A considerable body of knowledge exists about crustacean 

 mortalities, and estimates have been made of the prevalence of 

 shell and gill disease in several commercial species. 

 Information derived from experimental studies and observations on 

 laboratory-held animals, have suggested that several diseases 

 undoubtedly have a role in naturally-occurring mortalities. 

 Recent studies indicate, however, that ocean pollution and other 

 conditions that impact on healthy environments lead to increases 

 in the prevalence of visible signs of disease. Crustacean 

 landings suggest that in spite of disease and mortality, 

 reproduction and recruitment are sufficient to compensate for 

 population losses. There is increasing concern, however, about 

 the numbers of animals exhibiting external signs of disease that 

 render them unsuitable for marketing. 



The overall opinion of the Shell Disease Working Group is 

 that crustacean mortalities, although difficult to assess 

 visibly, are higher under conditions of poor water quality than 

 in ecosystems without measurable contaminant impacts. There is, 

 however, no published information on mortalities in wild 

 populations from unimpacted areas that could be attributed 

 directly to shell disease. This does not imply that deaths do 

 not occur — simply that observations of them in natural 

 populations would be difficult to make. By inference from 

 observations of captive or experimental populations, it 

 seems likely that highly stressed individuals may die from the 

 effects of advanced shell disease, secondary infections, and/or 

 predation. 



5.0 CONCLUSIONS 



Based on examination of available published and unpublished 

 information, and on extensive discussion, the Working Group has 

 reached these conclusions : 



° Shell disease represents a stage in the natural 

 relationship of crustaceans with chitin-utilizing bacteria and 

 fungi. An uneasy balance is usually maintained by the metabolic 

 processes associated with new shell formation at molting, and 

 subsequent shell maintenance and repair -- and the growth, 

 metabolism, and reproduction of microbes capable of degrading 

 chitin. The balance may be disturbed by environmental changes 

 that reduce the crustacean's capabilities (metabolic and 

 defensive) to maintain an intact shell, or that encourage 

 multiplication of marine microorganisms with chitinolytic 

 capabilities. 



° Mortalities from shell disease have been observed 

 in laboratory-held and impounded populations, occasionally in 

 large numbers. Systemic infections which may develop after 

 perforation of the chitinous integument, as well as destruction 

 of the gills, and adhesions which prevent molting, have been 

 considered to cause mortalities. Shell disease may predispose 

 crustaceans to predation or disease-related mortality, but there 



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