CCAMLR

This document analyses and relates long term information on population trends of inshore demersal fish and Antarctic shags of the South Shetland Islands. The analysis is complemented with comparable information on diet, foraging patterns and breeding output of shags from the Danco Coast, western Antarctic Peninsula, an area that has remained out of the influence of the commercial fishery. Instead of climate change processes, indicated as responsible for the diminution of other bird populations, the most reliable cause of the declining trend observed in shag colonies at the South Shetland Islands is the concomitant decrease in the abundance of two of their main preys, the nototheniids Notothenia rossii and Gobionotothen gibberifrons, owed to the intensive industrial fishing in the area in the late 1970s.

In this study, otolith microstructure analysis on 7 blue phase fingerlings and 26 brown phase fingerlings selected from a total sample of 364 early juveniles caught mainly in summer from 2003 to 2010 at Potter Cove, South Shetland Islands, allowed to provide insights on early life history timings and to validate annulus formation. Although in the literature it has been reported that N. rossii in the South Shetland Islands hatches in spring, our daily ring back counting in otoliths from the date of capture showed two main periods of larval hatching separated by 5-6 months, one in late summer between February and March and another in winter between July and August. The maximum and minimum ages estimated for the pelagic blue phase and demersal brown phase fingerlings were respectively 227 and 240 days, indicating a timing of demersal settlement of about eight months from hatching. The age/length frequency distribution of fish sampled in spring 2010 showed the simultaneous presence of two different cohorts, belonging to biological ages 0+ and 1+ that hatched respectively in summer and winter. The growth rate of N. rossii fingerlings in Potter Cove was estimated between 0.26 and 0.31 mm/day, which is equivalent to that reported for fingerlings caught at South Georgia, and considerably higher than the growth rates known for other nototheniids within similar size range. Based on the early life events associated with the two hatching periods in N. rossii, two main types of life cycles are hypothesised for South Shetland Islands fingerlings. All the pelagic blue phase fingerlings (age group 0+) hatched in July-August (winter cohort), entering in Potter Cove in February-March. The brown phase fingerlings (age group 0+) hatched in February-March (summer cohort), entering in the cove already in spring, i.e. from September-October onwards. Finally, early juveniles (age group 1+) hatched in July-August, entering in the cove the following year to spend the second winter inshore.

Following seals and baleen whales prior to the 1970s, demersal fish stocks were depleted off the South Shetland Islands by intensive industrial fishing during the late 1970s to early 1980s. Little has been reported since about how these stocks have fared, after international agreement closed this fishery in 1990. We report changes in size and abundance of the commercially exploited Notothenia rossii and Gobionotothen gibberifrons relative to the ecologically similar but unexploited N. coriiceps at Potter Cove, South Shetland Islands, over a 28-yr period, 1983-2010. 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 (1992-1994), a sharp increase in length and a continued decline in relative abundance indicated low recruitment. The length-frequency distribution of unexploited N. coriiceps throughout the whole period showed no change in modal size or mean length of the fish. We relate these patterns to the fishery and suggest that a further two decades will lapse before these stocks recover. Using the South Shetland fisheries as an example, current management rules for Southern Ocean fisheries, deemed to be precautionary and disallowing depletion beyond which a stock can recover in 2-3 decades, may be unrealistic in an ocean profoundly altered by numerous stock depletions and rapid climate change.