Abstract.- Knowledge of trophic 

 relationships is vital to understand- 

 ing any ecological community. The 

 trophic relationships of Antarctic 

 demersal fish are poorly known and 

 have been described quantitatively 

 by only a few researchers. 



Gut contents were analyzed on over 

 300 stomachs from fish collected dur- 

 ing the 1987-88 AMLR ground fish 

 survey of South Georgia I., Antarc- 

 tica. All fish were collected with a 

 bottom trawl during the austral sum- 

 mer. Fifteen species of demersal fish 

 (including those of commercial value) 

 were collected. Similarity analysis 

 was applied to the diet information 

 to describe trophic relationships in 

 the South Georgia community. 



The most abundant species of the 

 South Georgia demersal fish commu- 

 nity were classified into three groups 

 based on summer diets. The largest 

 group contained species heavily de- 

 pendent on krill Euphausia superba, 

 and included Champsoeephalus gun- 

 nari and Notothenia rossii. The sec- 

 ond group was comprised of pisci- 

 vores. Three of the four members of 

 this group (Dissostichus eleginoides, 

 Cfiaenocephalus acemtus, and Pseudo- 

 chaenichthys georgianus) are com- 

 mercially valuable. The food of their 

 prey often consisted of krill. The 

 third group contained a loose asso- 

 ciation of species which feed on ben- 

 thic organisms more than did other 

 fish species in the community. Noto- 

 thenia gibberifrons and Notothenia 

 squamifrons were the important 

 commercial species in this group. 



Krill was found to be the most im- 

 portant prey species to the fish in the 

 South Georgia system. However, 

 based on the analysis of diet overlap, 

 competition appeared to be unimpor- 

 tant in this community during the 

 austral summer. 



Selective reduction of populations 

 within the fish community by fishing 

 may have widespread repercussions. 

 Many of the commercially valuable 

 species feed on other fish species 

 which in turn feed on krill or benthic 

 organisms. The relatively simple but 

 highly interconnected food web in 

 the South Georgia system may have 

 a lower potential for fish yield than 

 previously thought. 



Manuscript accepted 6 May 1991. 

 Fishery Bulletin, U.S. 89:643-654 (1991). 



Trophic Relationships within the 

 Antarctic Demersal Fish Community 

 of South Georgia Island 



James E. McKenna Jr. 



Graduate School of Oceanography. University of Rhode Island 

 Narragansett, Rhode Island 02882 

 Present address: Florida Marine Research Institute 

 100 8th Avenue SE, St. Petersburg, Florida 33701 



The Antarctic ecosystem has been 

 physically isolated from the rest of 

 the world for at least the last 30 mil- 

 lion years, since the Drake passage 

 opened as South America moved 

 away from Antarctica (Kennett 1982: 

 726) and probably longer (Regan 

 1914). The fish assemblage of this 

 region appears to be the result of 

 evolutionary radiation from a limited 

 fauna that was present in the region 

 when Antarctica separated from 

 Australia (Eastman 1985, Kock 

 1985b). Over 60% of the species and 

 90% of the individuals belong to four 

 families in the suborder Notothenioi- 

 dei, and 95% of the species in this 

 group are endemic to the Antarctic 

 region (DeWitt 1971, Kock 1985b). 

 The four dominant families are the 

 Antarctic cods (Nototheniidae), 

 dragonfish (Bathydraconidae), icefish 

 (Channichthyidae), and spiny plunder- 

 fish (Harpagiferidae). These fish are 

 generally sedentary, benthic forms 

 found on the Antarctic continental 

 shelves (Norman 1938). An interna- 

 tional fishery has developed for 

 larger members of these families, 

 especially in the region around South 

 Georgia I. (Fig. 1) (Kock 1986). 



South Georgia is located just south 

 of the Antarctic Convergence and is 

 surrounded by a narrow, relatively 

 shallow continental shelf. The phys- 

 ical oceanography of the region is 

 very complex (Foster 1984) and may 

 contribute to the high productivity 

 and abundance of krill (Hempel 

 1985). 



There is a high degree of endemism 

 in Antarctic fishes (DeWitt 1971). 

 The assemblage of fish species that 

 has evolved in the Southern Ocean is 

 well adapted to the Antarctic environ- 

 ment (Eastman 1985). However, the 

 fish community in the South Georgia 

 area appears to be changing, possibly 

 in response to the fishing pressure 

 that it has experienced over the past 

 two decades (Kock 1985b and 1986, 

 McKenna and Saila 1991). Exploited 

 South Georgian stocks have been 

 declining since the late 1970s (Kock 

 1985b and 1986, Gabriel 1987). In the 

 early 1970s, the commercial trawl 

 fishery was supported mostly by the 

 catch of marbled rockcod Notothenia 

 rossii, which yielded hundreds of 

 thousands of tons each season (Kock 

 1986). By 1985, the stock was esti- 

 mated to be less than 10% of its pris- 

 tine size (Kock 1985b). The fishery is 

 presently supported by catches of 

 mackerel icefish Champsocephalus 

 gunnari, but its decline is also evi- 

 dent (Kock 1986, McKenna and Saila 

 1989). Abundance estimates of other 

 species in the South Georgia region 

 decreased by as much as two orders 

 of magnitude between 1975-76 and 

 1980-81 (Kock et al. 1985a). 



The community structure has 

 changed with decreases in stock sizes 

 (McKenna and Saila 1991). There has 

 been a significant shift from a com- 

 munity dominated by a few large- 

 bodied species to fewer, mostly small- 

 bodied species with more equitable 

 abundances. 



643 



