Zebrafish are used by the millions in biomedical laboratories worldwide, yet their capacity for pain and stress and their welfare needs remain underappreciated compared to mammalian models.
The enormous scale of zebrafish use in research means even small welfare improvements per individual aggregate to massive reductions in total suffering. Zebrafish exhibit avoidance learning, show stress responses measurable via cortisol and behaviour, and have nociceptive systems. Water quality, conspecific density, enrichment provision, and anaesthesia protocols all significantly impact welfare. The regulatory gap — where larvae are excluded from welfare protections applied to adult fish — leaves billions of animals in a policy blind spot that the science no longer justifies.