Variation in the distribution of Antarctic krill Euphausia superba around South Georgia

There is considerable spatial structure within the South Georgia krill-based ecosystem in terms of predator demand, commercial fishery pressure and krill abundance. Here we investigate the hypothesis that there is also spatial structure in the krill population composition, finding differences in length-frequency histograms between the eastern and western ends of the island. Between 1981 and 1997 the British Antarctic Survey carried out 6 major cruises where krill were taken from both ends of South Georgia. Cluster analysis revealed that the length-frequency histograms could be grouped into 4 main types with relatively simple biological characteristics: Cluster 1 contained small krill (mean size 23.9 mm) up to 2 yr old (1+ yr class); Cluster 2 (mean size 31.1 mm) contained a mixture of 1+ with some 2+ and 3+ yr classes; Cluster 3 contained medium-sized krill (mean size 41.4 mm) probably 2+ and 3+ yr classes; Cluster 4 contained large krill (mean size 50.3 mm) likely to be 3+ or older. Principal components analysis (PCA) provided good separation of these clusters using the first 2 axes (80% of the total variance). There were no obvious differences in the length composition of krill sampled from different water depths, although there were some indications that differences did occur between different water masses. Detailed inspection of the individual cruises revealed that the length-frequency histograms at the western end of the island contained more large krill than those from the eastern end of the island and also that when Weddell Sea water was found within a cruise then this contained the smallest krill. We consider that such differences may arise not only because krill may experience different conditions at each end of the island but also may originate from 2 separate sources which may have different population structures.

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