Epidemiology of bitter crab disease (Hematodinium sp.) in snow crabs Chionoecetes opilio from Newfoundland, Canada

Jul 7, 2005Diseases of aquatic organisms

Patterns of bitter crab disease in snow crabs from Newfoundland, Canada

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Abstract

Prevalences of bitter crab disease (BCD) in female snow crabs reached 19 to 26% in 1999.

Hematodinium sp. causes BCD, rendering infected snow crabs unmarketable due to bitter flavor.
The disease has become firmly established in Conception and Bonavista Bays, with lower levels on the Avalon grounds.
Infections were highest in female crabs and smaller male crabs, indicating vulnerability in these groups.
All naturally infected crabs died in a mortality study, with a 50% mortality rate observed in experimentally inoculated crabs.
Transmission likely occurs during the post-molt phase, with overt infections developing 2 to 4 months later.
A significant positive association was found between prevalence of BCD and factors such as depth and substrate type.

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The parasitic dinoflagellate Hematodinium sp. causes a condition known as bitter crab disease (BCD) in snow crabs Chionoecetes opilio and Tanner crabs C. bairdi. As the name of the condition implies, crabs infected with BCD are unmarketable due to their bitter flavor. We surveyed the distribution of BCD in 3 regions within the snow crab fishery of Newfoundland from 1997 to 2003. Over time, the disease has become firmly established in Conception and Bonavista Bays and persists at low levels on the Avalon fishing grounds. An epizootic occurred within Bonavista and Conception Bays in 1999 and persisted in Conception Bay in 2000, reaching prevalences of over 2% to 9% in trapped and trawled male crabs and from 19 to 26% in trawled and trapped female crabs, respectively. Infections were highest in females and small males, i.e. the unfished and pre-recruit portions of the fishery. In a mortality study, all of the naturally infected crabs died and 50% of the experimentally inoculated crabs died. Patterns in the molting cycle and prevalence of infection indicate that transmission occurs during the post-molt condition, and that overt infections probably develop 2 to 4 mo later with mortalities occurring at least 3 to 4 mo thereafter. The hydrography of this bay may have contributed to the epizootic as infections were centered within the deeper confines of the bay. Analysis of various abiotic factors uncovered a significant positive association between prevalence, depth and mud/sand substrates; the nature of this relationship was not apparent but may be related to diet or alternate hosts. Lastly, given the increase in BCD in snow crabs in Newfoundland, we recommend that fishery management programs for Chionoecetes fisheries employ non-selective gear to monitor for Hematodinium infections in female and juvenile crabs because these under-sampled members of the population may forewarn of impending recruitment declines that might otherwise remain unexplained.

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