Abstract
Farmed fish are typically reared at densities much higher than those observed in the wild, but to what extent crowding results in abnormal behaviours that can impact welfare and stress coping styles is subject to debate. Neophobia (i.e. fear of the ‘new’) is thought to be adaptive under natural conditions by limiting risks, but it is potentially maladapted in captivity, where there are no predators or novel foods. We reared juvenile Nile tilapia (Oreochromis niloticus) for six weeks at either high (50 g l−1) or low density (14 g l−1), assessed the extent of skin and eye darkening (two proxies of chronic stress), and exposed them to a novel object in an open test arena, with and without cover, to assess the effects of density on neophobia and stress coping styles. Fish reared at high density were darker, more neophobic, less aggressive, less mobile and less likely to take risks than those reared at low density, and these effects were exacerbated when no cover was available. Thus, the reactive coping style shown by fish at high density was very different from the proactive coping style shown by fish at low density. Our findings provide novel insights into the plasticity of fish behaviour and the effects of aquaculture intensification on one of the world's oldest farmed and most invasive fish, and highlight the importance of considering context. Crowding could have a positive effect on the welfare of tilapia by reducing aggressive behaviour, but it can also make fish chronically stressed and more fearful, which could make them less invasive.
Highlights
Food security relies on aquaculture intensification to maximize fish production and increase economic viability [1,2] but to what extent crowding may affect behaviours and compromise welfare is& 2018 The Authors
Experiments on Atlantic salmon (Salmo salar L) and African catfish (Clarias gariepinus) indicate that fish reared at high density become less aggressive and more willing to rise to the surface to feed than fish reared at low density [7,8,9], but the opposite has been reported for rainbow trout (Oncorhynchus mykiss), with fish becoming more aggressive as stocking density increases [3]
Fish reared at high density were more cautious and less likely to leave the starting box than fish at low density (50% left), but as expected, this was only manifested when the starting box remained covered during testing (Cox estimate for low density 1⁄4 2.06, s.e. 1⁄4 0.55, z 1⁄4 3.73, p, 0.001; n 1⁄4 58, number of events 1⁄4 24, LRT 1⁄4 19.59, d.f. 1⁄4 1, p, 0.001; figure 2a)
Summary
Food security relies on aquaculture intensification to maximize fish production and increase economic viability [1,2] but to what extent crowding may affect behaviours and compromise welfare is. Stocking density is one of the parameters most managed by fish farmers, but what constitutes 2 an acceptable stocking density differs from species to species, and is subject to debate [3,4,5,6]. Stress-related behaviours may increase with increasing density in some individuals but decrease in others [4]
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