Spring mortality of the cyclopoid copepod Oithona similis in polar waters

Abstract

The cyclopoid copepod Oithona similis is highly abundant and ubiquitous in the marine epipelagic environment, yet rates of mortality in this species have rarely been quantified; indeed we are lacking such measurements for Copepoda in general in cold waters. In the present study we examined O. similis stage structure, egg production and mortality rates across the Scotia Sea, Southern Ocean, sampling from the ice edge to the Polar Front in the austral spring of 2006. The population stage structure near the retreating ice edge was indicative of a recruitment pulse moving through the younger stages; therefore, the assumptions of the vertical life table (VLT) approach were not met and mortality was not estimated for those stations. At all other stations the assumptions of VLT were largely met, and mortality rates were determined as across-station averages. The highest rates of mortality occurred across the egg to NII stages at around 0.04 d–1, falling to <0.03 d–1 in subsequent stages, and then increasing again to 0.11 d–1 across copepod stage V to adult males. The ratio of adult males to females suggested that males have a mortality rate ~12 times greater than females (i.e. the adult male to female abundance ratio is 0.08). It is unlikely that these differences can be attributed simply to the males’ shorter physiological longevity (longevity when free of predators); the primary cause is likely elevated predation mortality due to the risks associated with mate locating behaviour.