Life cycle strategy of Hysterothylacium aduncum to become the most abundant anisakid fish nematode in the North Sea

The present study demonstrates the influence of physical systems (mixed and stratified waters) on the occurrence and distribution of the anisakid nematode Hysterothylacium aduncum in commercially important gadiform fish species from the central and northern North Sea. Stratified waters are essential in structuring marine food webs and are therefore also important for the transfer of fish parasites to their host species. During two research cruises in 2001, 308 haddocks (Melanogrammus aeglefinus) and 203 whitings (Merlangius merlangus) were caught in the central and northern North Sea and were examined for anisakid nematode parasites and food composition. Additionally, the most abundant pelagic invertebrates, hyperiid amphipods, of the investigated area were sampled and examined with regard to their parasite infestation rates. The influence of stratified waters on the infestation of the gadiform fish species by H. aduncum was shown by ecological and parasitological investigations. In stratified areas haddocks and whitings feed specifically and invariably on pelagic hyperiids. The number of ingested hyperiids in these areas was in both fish species correlated with infestation rates of H. aduncum. Therefore, hyperiids were identified as the obligatory intermediated host of H. aduncum, since these crustaceans carried high numbers of nematode larvae in their haemocoel. The results of the present study demonstrate that haddocks and whitings from the stratified waters are highly parasitised, both in terms of the numbers of individuals of larval and adult stages of H. aduncum.