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

Abstract

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.

Invasive crayfish impacts on native fish diet and growth vary with fish life stage

“…fish exhibited lower growth rates, and this was maintained in the Evenlode until fish were aged 2?. Signal crayfish can prey directly upon small fish (Guan and Wiles 1997) and consequently small fish may spend more time engaged in predator-avoidance, limiting foraging opportunities (Light 2005) and reducing growth rates in chub (Allouche and Gaudin 2001). Whilst our results were correlative, a consistent pattern of reduced juvenile chub growth was detected in our four datasets: a space-for-time approach in Evenlode and Cherwell, and a before-after invasion approach in Rother and Chad Brook.…”

Ecosystem services provided by waterbirds

“…5). Perceived risk of predation affects fish behaviour and growth rates (Allouche & Gaudin, 2001;Wacht-Katz et al, 2010), and leads them to avoid surface layers and shallow littoral zones during the daytime where they would be visually exposed to piscivorous birds (Gliwicz & Jachner, 1992;Lewin, Okun & Mehner, 2004, Mehner, Kasprzak & Holker, 2007. Thus, the fear of predation by birds can have dramatic cascade effects on food webs in aquatic ecosystems.…”

In situ measurements of turbulence in fish shoals

“…The main reasons for locomotion in habitats with only weak background currents are feeding (Neumann et al 1996;Schmidt-Nielsen 1997;Cech and Kubecka 2002), predator avoidance (Allouche and Gaudin 2001), and social interaction (Hoare et al 2000;Behrmann-Godel et al 2006). For swimming animals, the costs of locomotion are determined not only by the drag force and by the generation of TKE, but also by the intensity of background turbulence in the ambient water.…”