In a fish school, position means a lot. Animals at the front will have first access to food; animals in the center are well protected from predators, and those pulling up the rear get to save a little metabolic energy as they draft off those in front of them. But who ends up where in a school, and why? This is precisely the question that Shaun Killen of the University of Montpellier and his colleagues Stefano Marras, John Steffensen and David McKenzie recently set out to answer.
Given that it is physically, and thus physiologically, more demanding to swim at the front of a school than in the middle or at the back, the researchers hypothesized that an animal's aerobic scope – the difference between its resting and maximal metabolic rate – was probably important in determining its position. To test this idea, Killen and colleagues used wild-caught, juvenile grey mullet to quantify schooling behavior of individual animals in a flume. First, a focal fish was introduced into the swim tunnel with seven companion fish and the group was allowed to acclimate to slow-speed swimming for 5 h. After the acclimation phase, the team recorded the fish schools from above using a digital video camera for 20 min at each of three speeds. For each 20 min block they analyzed the videos in 5 s increments to determine the typical position of the focal fish and how much it varied within the small school. Repeating this procedure for 20 focal fish, the authors also measured the tailbeat frequencies of the focal fish and used respirometry to measure their standard and maximal metabolic rates.
Analyzing the results, the authors realized that while there were no significant relationships between standard metabolic rate and mean position within a school, there were significant relationships between an animal's aerobic scope and its mean position. Specifically, animals with a higher aerobic scope were more likely to be found near the front of a school at higher speeds. Moreover, although most fish maintained similar positions as the school's speed varied, some did not, and those that tended to move toward the back of a school when speeds increased were fish with a lower aerobic scope. The researchers also demonstrated that fish near the back of a school were able to get away with lower tailbeat frequencies at intermediate and high speeds than fish at the front of the school.
So aerobic capacity appears to influence where in a school a fish ends up, especially if the school is swimming at a relatively high speed. Killen and his colleagues raise the important question of whether their results apply to larger schools, and even more intriguingly suggest that aerobic ability could influence what school a fish might join. Studying aerobic capacity, and its variability among different schools, would be an exciting next step.