Protistan parasites as mortality drivers in cold water crab fisheries

From a historical perspective, several protistan taxa, including the recently re-aligned Microsporidia, have been associated with or identified as causes of mortalities in crustacean populations. Depending upon the host species, associated protistan prevalences could be as low as 5% or approach 100%. It has generally been assumed that reported prevalences translated directly into significant mortalities that could impact the distribution and abundance of affected populations. However, this assumption may be incorrect especially when the dynamics of host–pathogen–environment interactions are not entirely understood. We will discuss the presumed impact of several protistan pathogens on temperate and cold water commercial crab species. By using selected examples such as a ciliate in the Dungeness crab (Cancer magister) and Hematodinium sp. infections in North Pacific crabs, we will attempt to contrast differences between prevalence and mortality, acute and chronic infections/mortalities, age or size selectivity of affected population, and geographically restricted and widespread epizootics. We will also briefly discuss the potential impact of environmental changes such as climate change and ocean acidification on both host and protistan pathogen.

Major biotic processes (blue) affecting fish production (green) and the abioticfactors (yellow) that modify these processes. The four major hypotheses concerning control of fishery abundance are related to the major processes controlling production and mortality of fish life history stages: reproductive output, starvation, predatory (including cannibalistic) losses, transport losses and disease (red). To represent an actual fishery environment, several such interlocking diagrams would be needed to depict multiple species (with permission and modified from White, 1985).Figure optionsDownload as PowerPoint slideHighlights
► We review the potential of protistan pathogens as drivers of significant mortality in wild coldwater crab species.
► We submit that the data are not clear because baseline disease data are generally absent.
► We indicate that where data may be available, epizootiological studies are incomplete with respect to the factors examined.
► We propose that disease data needs to be gathered with the final intent to model disease impacts on populations.
► We also propose that the current epidemiological triad model may be impeding studies because it is too simplistic in nature.