Krill demography and large-scale distribution in the Western Indian Ocean sector of the Southern Ocean (CCAMLR Division 58.4.2) in Austral summer of 2006

Krill demography was studied during a large-scale survey of the South Western Indian sector of the Southern Ocean conducted in late January to late February 2006 (BROKE-West). The survey progressed from 30oE in late January to 80oE in late February and was bounded in the North by 62°S. The average krill density calculated from the catches of the RMT 8 net for four geographical strata ranged from 0.84-68 individuals per 1000 m-3, with overall mean of 6.7 individuals per 1000 m-3. Krill distribution and its population structure were analysed using cluster and mixture analysis of the length frequency distribution, showing small sized krill (modal size of 25-30 mm) broadly distributed in the centre of the western area whereas middle and large sized krill (modal size of 35-45 mm and 45-55 mm, respectively) mainly distributed in the middle and eastern half of the survey area. These findings are discussed in the context of the observed geographic and oceanographic structures; the continental slope area, fronts, currents and gyres. Proportional recruitment indices ranged 0.089-0.226 for R1 (age 1+ recruitment) and 0.204-0.440 for R2 (age 2+ recruitment) in the four strata. Recruitment analysis revealed differences between western and eastern halves of the surveyed area, suggesting the possible existence of separate self-sustaining populations linked to the Weddell Gyre and Prydz Bay Gyre systems. Krill larvae were mainly observed along the shelf break, implying the early onset of spawning in mid-November. The series of coastal polynyas developing in the early spring may be one of the important habitats allowing use of early season phytoplankton growth for early maturation and spawning, and those larvae would be transported westwards by the coastal current along the shelf slope.