Episodic recruitment in Antarctic krill Euphausia superba in the Palmer LTER study region

As part of the Palmer Long-Term Ecological Research (LTER) program, the size structure of the population of Antarctic krill found between Anvers and Adelaide Islands west of the Antarctic Peninsula has been analyzed for patterns in recruitment success. The data were from a series of 11 cruises, 1 in spring (November 1991) and 10 in summer (January 1993 to January 2002). A maximum-likelihood fitting procedure was used to fit a mixture of normal distributions to the length-density distributions derived from net data. The recruitment index was calculated as the proportion of age-class 1 krill of the total. Antarctic krill in the Palmer LTER region showed a pattern of episodic recruitment, with 2 strong year classes in succession followed by 3 or 4 moderate or poor year classes. The 2 strong year classes represented 85 to 90% of the krill caught for 5 or 6 yr, and as the absolute abundance of the year class declined due to mortality, so did the abundance of Antarctic krill in the region. The recruitment index was positively correlated with the absolute value of a seasonal El Nino /Southern Oscillation (ENSO) index, with strongest recruitment during the neutral or moderate periods of ENSO. The mechanism underlying the strong link between the recruitment index and ENSO is most likely the effects of seasonal sea-ice dynamics on both reproduction and winter-over survival of the resulting larvae as previously documented.