Stage-specific mortality of Calanus finmarchicus, Pseudocalanus elongatus and Oithona similis on Fladen Ground, North Sea, during a spring bloom

We used Woods Population Surface Method to estimate stage-specific mortality rates for populations of Calanus finmarchicus, Pseudocalanus elongatus, and Oithona similis from abundance data obtained during the Fladen Ground Experiment (FLEX 1976) in the northern North Sea. Temperatures from the day and night depth distributions of each developmental stage were used to force temperature functions for the duration of developmental stages. In addition, we considered the influence of potential food-limitation of development, which had only minor effects on the mortality estimates. We show that the risk of mortality changes substantially over the life span of the animals, with notable differences among species. O. similis, in particular, shows negligible losses after naupliar stages 1 and 2, which may account for its numerical dominance in many ocean regions. Differences among taxa are poorly explained by allometry alone and are instead related to species- and stage-specific differences in behavior. The vertical migration behavior of late developmental stages of C. finmarchicus (i.e. their choice of habitat) might help to reduce mortality risk.