Spatial distribution of predator/prey interactions in the Scotia Sea: implications for measuring predator/fisheries overlap

The measurement of spatial overlap between predators and fisheries exploiting a common prey source is dependent upon the measurement scale used and the use of inappropriate scales may provide misleading results. Previous assessments of the level of overlap between predators and fisheries for Antarctic krill Euphausia superba in the South Shetland Islands have used different measurement scales and arrives at contradictory conclusions. At-sea data from observations of krill predators during the CCAMLR 2000 krill survey were used to identify the areas of potential overlap with fisheries in the Scotia Sea and to determine the scale at which such overlap should be measured. The relationship between auto-correlation and sampling distance was used to identify the characteristic scales of the distribution of predators, krill and krill fisheries and an effort-corrected index of relative abundance as a function of distance from land was used to identify the characteristics of areas of high potential for overlap. Despite distinct differences in foraging ecology a group of krill-dependent species including chinstrap penguin Pygoscelis antarctica, (Antarctic) fur seal Arctocephalus sp. (gazella) and white-chinned petrel Procellaria aequinoctialis showed similar patterns of distribution; the relative abundances were highest at 60 - 120 and decreased sharply t distances greater than 150 km from land. There was more inter-specific differences in the characteristic scales which were of the order of 50 - 100 km. Antarctic krill had a characteristic scale of c 200 km and the relationship with distance from land showed a log-linear decline. Krill fisheries operated at a scale of 150 km and almost all of this operation took place within 100 km of land. The requirement of land for breeding and the biological and oceanographic conditions that produce high concentrations of krill associated with those islands produce a system in which the demand for Antarctic krill from fisheries and predators is essentially co-extensive. The areas of greatest potential overlap are within 150 - 200 km of land and the extent of any such overlap in these areas should be assessed at scales of 70 - 100 km to accommodate the scales of operation of the predators involved.