River lampreys are ancient, jawless fish that serve ecological roles as filter feeders and prey species. Their welfare in the context of water quality management and weir obstruction is an emerging concern.
Lamprey welfare is at the boundary of certain and uncertain sentience — as jawless vertebrates, they have more neural complexity than bivalves but less than teleost fish. Their welfare is relevant both directly and ecologically: lampreys serve as prey species for otters, herons, and other wildlife whose welfare is clearer. Obstructions to lamprey migration cause reproductive failure and population decline; water quality deterioration affects their larvae's ability to filter-feed. The conservation actions that protect lampreys — weir removal, water quality improvement — benefit the entire riverine community including species with clearer welfare significance.