Temperature-dependent growth of Antarctic krill: predictions for a changing climate from a cohort model

In the Southern Ocean, Antarctic krill Euphausia superba are the dominant prey item for many predators, and a changing climate may affect the biomass of krill available to both preda- tors and the krill fishery. We projected growth trajectories for individual krill within cohorts and esti- mated how total biomass in an area available to both predators and the fishery may vary from year to year simply due to fluctuations in temperature. We used an existing temperature-dependent growth model and a time series of temperature data (1970 to 2004) for 2 regions in the Southern Ocean: (1) around the Antarctic Peninsula, and (2) around the island of South Georgia. The growth model predicted increasing individual size within a cohort (in terms of length and weight) with increasing temperature in the cooler Antarctic Peninsula region and decreasing individual size with increasing temperature in the warmer South Georgia region. Years with many cohorts of small individuals in the population resulted in biomass well below average, whereas years with many cohorts of large indi- viduals resulted in biomass well above the average, suggesting that temporal changes in Southern Ocean temperatures may have profound effects on the total biomass in an area that is available to both predators and the fishery. Moreover, the effects of a potentially warming Southern Ocean on krill biomass will likely be more pronounced in the warmer regions occupied by krill.