Association between nematode Hysterothylacium aduncum invasion of cod larvae and growth

Parasitic nematodes of the superfamily Ascaridoidea are distributed worldwide also with numerous representatives in fish. They have an important role to play in the aquatic environment and may affect survivability of fish. The life cycle of many of these fish infecting roundworm species includes invertebrates and fish species and for some species also higher vertebrate hosts.

We have recently demonstrated that fry of North Sea cod has a high prevalence of infection with regard to the nematode Hysterothylacium aduncum and it was indicated that these infections could affect survival of cod and thereby affect the cod stock in the North Sea. The objective of the present study was to elucidate if infections are associated with a decrease or an increase of fish size when examining fish of the same age.

We investigated effects of H. aduncum infections on the growth rate of cod larvae by using the otolith reading method. In our study, the prevalence of infection with H. aduncum in North Sea cod Gadus morhua larvae was studied during the years 1992-2001. A subsample of 65 cod was selected based on the body length (range 20 to 45 mm) with 32 infected and 33 uninfected fishes.

For ageing the cod larvae, lapillus otoliths were removed, polished and the number of growth zones in each otolith counted by light microscopy. Each growth zone indicates one day of the fish life span. Covariance analysis demonstrated highly significant differences (p≤ 0.001) between the growth rate of infected and uninfected cod larvae; the infected larvae showed a higher growth rate compared to uninfected ones of similar ages.

The higher growth rate in infected fish could be a result of higher ingestion of copepods which are serving as intermediate hosts of these parasites. The increased growth could reflect that fish larvae with a higher ingestion rate of copepods will have a higher probability of infection. Further studies should elucidate if the higher infection (due to an increased feeding rate) may lead to a decreased survivability at a later stage of life.