Parasite faunas of farmed cod and adjacent wild cod populations in Norway: a comparison

Abstract

Atlantic cod Gadus morhua L. is host to more than 120 parasite species. Background abundance of these parasite species on adjacent wild hosts determines the infection pressure on cod farmed in open pens. In 2006, 2007 and 2008, 343 cod were collected from 4 locations along the coast of Norway: Øksfjord, Kvarøy, Brønnøysund and Ålesund. Freshly killed wild local cod, wild migratory cod, hatchery-reared farmed cod and wild-caught farmed cod were given a complete autopsy according to a standardized protocol. A total of 343 cod were examined, from which 48 parasite taxa, including 37 named species, were recorded. Wild local cod had the most diverse parasite fauna. Wild-caught farmed cod had a more diverse parasite fauna than the wild migratory cod, and the latter had 2 more parasite taxa than the hatchery-reared cod. The most common parasites in hatchery-reared cod were the digenean Cryptocotyle lingua, the monogenean Gyrodactylus marinus and the protozoans Spironucleus torosa and Trichodina spp. Other parasites occurring frequently in hatchery-reared cod were the parasitic copepod Cresseyus confusus, the myxosporean Zschokkella hildae and the nematode Hysterothylacium aduncum. The nematode and digenean fauna of the hatchery-reared fish was sparse compared to wild cod and the wild-caught farmed cod. Caligid copepods were very rare on the hatchery-reared cod. These results support the hypothesis that food-borne parasites, such as nematodes and mature stages of digeneans, are most unlikely to become a health problem for farmed cod, and that parasites with simple life cycles and pelagic transmission stages, such as monogeneans and trichodinids, may dominate the parasite fauna of farmed cod in the future.