Krill population dynamics in the Scotia Sea: variability in growth and mortality within a single population

Understanding the demographics of Antarctic krill over large scales may be complicated by regional differences in the processes that govern population structure. The influence of regional differences in growth and mortality on population size structure was examined using data on the length–frequency distribution of krill in the Scotia Sea using samples from the South Shetland Islands and South Georgia collected annually from 1991 to 2000. A correction function, which took account of the higher growth rate at South Georgia, produced a consistent similarity in the position of the modal size classes that was not present in the raw data. Optimising the mortality rate, to minimise the differences in the growth corrected length–frequency distribution, suggested a higher mortality rate at South Georgia that the South Shetlands. The intra-specific variations in growth and mortality rates are consistent with published values and with other Euphausiids species. Having accounted for the demographic plasticity, it is apparent that strong recruitment of the smallest size class of krill is represented in both populations simultaneously. It appears that first-year krill are advected into different regions of the Scotia Sea where the resultant population size structure is determined by regional differences in growth and mortality. The majority of the commercial harvest of krill in the Antarctic occurs in a relatively small number of regional fisheries within the Scotia Sea and is managed using population models based on asingle set of demographic parameters. Where substantial differences in these parameters exist between fishing areas, the calculation of catch limits should take these differences into account.

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