Patterns of Mortality in a Lobster Pound

Data from a Maine Lobster Pound Association (MLPA) survey conducted in 2006 suggest that there has been an increase in the mortality of lobsters held over winter in pounds. Estimates among survey respondents place the loss at 14%-27% in 2006. This represents hundreds of thousands of dollars in lost revenue and places this industry in economic jeopardy. These losses have occurred despite no clear evidence of a disease epizootic, no notable change in handling and shipping practices, and in the face of widespread prophylactic antibiotic use. In the fall of 2006, a pilot study was conducted by the Maine Aquatic Animal Health Laboratory and the Lobster Institute to evaluate the overall health of lobsters prior to impoundment, and to identify any factors that could be contributing to the reported mortality in pounds. The results of this preliminary work indicated that a bacterium, Photobacterium indicum, was present in 3%-5% of the sample size and in greater than 90% of morbid and dead lobsters. In order to investigate the role of Photobacterium indicum as a potential pathogen of wild and impounded lobsters, a pound stocking experiment was designed to monitor mortality events of lobsters during a normal impoundment cycle. In phase I of the experiment, lobsters underwent health assessment and bacteriological screening prior to tagging and placement into the pound. Unbroken tags collected on surveillance days were considered evidence of mortality. Impounded lobsters were also resampled on three separate occasions during the study to monitor the temporal pattern of bacteremia and mortality. In phase II of the experiment, 100 lobsters were sampled in a multivariate, physiological parameter study for evidence of stress. The lobsters were placed in individualized compartments within the pound and then retrieved 7 days later for resampling. The results of phase I indicate that mortalities tend to occur within 5 - 1 4 days of impoundment and then subside to low levels. Spikes in mortality may be coincident with feeding dates and increases in lobster density. Approximately 19.9% of 483 sampled lobsters were transiently bacteremic. Photobacterium indicum was cultured at a prevalence of 0.8%). Repeat samples indicate a mean prevalence of 17.2% bacteremia, with a 1.8% mean prevalence of Photobacterium indicum. Aerococcus viridans, the causative agent of Gaffkemia, was not detected in this experiment or in the 2006 pilot study. The logistic regression model predicts an increased trend in mortality during impoundment for female, soft shelled lobsters. Survival analysis curves indicate that there is no significant difference in mortality during impoundment based on origin, vigor or bacteremia. Phase II results indicate a significant difference in analyte levels between initial and repeat samples. In conclusion, there is no evidence of a bacterial pathogen as the proximate cause of mortality in impounded lobsters at this time. The prophylactic treatment of lobsters with oxytetracycline supplemented feed, approved by the FDA for use in lobsters to prevent the disease gaffkemia, is contraindicated. Further research is needed to understand the role of physiological stress on the survivability of lobsters during impoundment. and long distance transport