Zebrafish are the second most commonly used vertebrate in biomedical research. Their welfare in laboratory settings is regulated but inadequately understood, given the enormous numbers involved.
Zebrafish welfare in research is complicated by the enormous numbers used and the regulatory gap around embryos and larvae. The exemption of early life stages from welfare regulation means that millions of zebrafish are subjected to experimental procedures without the ethical review that would apply to equivalent mammalian experiments. Adult zebrafish in high-density housing experience social stress from territorial aggression that causes fin damage and stress. The nociceptive capacity of zebrafish means that experimental procedures causing tissue damage involve welfare-relevant suffering. The Three Rs principles of replacement, reduction and refinement apply as much to zebrafish as to mammals, but implementation is less developed. Improving zebrafish welfare requires both regulatory attention to currently unprotected life stages and better husbandry standards for adult fish.