Effects of epizootic shell disease in American lobster Homarus americanus determined using a quantitative disease index.

The incidence of epizootic shell disease in American lobster Homarus americanus has increased in southern New England, U.S.A., in the last decade, but few longitudinal studies have followed the disease progress in individual lobsters or demonstrated direct effects on mortality or growth. Diseased lobsters were held in the laboratory for 1 yr, and the progression of disease and its effects on molting, mortality, and growth were monitored. A quantitative disease index (QDI) was developed by measuring disease lesions in digital images of the carapace and expressing the result as a proportion of shell area. Some lobsters died due to high temperatures, but at least 13 of 55 lobsters (24%) died as a direct result of disease-related problems, mostly during molting, and there was a significant relationship between mortality and high values of the QDI. Lobsters that molted successfully were free of disease lesions, but many had exoskeletal deformities. There was no relationship between pre-molt size and disease severity, but molt increment was significantly correlated with premolt carapace length (CL) and negatively correlated with QDI. However, percentage growth was negatively correlated with QDI, but not with pre-molt CL. These significant lethal and sublethal effects of epizootic shell disease should be considered in lobster management.