Small fish including zebrafish are increasingly used in biomedical research. Their welfare is regulated less strictly than mammalian models despite growing evidence for their pain capacity.
Zebrafish welfare sits at the intersection of the fish pain debate and research animal regulation. Research has demonstrated that zebrafish respond to noxious stimuli with analgesic-reversible avoidance behaviour, gill movement changes, and altered cortisol — consistent with pain responses. EU regulations cover adults but exempt larvae during the period they are most heavily used in high-throughput screening. The welfare implications of screening millions of larvae through chemical libraries are significant if larvae have meaningful pain capacity. Three Rs principles (Replace, Reduce, Refine) apply to zebrafish research but refinement guidance specific to fish is less developed than for mammals. Welfare improvements including temperature optimisation, appropriate anaesthesia, and humane endpoint criteria are available but inconsistently applied.