Collective behaviour of fish in the presence and absence of flow

Spatial patterns in moving fish shoals originate from social interactions and are driven by factors that benefit individuals, such as predator avoidance or reduced energy expenditure. How shoaling is influenced by the presence and absence of a flow field is only partially understood. Adopting a reductionist approach, time spent shoaling, shoal structure and information transfer between pairs (the smallest subsystem of a shoal) of Eurasian minnows, Phoxinus phoxinus, were investigated in a recirculating flume in the absence (control) and presence of flow (low and high). Minnows spent more time shoaling under high-flow conditions. The pairs tended to swim in a tandem and side-by-side configuration under static and high flow, respectively, while under low flow, the neighbour's position was more uniformly distributed around the focal fish. Our approach involved analysis of fish accelerations and velocity correlations and indicated that, with the potential for hydrodynamic masking in flowing water, the relative positions adopted reflect the individual's propensity to increase information transfer with its neighbour by visual means so that an energy-efficient configuration can be maintained. Conversely, when the energetic benefits of a side-by-side configuration are lost in the absence of flow, fish limit the costs of the partial visual impediment (on one side) imposed by the presence of a close neighbour by employing a ‘follow-the-leader’ tactic.