Impact of Epizootic Shell Disease on the American Lobster in Southern New England

Epizootic shell disease was first identified in 1997 in American lobsters caught in Block Island Sound and Narragansett Bay, Rhode Island.  Since then the disease has spread throughout most of the southern New England lobster stock where it is now found at prevalence levels as high as 20-30%.  During the same time period the lobster population in southern New England experienced a dramatic decline in abundance.  Stock assessments of the American lobster have suggested low recruitment and increased natural mortality rate as the likely causes of the stock decline.  However, the extent to which shell disease contributes to the presumed increase in natural mortality and the overall population decline remains unknown.  Epizootic shell disease is characterized by an extensive erosion and melanization of the cuticle and is readily identified through gross inspection in the field.  The external signs of the disease provide an indicator of infection and a measure of disease severity.  These data were recorded as part of a long-term tagging study conducted in eastern Long Island Sound.  We used logistic regression to estimate disease-associated mortality by disease severity state, sex, and reproductive status (ovigerous or non-ovigerous). Our analysis shows significant mortality associated with epizootic shell disease and suggests that natural mortality rate for the southern New England lobster stock has more than doubled from the previously assumed rate.  This study also shows that ovigerous females are most affected by the disease. Female lobsters rarely molt when ovigerous and this likely contributes to mortality in affected ovigerous females. Our data identifies shell disease in lobsters as small as 35mm carapace length suggesting significant disease associated mortality is likely taking place prior to these animals recruiting to the fishery.  Shell disease is likely reducing reproductive output through lost clutches as well as reducing spawning stock biomass through disproportionately elevated female mortality. These findings corroborate the stock assessments suggestion that natural mortality rate has risen in the southern New England stock and identifies a path by which disease may be affecting the stock-recruitment relationship in the region.