Parasites as biological tags for stock identification of Atlantic horse mackerel Trachurus trachurus L.

Forty-five different parasite taxa were recorded from 1919 Atlantic horse mackerel Trachurus trachurus caught at 20 stations in a study area from off the coast of Morocco to south-west Norway, and throughout the Mediterranean Sea. Eleven taxa are new host records, and one is probably a new species. The geographical distribution and biology of each parasite and its value for the stock identification of T. trachurus are described and discussed. The most effective biological tags were the larval nematodes Anisakis spp. and Hysterothylacium aduncum. The distinctive pattern of infection with these nematodes observed in samples from the single North Sea station clearly distinguishes this from all other stations and supports the current management strategy which treats the North Sea population as a separate stock. The distinction between the putative “western”, “southern” and “mauritanian” stocks is less clear, with evidence of considerable mixing between them. The highly localised distributions of some parasites in the Mediterranean part of the study area suggest that T. trachurus populations there appear to comprise three main stocks—western, central and eastern. There is also strong evidence of the migration of fish from Atlantic populations into the extreme western part of the Mediterranean.