Meta-study: how density affects fish welfare
To fill knowledge gaps about impacts of stocking density and the mechanisms behind it, researchers from two Chinese and one German university conducted a comprehensive meta-analysis of more than 200 research articles.
In an article published in Aquaculture, the researchers delve into the debate over whether high or low stocking density is most negative for fish welfare. They also point out that the extent to which physiological stress contributes to welfare deterioration compared to aggression is poorly quantified.
"However, the assumption that lower density always results in better welfare is not universally valid," they quickly state.
They point out that studies have shown that low density can also be a significant stress factor, partly because unstable social hierarchy can make it possible for dominant individuals to exercise sustained aggression, which in turn can cause severe stress, starvation and high mortality in subordinates.
"To close the knowledge gaps about how density affects fish welfare and the mechanisms behind it, we conducted a comprehensive meta-analysis based on 2,507 paired observations from 214 publications covering 108 fish species," they write.
Results
The analyses showed that high density, compared to low density, resulted in significantly impaired growth performance, documented by reduced specific growth rate (SGR) (-9.4%), lower daily feed intake (DFI) (-13.6%) and higher feed conversion ratio (FCR) (+8.8%).
"Mechanistically, this is driven by a clear physiological stress response, including increased cortisol levels (+11.3%) and reduced digestive function (-15.5%). Contrary to what one might expect, the frequency of aggressive behaviour (FAB) decreased significantly (-38.6%), suggesting that crowding-induced physiological stress – rather than aggression – is the main cause of the deteriorated welfare," they write.
Furthermore, subgroup analyses showed that predatory fish had better tolerance to stress at high density, with a relatively small reduction in SGR (-7.6%), a clear decrease in FAB (-40.6%), and non-significant changes in cannibalism rate (CR), coefficient of variation for final weight (CV), DFI and cortisol levels.
Omnivores, on the other hand, experienced a greater SGR decline (-12.1%), along with increases in CR (+469.5%), CV (+26.7%), and cortisol levels (+19.3%), as well as a significant reduction in DFI (-21.9%).
“Finally, we identified indoor production systems and precise feeding rates as possible strategies to reduce stress at high densities. The findings provide a mechanistic understanding and practical solutions to improve welfare in intensive aquaculture,” the researchers write.
Conclusion
"The documentation from the meta-analysis in this study indicates that high density, compared to low density, has a more pronounced negative effect on fish welfare – particularly shown through a significant suppression of specific growth rate (SGR)," is one of the conclusions from the study.
