Shell disease in impounded populations of the American lobster, Homarus americanus, in Nova Scotia, Canada

The overall research goal was to identify risk factors for the development of shell disease in impounded populations of the American lobster, Homarus americanus. Shell disease, a disease syndrome characterized by progressive erosions and necrosis of the exoskeleton of aquatic crustaceans, continues to be one of the most prevalent and costly conditions affecting captive populations.

An epidemiological study was designed to evaluate the association between physiological and environmental factors and the development of shell disease on an individual lobster level. Initial health assessments on fresh caught post-harvest lobsters from Southwest Nova Scotia dealers showed significant differences between dealers, depending on the location of their respective fishing grounds (lower total protein levels, softer shells, and a higher proportion of intermolt lobsters in the primarily mid-shore dealer compared to the primarily inshore dealer).

Monthly observations of frequency and characteristics of shell disease lesions were performed during the storage period. Logistic regression results showed total protein as the strongest predictor of shell disease (odds ratios ranged from 4.3 to 32.8 for lobsters with low total protein levels, versus lobsters with high protein levels). The lobsters' molt stage also emerged as an important predictor for shell disease (odds ratios of 2.1 and 2.4 at models for 30 and 90 days storage) with a trend towards a relationship in the 60 and 120 day models, suggesting that the timing of the fishery in Southwest Nova Scotia in relation to the molt cycle is important in determining the post storage prevalence of shell disease in impoundment facilities. Shell hardness showed significance only in the 30 day model (odds ratio of 1.6), suggesting that this factor may only be important early in the storage period while sludge removal from the surface of the lobster was significant in 2 of the final models (odds ratios of 1.7 and 2.0 at 90 and 120 days, respectively), with a trend towards significance in the remaining two models, suggesting that the sludge offers some protective benefits to the captive lobsters.

From the perspective of a long term storage facility, the quality of the lobsters entering the facility is critical and, as this study shows, determines the extent to which shell disease will develop.