Schooling behaviour in fishes reduces the energetic costs of swimming for the school's members by essentially allowing each fish to ‘draft’ off of others in the group. This phenomenon has been thoroughly studied in laboratory settings under conditions of laminar flow – that is, steady, unidirectional flow with minimal turbulence. However, in the natural environment, water flow can often be turbulent owing to weather, underwater structures, water current dynamics and other factors, which could make it difficult for fish in schools to maintain optimal positions relative to their fellow group members (and thus to reap energetic rewards). Lewis Halsey, of the University of Roehampton in the UK, along with colleagues from the Centre for the Environment and the University of Glasgow, wanted to determine whether schooling behaviour remains energetically advantageous in turbulent conditions. The group further wanted to test whether the size of a school is important in conferring energy cost savings.

To address these questions, the researchers placed European sea bass in a special swim flume individually or in groups of three or six fish. Three propellers at one end of the flume allowed the scientists to subject each fish or group of fish to three different water flow rates (20 min at each rate) which, because of the flume design, corresponded to three different levels of turbulence. Using video footage of each trial, the team recorded the three-dimensional position of each fish within the flume, analysed how much (or little) it maintained its position, and determined its tail beat frequency at each flow rate and level of turbulence. After the experiments, to more precisely quantify the turbulence experienced by each fish, the team used a velocimeter to measure the rate and direction of water flow at many different locations within the flume under the three experimental conditions.

Halsey and his colleagues found that mean flow rate did not affect tail beat frequency in fish swimming alone or in groups of three. In groups of six fish, tail beat frequency decreased with increasing flow rates, which could reflect more effective drafting among fish in larger schools. Increases in turbulence increased the tail beat frequency in all group sizes, suggesting that swimming generally becomes costlier as flow becomes more uneven. However, fish in larger groups had lower tail beat frequencies in turbulent conditions than fish swimming alone or in smaller groups, indicating that schooling did help conserve energy as it does under laminar flow conditions. Notably, though, the energy cost savings afforded by schooling behaviour in turbulent flow were much lower than those estimated in past studies with laminar flow.

Overall, the researchers found that swimming in schools is energetically advantageous for sea bass in dynamic water conditions. However, the metabolic advantage of schooling appears to be smaller in turbulent conditions than in ideal conditions. In addition, the size of a school affects the degree to which changes in water flow rates and patterns influence the energy expenditure of fish within a school. Although more research is needed to better understand the energetic costs and benefits of group swimming in nature, this study shows that when the going gets rough, sea bass should probably stay in school.

L. G.
J. D.
S. S.
How does school size affect tail beat frequency in turbulent water?
Comp. Biochem. Physiol. A