Non-consumptive mortality in copepods: occurrence of Calanus spp. carcasses in the Arctic Ocean during winter

Abstract

During a research cruise to the Arctic Ocean in January 2012, we observed high occurrence of carcasses of Calanus spp. We analysed live samples to separate living from dead individuals. In Rijpfjorden, an Arctic fjord in north-eastern Svalbard, 9–14% of the Calanus population were observed to be dead. At Sofiadjupet, an oceanic basin located at 81.4N at the southern edge of the Arctic Ocean, 94% of the Calanus at depth (between 300 and 2000 m) were dead. This is the first estimation of the contribution of carcasses to high Arctic copepod populations. Lipid sacs were preserved in part of the observed carcasses and the lipid sac area was significantly smaller in dead Calanus than in living individuals. We discuss possible causes for the observed mortality, its implication for population dynamics and energy transfer and review previous accounts of non-predatory mortality of marine copepods. In our case, partial consumption by predators and death after reproduction are unlikely causes of mortality, thus other stressors (starvation, environmental conditions, parasitic or viral infection) may be responsible. Our analysis shows that we have a poor understanding of factors other than predation causing mortality in copepod populations.