Bitter crab disease dynamics in eastern Bering Sea Tanner and snow crab: An underestimated and emergent stressor

Abstract

Tanner crab (Chionoecetes bairdi) and snow crab (C. opilio) populations in the eastern Bering Sea have reached historic lows in recent years, and declines have been linked to recruitment failures and mortality events. Bitter crab disease, caused by a parasitic dinoflagellate (Hematodinium sp.), contributes to high mortality rates in Tanner and snow crab, and outbreaks have the potential to reduce recruitment and population productivity. Here, we employed a polymerase chain reaction assay to detect Hematodinium sp. in Tanner and snow crab hemolymph samples to: 1) evaluate testing accuracy of visual disease detection methods; 2) estimate bitter crab disease prevalence from 2015 to 2017 in eastern Bering Sea monitoring sites; and 3) identify factors influencing the likelihood of Hematodinium sp. infection. Our results indicated that visual diagnostic methods failed to detect 93 % of infections, and underestimated disease prevalence by up to 90 %. Infection risk was highly dependent on host size, sex and sampling date. Small Tanner crab (<30 mm carapace width) were nearly twice as susceptible to infection, and female snow crab were 9 % more likely to be infected than males. Most notably, bitter crab disease prevalence exceeded 50 % at two monitoring sites during the study period, and annual disease prevalence increased by approximately 10 % per year in both populations. We emphasize the severe population-level consequences for these high prevalence levels in eastern Bering Sea Tanner and snow crab stocks. Our approach highlights the critical importance of continued monitoring and mechanistic modeling of bitter crab disease in severely depressed crab populations.