The relationship of Hematodinium infection prevalence in a Scottish Nephrops norvegicus population to season, moulting and sex

The two major field methods (body colour and pleopod index) for the diagnosis of infection in the Norway lobster, Nephrops norvegicus by the dinoflagellate parasite Hematodinium were compared. Using the more reliable method. the pleopod index. which scores infection severity on a four-point scale. infection prevalence data were collected from a single fishing ground in the Clyde Sea area, western Scotland over a continuous period of 31 months. Peak infection prevalence occurs during the spring and is highest in small N norvegicus [mean carapace length 38.1 (+/-0.67)mm in females and 30.9 (+/- 0.50) mm in males] and in females. Mean infection severity increased from 1.05 +/- 0.03 during the low season (July-December) to 2.59 +/- 0.19 by the end of the main infection season (May), highlighting the progressive nature of patent infection over time. Infection prevalence is synchronous between the sexes in some seasons but not in others. Additionally, the proportion of recently moulted, infected males in the late season extends the duration of peak infection prevalence in certain years. Data from two adjacent fishing grounds in the Clyde Sea area also show that infection prevalence depends upon the population structure at a given site the site with smaller animals showing the higher prevalence. An evaluation of methods for monitoring Hematodinium infection suggests that the pleopod method gives the most accurate estimation of prevalence in field-caught lobsters. (C) 2001 International Council for the Exploration of the Sea.