Quantifying the welfare impact of air asphyxia in rainbow trout slaughter for policy and practice.

The effective improvement of animal welfare requires quantitative methods to compare diverse impacts across practices and policies on a common, relatable scale. The Welfare Footprint Framework (WFF) fulfills this need by providing a standardized welfare impact measure: cumulative time in affective states of varying intensities. To this end, WFF estimates rely on documented syntheses of existing research, including behavioral, neurophysiological and pharmacological indicators. We apply this framework to quantify the welfare impact of air asphyxia during fish slaughter, using rainbow trout as a case study. Based on a review of research on stress responses during asphyxiation, we estimate 10 (1.9-21.7) min of moderate to intense pain per trout or 24 (3.5-74) min/kg. Cost-effectiveness modelling shows that electrical stunning could avert 60-1200 min of moderate to extreme pain per US dollar of capital expenditure, but commercial performance remains variable. Percussive stunning demonstrates reliable effectiveness, but still faces implementation challenges. These findings provide transparent, evidence-grounded and comparable metrics to guide cost-benefit decisions and inform slaughter regulations and practices in trout (and potentially other species). With over a trillion fish slaughtered annually, they also demonstrate the potential scale of welfare improvements achievable with effective stunning methods.