A new study, undertaken by researchers from Swansea University’s Centre for Sustainable Aquatic Research (CSAR), looks into how crowding in aquaculture can result in abnormal behaviours that can impact welfare, and they conclude that it can have positive, as well as negative, effects.

During their trials they raised juvenile Nile tilapia (Oreochromis niloticus) for six weeks at either high (50 g l 21 ) or low density (14 g l 21 ), and assessed the extent of skin and eye darkening (two signs of chronic stress).

They also exposed the fish to a novel object in an open test arena, with and without cover, to assess the effects of density on neophobia – the fear of the ‘new’ – and stress coping styles.

Their results, published in the Royal Society Open Science journal this week, showed that: “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.”

Conclusions

In welfare terms they conclude while neophobia is “thought to be adaptive under natural conditions by limiting risks… it is potentially maladapted in captivity, where there are no predators or novel foods" and that crowding could have a positive effect on the welfare of tilapia by reducing aggressive behaviour.



In terms of ecological impacts, they note that, while high stocking densities can make fish chronically stressed and more fearful, the fact that it gives them a reactive style of coping with stress should make them less invasive should they escape from captivity – "which is important for a species like Nile tilapia, which have been translocated all over the world and can pose a major threat to native biodiversity".

“Tilapias rank among the oldest and most widely farmed fish worldwide, but are also included in the ‘100 world’s worst invasive alien species’, so knowledge about how these species respond to novelty when they escape from fish farms and become feral might be important for reducing impacts,” they observe.