The water in rivers and seas can vary from crystal clear to densely clouded. Turbidity, the cloudiness of water caused by small particles such as algae, mud and silt, is increasing in many rivers and coastal waters due to farming, mining, construction, and other pollution. Because cloudy water reduces visibility, it can make it more difficult for aquatic animals to see their food, friends, or enemies.
Researchers from Bristol's School of Biological Sciences looked at how shoals of three-spined sticklebacks respond to their water becoming cloudier and observed their reactions when food suddenly appeared.
Sticklebacks are common freshwater fish, and are often found in small shoals in ponds, streams and rivers across the Northern Hemisphere, where they can experience waters that range from clear to deeply murky.
The study found in murkier water fish took longer to move towards the food than those in clear water, but individuals in larger groups responded more quickly overall.
The way the fish moved together also influenced their response. In small groups living in murky water, the fish reached the food more quickly when they stayed close together and swam in the same direction. By contrast, in larger groups in clear water, tighter coordination slowed the group's response.
Before 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.
The findings suggest that, despite reduced visibility, sticklebacks can compensate by increasing cooperation and coordination with one another.
This enhanced social behaviour may help them overcome the challenges of murky environments and respond more effectively to important opportunities such as finding food.
Christos Ioannou, Professor of Behavioural Ecology in the School of Biological Sciences, and corresponding author, said: "When we put an unexpected piece of food in the water, it took the fish longer to find it when the water was cloudier. This makes sense and has been shown before in different water-living species.
"However, we were excited to see that the sticklebacks were able to compensate for this challenge by changing their shoaling behaviour. Their shoals became more coordinated, with the fish moving and aligning themselves together much more closely in poor conditions. This in turn meant the fish were able to find the food faster than if they weren't coordinating."
Dr Sean Rands, Senior Lecturer and a co-author of the study, commented: "It's interesting that the fish are changing their shoaling behaviour to match the environment. When we are studying grouping behaviour in animals, researchers often assume that there is a fixed set of behavioural rules that are used in all environments.
"Finding that these rules are able to change in response to the challenges animals experience is reassuring, as it shows that these little fish have some extra tricks for the changes that humans are causing to their homes.
"At the moment, we're seeing our weather flipping between hot, dry periods to heavy rains, which mean our rivers and streams experience sudden surges of muddy, highly turbid water. It is good to know that the fish can respond to these big changes."
Paper
'Plasticity in collective behaviour helps fish shoals mitigate limited visual information from water turbidity' by Christos C. Ioannou, James Cordery, Sean A. Rands in Proceedings of the Royal Society B
