stick with familiar environments as much as possible due to basic survival instincts. The results might mean that fish within schools are only social within the school and not likely to interact with other schools. A question from the first experiment that should explored would be if the opposing school size were even more polarized, would there be significant results? Future investigations to expand on the color experiment would be to use blue and green dividers, because these are sensitive colors that the fish can likely distinguish between.


Alexander, R. D. 1974. The Evolution of Social Behavior. Annual Review of Ecology and Systematics 5:325“383.

GRIFFITHS, S. W., and A. E. MAGURRAN. 1997. Schooling preferences for familiar fish vary with group size in a wild guppy population. School of Biological and Medical Sciences 264:547“551.

Hemmings, C. C. 1966. OLFACTION AND VISION IN FISH SCHOOLING. J. Exp. Biol 45:449“464.
Department of Zoology, University of Cambridge, and Stazione Zoologica, Naples

Keenleyside, M. H. 1955. Some Aspects of the Schooling Behaviour of Fish. Behaviour 8:183“247.

Levine, J. S., and E. F. Macnichol. 1982. Color Vision in Fishes. Scientific American 246:140“149.

Pavlov, D. P., and A. O. Kasumyan. 2000. Patterns and mechanisms of schooling behavior in fish: A review. Journal of Ichthyology.

Pope, R. M., and E. S. Fry. 1997. Absorption spectrum (380“700 nm) of pure water II Integrating cavity measurements. Applied Optics 36:8710.

Reuter, H., M. Kruse, A. Rovellini, and B. Breckling. 2016. Evolutionary trends in fish schools in heterogeneous environments. Ecological Modelling 326:23“35.

Scott, G. R., and K. A. Sloman. 2004. The effects of environmental pollutants on complex fish behaviour: integrating behavioural and physiological indicators of toxicity. Aquatic Toxicology 68:369“392.

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