Fish swimming in schools save energy regardless of their spatial position

Abstract

For animals, being a member of a group provides various advantages, such as reduced vulnerability to predators, increased foraging opportunities and reduced energetic costs of locomotion. In moving groups such as fish schools, there are benefits of group membership for trailing individuals, who can reduce the cost of movement by exploiting the flow patterns generated by the individuals swimming ahead of them. However, whether positions relative to the closest neighbours (e.g. ahead, sided by side or behind) modulate the individual energetic cost of swimming is still unknown. Here, we addressed these questions in grey mullet Liza aurata by measuring tail-beat frequency and amplitude of 15 focal fish, swimming in separate schools, while swimming in isolation and in various positions relative to their closest neighbours, at three speeds. Our results demonstrate that, in a fish school, individuals in any position have reduced costs of swimming, compared to when they swim at the same speed but alone. Although fish swimming behind their neighbours save the most energy, even fish swimming ahead of their nearest neighbour were able to gain a net energetic benefit over swimming in isolation, including those swimming at the front of a school. Interestingly, this energetic saving was greatest at the lowest swimming speed measured in our study. Because any member of a school gains an energetic benefit compared to swimming alone, we suggest that the benefits of membership in moving groups may be more strongly linked to reducing the costs of locomotion than previously appreciated.

Keywords: Collective behaviour; Ecophysiology; Energetics; Hydrodynamics.

Fig. 1

a Positions of the focal fish (red dot) relative to its closest neighbour. At negative values, the focal fish was swimming behind its closest neighbour, 0 represents swimming side by side, and at positive values, the focal fish was ahead of its closest neighbour. Note that for clarity, the other six school members are not shown in the figure. b Reduction in TBF (%) of focal fish when swimming in different positions in a school, compared to swimming alone. Bar values refer to the position represented above in panel a. Values are mean ± S.E. c Reduction in the estimated metabolic rate (MO₂) compared to solitary swimming in juvenile golden grey mullet while swimming in various positions relative to a neighbour in a school. Values are mean ± S.E.

Fig. 2

a Reduction in TBF (%) of focal fish when swimming at three different water speeds in a school relative to swimming alone. Values are mean ± S.E. b Reduction in the estimated metabolic rate (MO₂) compared to solitary swimming in juvenile golden grey mullet while swimming at various speeds. Values are mean ± S.E.

Fig. 3

Mean TBF (Hz) of the focal fish when swimming alone (black bars), in a school occupying the position rank #1 (grey bars) and in a school occupying position rank #2 to #8 (pooled together), at three different swimming speeds. Values are mean ± S.E.