Impacts of the parasitic dinoflagellate Hematodinium sp. on Irish crustacean fisheries

Abstract

Infection of Cancer pagurus by a parasitic dinoflagellate of the genus Hematodinium is described for the first time in Ireland. An industry-based monitoring programme was established to determine seasonality of infection intensity and prevalence in the country’s three largest brown crab Cancer pagurus fisheries in the southwest, north and southeast. Velvet crabs Necora puber, European lobsters Homarus gammarus and spider crabs Maja brachydactela were also screened for Hematodinium sp. Infections were scarce in N. puber and absent in both H. gammarus and M. brachydactela samples. In relation to C. pagurus, the parasite was present in all areas for the majority of sampling periods, with highest prevalences recorded in pre-recruit animals of both sexes. Microscopic examination of haemolymph revealed trophont, plasmodial and dinospore stages of the parasite. Overall prevalence in males (16 %) was higher than females (9 %). Prevalence of Hematodinium sp. infection ranged from 0 - 51 % but a distinct seasonal trend was not apparent. Infection intensity was seasonal with significantly higher peaks occurring in late autumn/early winter months than in other quarters, which corresponded with industry reporting moribund and dead pink-shelled crabs in commercial catches. It is postulated that seawater temperature or a temperature linked process is a key factor in the triggering the final stages of infection as significant autumn peaks were followed by a reduction in infection intensity as temperature decreased in the late winter/early spring months with no increase in intensity again until the following autumn. It is proposed that infection intensity rather than prevalence provides a more appropriate indication of the period when there is greatest potential for biological and economic impacts and discuss the parameter’s application as a fisheries management tool.