Fish are the most numerous vertebrates used in laboratory research, and growing evidence for fish pain and stress responses is driving improvements in how their welfare is assessed and protected.
The welfare of laboratory fish has been systematically undervalued based on assumptions about fish pain capacity that are increasingly difficult to defend. Fish show behavioral and physiological responses to harmful stimuli that parallel mammalian pain responses, use analgesics to reduce those responses, and show learning and avoidance behavior that suggests pain affects future decision-making. The scale of fish use in research — orders of magnitude greater than mammalian species — means that even marginal improvements in fish welfare protocols would affect more animals than all mammalian welfare improvements combined. Applying the Three Rs rigorously to fish research represents one of the highest-impact opportunities in laboratory animal welfare.