Distribution and seasonal prevalence of Hematodinium sp. infection of the Norway lobster (Nephrops norvegicus) around the west coast of Scotland

Around Scotland, coastal stocks of the commercially important Norway lobster (Nephrops norvegicus), particularly those within the Clyde Sea Area (CSA), have for some years been known to be infected by syndiniid dinoflagellate endoparasites of the genus Hematodinium. The prevalences of infected Norway lobsters in trawl samples from the CSA are reported for 1992–1995, together with similar data from a range of Scottish west coast sites in 1992, and from the Irish Sea in 1994 and 1995. In the CSA, infection prevalences declined after 1992 and were higher than those recorded in other sampled areas. Infection by Hematodinium was confined to the first half of the year, peaking in the spring and early summer, with medium-sized and female lobsters showng the highest prevalence. Comparison between CSA sites suggests that infection may be influenced by factors directly related to Nephrops age rather than size. In addition to the usual diel variability in catch, in spring male Nephrops showed diel variability in Hematodinium prevalence in trawl samples (being higher in the midnight samples). Females showed no diel variation in prevalence. Underwater television surveys of Nephrops burrow density and data on commercial landings indicate a decrease in Nephrops abundance in the last decade, which might in part reflect the higher levels of infection by Hematodinium during this time.