Zebrafish have become one of the most widely used vertebrate model organisms in biomedical research, yet their welfare needs are often overlooked compared to mammals. Growing evidence suggests zebrafish experience pain and stress, requiring refined husbandry and experimental protocols.
The rapid expansion of zebrafish use in research has outpaced development of welfare standards. Because zebrafish are small, short-lived, and visually different from mammals, their capacity for suffering is frequently underestimated. Research now confirms they have functional nociceptors, endogenous opioid systems responsive to analgesics, and learned fear responses — all indicators of pain processing. The 3Rs (Replacement, Reduction, Refinement) framework applies fully to zebrafish. Refinement priorities include optimal water quality (pH 7-7.5, temperature 26-28°C), species-appropriate social housing, enriched tank environments, and analgesic protocols for invasive procedures. Humane endpoints — defined criteria for euthanasia before maximal distress — should be specified in every protocol.