Mortality rates are among the most widely used welfare indicators in aquaculture. Understanding their limitations and complementary measures helps build a more complete picture of farm fish welfare.
Mortality data is simultaneously the most available and the most incomplete welfare indicator in aquaculture. Its economic significance means it is reliably recorded, but it captures only the endpoint of severe welfare failure — the dying — while missing the suffering of fish that are ill, injured, or chronically stressed but alive. A farm with low mortality but high levels of fin damage, parasite burden, or abnormal swimming behaviour has poor welfare despite a good headline metric. Mortality timing and pattern provide additional information: acute spikes indicate specific welfare events (treatment, handling, disease outbreak) while chronic low-level losses suggest persistent environmental problems. Comparison between sites, species, and production methods contextualises mortality data — high losses in some conditions are nearly universal and reflect systemic problems with production systems themselves. Best practice welfare assessment combines mortality with fish-based measures (fin damage, body lesions, parasite counts, body condition scoring) and environment-based measures (water quality parameters) to build a multi-domain welfare profile.