European eels have declined by 95% since the 1980s — one of the most severe fisheries collapses in history with direct welfare implications for remaining wild individuals.
Wild eel welfare operates at both individual and population scales. Individual eels parasitised by A. crassus face impaired swim bladder function that compromises their ability to complete the spawning migration — a physiological welfare harm that may prevent them from achieving the singular reproductive purpose of their lives. At population level, the 95% collapse represents the cumulative welfare loss of an enormous number of failed migrations, barrier-caused mortalities, and parasitism. Eel pass installation, barrier removal, and parasite management are the primary welfare-aligned interventions.