Parasites of wild cod postlarvae (Gadus morhua L.) in the North Sea

The Atlantic cod, Gadus morhua L. is a common and commercially important fish species in the North Sea, but estimated stock size and recruitment has been on an overall declining path since 1980. Atlantic cod is in this sense believed in the future to become one of the most intensively cultivated gadoid species in the North Atlantic and has been the subject of several parasitological studies.

Past research is however primarily concentrated around adult or juvenile fish and our knowledge concerning the larval stage is very sparse. This is in spite of the general belief that at this stage, fish larvae encounter highest levels of mortally and predation. In this sense effects of parasitization could be speculated to have a higher impact.

In this study we investigated the occurrence of metazoan ecto- and endoparasites in a school of 209 cod post larvae (9mm – 39mm) caught in the North Sea of Denmark. Skin, fins, body cavity, gut and intestinal tract were checked and parasites were identified either through morphology or with the use of PCR.

A total of 58 parasites were found counting 38 individuals of the ectoparasitic copepod Caligus elongatus, 7 individuals of the endoparasitic nematode Hysterothylacium aduncum, 5 trematodes (1 Lecithaster levinseni, 4 Hemiurus) and 5 individuals of a tetraphyllidean plerocercoid cestode larva. Intensity, abundance and prevalence will be presented as well as area of infection.

Brief introduction to the parasites and known literature on the parasitic effect will be given as well as a discussion on the possibilities of increased mortality due to infection.