Just like 20-somethings yelling or waving in a crowded, strobe light-filled club, three-spined sticklebacks (Gasterosteus aculeatus) can change their social behavior for the benefit of the group. These small fish will change how they swim to help find food when they are in cloudy water. The findings are detailed in a study published today in the journal Proceedings of the Royal Society B.

There are eight known species of sticklebacks (order Gasterosteiformes). These small fish live in fresh, brackish, and marine waters all over the Northern Hemisphere and as far north as the Arctic Ocean. They can grow to a maximum length of roughly seven inches and are known for a row of two to 16 spines on their backs.

Since they live in everything from salty seas to fresh lakes, their habitats can range from crystal clear to densely clouded. Turbidity is when small particles like mud, algae, or silt make water cloudy. While it is a natural process, farming, mining, construction, and other pollutants are increasing turbidity and reducing visibility in rivers and coastal waters around the world. This can make it more difficult for aquatic animals like sticklebacks to see their food, friends, or enemies.

“At the moment, we’re seeing our weather flipping between hot, dry periods to heavy rains, which mean[s] our rivers and streams experience sudden surges of muddy, highly turbid water,” 

Dr. Sean Rands, a study co-author and behavioral ecologist at the University of Bristol, said in a statement. “It is good to know that the fish can respond to these big changes.”

For this study, the team observed how three-spined sticklebacks responded to their water becoming cloudier and noted their directions when food suddenly appeared in the water. 

In murkier water, the fish took longer to swim towards the food than those in clear water. However, individuals in larger groups responded more quickly in both clear and cloudy water. 

The way that the sticklebacks moved together also influenced their response to the food. The smaller groups living in murky water got to the food more quickly when they swam close together in the same direction. By contrast, tighter coordination slowed the group’s response for larger groups of fish in clear water.

Before the food was introduced, fish in murky water tended to stick closer together, align their movements more closely, and respond more eagerly to the behavior of group members that were nearby.

According to the team, these results suggest that sticklebacks can compensate for reduced visibility in the water by increasing cooperation and coordination with one another. 

“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,” added study co-author and behavioral ecologist Christos Ioannou. “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.” 

This enhanced social behavior may help them overcome the challenges of living in murky environments and respond better when there is an opportunity to find a meal. 

“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,” said Rands. 

 

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