Murky Water Changes How Stickleback Shoals Search for Food

Three-spined sticklebacks took longer to swim toward food that unexpectedly appeared in cloudier water, but the fish compensated by becoming more closely connected and coordinated within their shoals, according to a peer-reviewed study by University of Bristol researchers published in Proceedings of the Royal Society B. The changes in collective behaviour reduced the disadvantage associated with reduced visibility without fully eliminating the delay in locating food.

Sticklebacks are common freshwater fish that often form small shoals in ponds, streams and rivers across the Northern Hemisphere, where they can encounter water ranging from clear to deeply murky. Turbidity, the cloudiness of water caused by suspended particles such as algae, mud and silt, can make it harder for aquatic animals to see food, other animals or potential threats. The release notes that turbidity is increasing in many rivers and coastal waters because of farming, mining, construction and other forms of pollution.

Researchers from Bristol’s School of Biological Sciences examined how shoals of three-spined sticklebacks responded as their water became cloudier and then observed their reactions when food suddenly appeared. In murkier conditions, individual fish generally took longer to move toward the food than fish in clear water. Individuals in larger groups, however, responded more quickly overall. The results indicate that both group size and the degree of coordination within a group influenced how effectively the fish responded when visibility was limited.

The relationship between coordination and response time differed across group sizes and levels of turbidity. In small groups in murky water, fish reached the food more quickly when they stayed close together and swam in the same direction. In larger groups in clear water, by contrast, tighter coordination slowed the group’s response. Before the food appeared, fish in murky water tended to swim closer together, align their movements more closely and respond more readily to the behaviour of nearby group members, particularly in smaller groups.

These observations show behavioural plasticity, meaning that the fish adjusted their social responses to changing environmental conditions rather than following one fixed pattern of grouping. The study’s central result is that increased cohesion, alignment and responsiveness to neighbours helped sticklebacks mitigate some of the information lost through turbid water. The compensation was not complete: the release reports that fish in murkier water still took longer to reach the unexpected food than those in clear water.

Corresponding author Christos Ioannou, Professor of Behavioural Ecology, said the results showed that the fish “were able to compensate for this challenge by changing their shoaling behaviour.” According to Ioannou, their shoals became more coordinated in poor conditions, with fish moving and aligning themselves together more closely, allowing the coordinated groups to find food faster than if the animals had not coordinated.

Co-author Sean Rands, Senior Lecturer, said the findings show that behavioural rules governing animal grouping can change in response to environmental challenges. He noted that shifts between hot, dry periods and heavy rains can produce sudden surges of muddy, highly turbid water in rivers and streams, creating conditions in which flexible social behaviour may matter. The study therefore documents how the tested sticklebacks modified shoaling and foraging responses under different water-clarity conditions, linking their collective behaviour to the challenge of finding food when visual information is restricted.

Subject of Research: How water turbidity changes three-spined stickleback shoaling and food-search behaviour

Article Title: Muddying the waters makes fish more sociable, study finds

Article References: Muddying the waters makes fish more sociable, study finds. Original publication (Bibliographic details unavailable.)

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Keywords: three-spined sticklebacks, shoaling behaviour, water turbidity, collective behaviour, behavioural plasticity, animal coordination, foraging behaviour, aquatic pollution, group size, fish shoals, Muddying, waters