The role of fish in the Potter Cove ecosystem and status of the commercially exploited species.

MOREIRA, E.; ALESCIO, N. S.; BARRERA ORO, E. R.

Buenos Aires

Workshop; 4th IMNCOAST- Workshop; 2012

Ministerio de relaciones exteriores y culto

The role of fish in the Potter Cove ecosystem and the status of the commercially exploited species have been studied in the frame of the Ichthyology Project of the Instituto Antártico Argentino since 1983. In the inshore waters of the cove, the ecological role of demersal fish is more important than that of krill. The main fish families represented are the Nototheniidae, Channichthyidae, and Bathydraconidae. The fish species are major consumers of benthos, mainly gammaridean amphipods, and also feed on zooplankton, mainly krill in summer. Algae are consumed regularly throughout the year intentionally and even selectively by fish, rather than by accident. There is a relatively low dietary overlap among the fish species, explained by the about equally divided occurrence of generalised and specialised feeders, with no evidence of food competition. The fishes? trophic niches seem to be separated by depth and prey taxa. In these coastal waters, the role of krill is less significant; fish are links between lower and upper levels of the food web and are common prey of other fish, birds and seals. In offshore waters, out of the cove, demersal fish depend less on benthos and feed more on zooplankton (mainly krill) and nekton, and are less accessible as prey of birds and seals. The stocks of the nototheniids Notothenia rossii and Gobionotothen gibberifrons were depleted off the South Shetland Islands by intensive industrial fishing during the late 1970-early 1980s. Fish samples collected in Potter Cove over a 28-year period, 1983-2010 allowed to compare the status of these exploited species with that of the ecologically similar but unexploited N. coriiceps. N. rossii abundance declined from 1983 to 1991, and an increase in mean size during 1983-84 is consistent with weak cohorts during preceding years. Modal age changed from 2-3 to 6-7 yr. Length data of G. gibberifrons, available from 1986, exhibited a similar pattern, showing a decrease until 1991-92. After a period of relative stability (1992-94), a sharp increase in length and a continued decline in relative abundance indicated low recruitment. The length-frequency distribution of N. coriiceps through the whole period showed no significant change in modal size or mean length of the fish. A concomitant declining trend in the number of breeding pairs of the Antarctic shag Phalacrocorax bransfieldensis, a major predator of demersal fish, has been observed in the last fifteen years at several localities of the South Shetlands, close to Potter Cove. This is apparently explained by changes in inshore fish availability. We relate these patterns to the fishery and suggest that a further two decades will lapse before these fish stocks recover.