Effects of avian predation threat, water flow and cover on growth and habitat use by chub, Leuciscus cephalus, in an experimental stream

Trophic interactions, including "top-down" predator-prey interactions, are particularly important in influencing the structure of fish. communities. While the varied impacts of piscivorous fish have been well investigated, the effects of fish-eating birds on riverine fish behaviour and population dynamics still remain controversial, mainly because they are undervalued. Summer experiments were conducted in an experimental outdoor stream to evaluate the effects of avian predation threat, stream flow, and overhead cover on growth and behavioural tactics of wild juvenile chub (Leuciscus cephalus). Groups of fifteen chub maintained in riffle-pool sequences were submitted to combinations of different conditions, namely absence or presence of a simulated fish-eating bird, low or high flow, and absence or presence of medium or high cover. In the absence of predation threat, chub foraged in the riffles and maximized feeding opportunities. Under predation threat, they sheltered, foraged less and grew slowly and as expected, they increased their use of the riffles at high flow as water turbulence is an efficient shelter from birds but only in the absence of cover. In the presence of cover, fish sheltered exclusively under pool covers and were more prone to take risks at low flow because of higher costs in terms of lost feeding opportunities associated with these conditions. This result indicates that flow velocity altered cover use tactics through its impact on food supply, suggesting that it may affect the outcome of predator-prey relationships. So, chub use cover in a flexible manner according to both the benefits in terms of predator avoidance and the costs in terms of lost feeding opportunities. A striking finding of the experiments is the drastic reduction in the range of growth variances amongst fish when they are maintained under predation threat, suggesting a homogenization of fitness between individuals. From all our results, we argue that in lowland streams, under summer field situations, fish-eating birds may affect local prey population dynamics more through sub-lethal effects on growth rates than directly through death rates.