Density-dependent changes in neophobia and stress-coping styles in the world's oldest farmed fish

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.

Invasive Nile tilapia dominates a threatened indigenous tilapia in competition over shelter

“…Housing tanks were separated by perforated dividers, preventing movement of individuals between tanks, while allowing olfactory cue transmission. All housing tanks were held at similar densities, 55-65 individuals per tank, because housing density can affect aggression and boldness in tilapia (Champneys et al, 2018). Lighting was maintained on a 12:12 h light:dark cycle to mimic natural conditions in the tropics.…”

Consistent individual competitive ability in rainbow trout as a proxy for coping style and its lack of correlation with cortisol responsiveness upon acute stress

“…In this regard, fish are known to have a high level of behavioral plasticity. For example, boldness and other traits such as metabolic rates, performance or stress responsiveness, which are often used for characterizing fish SCS in behavioral tests, are known to be affected by the social environment and other environmental factors [25,[35][36][37][38][39]. On the other hand, also aggression and dominance behavior depend on the social environment, and it is known that fighting for dominance in salmonids and other fish is usually reduced when the stocking density is relatively high [40][41][42][43].…”

Conspecific density and environmental complexity impact behaviour of turquoise killifish (Nothobranchius furzeri)

“…Specifically, two experiments were conducted to assess the impact of (a) conspecific density and (b) structural complexity on fish body size, spontaneous locomotor activity parameters, boldness and exploratory behaviour, habitat choice, sociability, aggressiveness, feeding behaviour and the antipredator response of young adults. In other fish species, early rearing conditions are known to impact patterns of gene expression and neurophysiology with potential behavioural consequences (Champneys et al ., 2018). Therefore, we also expect enrichment of the rearing environment to impact behaviour in N. furzeri .…”