The effect of tank cover on welfare of farmed Nile tilapia

Welfare in aquaculture is a pressing topic. One of the main measures to improve the life of farmed fish is implementing environmental enrichment, which can include the addition of structural covers to rearing tanks. Here we test the effects of tank covers (fully covered, 50 % covered and uncovered) in an all-male population (N = 75 tagged individuals: 5 individuals per tank, 5 tanks per treatment) of farmed Nile tilapia using behavioural and physiological welfare indicators. Video recordings were performed over 21 days to evaluate overall swimming (swim bursts, freezing on the bottom or hovering in the water column) and aggression events (bites and chases). At the end of the experiment, blood was collected to assess circulating cortisol. Results showed a clearly lower welfare in the partially covered tanks regarding behavioural and physiological variables. The present study highlights the need to correctly study 1) the ethology of the farmed species and 2) the enrichment strategy to be implemented, before any measure is taken. While environmental enrichment is undoubtedly a much needed area of research and a promising solution to improve the welfare of farmed fish, a proper ethological assessment must be made a priori to avoid detrimental effects of poorly devised approaches.

Automated summary

This study tested the effects of tank covers on the welfare of farmed Nile tilapia, showing that partially covered tanks had lower welfare outcomes for both behavioral and physiological variables. Proper ethological assessment is crucial before implementing enrichment strategies to avoid negative effects on farmed fish welfare.