652 



Fishery Bulletin 89(4), 1991 



This study suggests that resources are effectively 

 partitioned in the austral summer. However, the Ant- 

 arctic undergoes strong seasonal changes, which affect 

 the abundance and availability of krill as well as other 

 aspects of the ecosystem. Krill spawn in summer and 

 the larvae migrate vertically from great depths 

 (>500m) as they develop over the winter, recruiting 

 to the population the next summer (Marshall 1979). 

 Adult and juvenile krill do most of their growing in 

 summer when they may be superabundant, occurring 

 in dense 'swarms' (Everson 1984a). During the un- 

 productive winter, krill grow little or may even shrink 

 in size (Ikeda and Dixon 1982), but their distribution 

 at that time is poorly known. 



The trophic structure of the Antarctic fish commu- 

 nity may change in response to these seasonal events. 

 Prey switching and niche shifts offer two mechanisms 

 to deal with these seasonal changes. Although diet 

 diversities were low for South Georgia fishes, all spe- 

 cies (with the exception of P. breviceps) consumed at 

 least one alternative food resource regularly. This in- 

 dicates that there is the potential for prey switching 

 according to the availability of the prey resources in 

 the environment. Seasonal prey switching has been 

 inferred from changes in the diets of related species 

 living along the Antarctic Peninsula (Daniels 1982). 



Niche shifts may occur to reduce competition at times 

 when krill is limiting. As krill becomes more limiting, 

 behaviorial changes, such as a shift to more specialized 

 feeding on alternative prey or in specific habitats, may 

 occur. To test these hypotheses a seasonal time series 

 of diets and the availability of prey is needed. 



Possible effects of commercial fishing 



Selective removal of species by fishing will effect the 

 demersal fish community. Prey of removed predators 

 may benefit from reduced predation, as has been sug- 

 gested for Champsocephalus gunnari at Kerguelen I. 

 (Duhamel and Hureau 1985). One competitor may 

 benefit by increased fecundity if the other is removed 

 by fishing (Beddington and May 1982). Competition 

 may be intensified at a lower trophic level if a predator 

 on one species of a competing pair is reduced (Miller 

 and Kerfoot 1987, Abrams 1987, Boisclair and Leggett 

 1989). If a niche shift had taken place in the past to 

 reduce competition, a species may expand to occupy 

 more of a niche when its competitor is removed (Con- 

 nell 1980, Beddington and May 1982). However, it is 

 difficult to predict the response of the community 

 without more data on the life histories and seasonal 

 dynamics of these fish and their prey. 



Ecological efficiency is another topic to be considered 

 when harvesting an Antarctic community. The South 

 Georgia community is highly productive (Hempel 1985). 



The diversity of the system is low and the food web 

 is relatively simple (Beddington and May 1982). How- 

 ever, the cost of activity and survival in the Antarctic 

 is high (Hempel 1985) and the loss of energy at each 

 trophic transfer places a limitation on the biomass of 

 fish available for harvest. Many of the commercially 

 valuable species are large piscivores (Chaenocephalus 

 aceratus, Pseudochaenichthys georgianus, Dissostichus 

 eleginoides, Notothenia rossii) one or more steps re- 

 moved from secondary production. The availability of 

 krill to these fish and their prey, and the fish's ability 

 to emphasize krill in their diets, will strongly influence 

 the yield of fish from the South Georgian community. 



Conclusions 



The most abundant species of the South Georgia 

 demersal fish community were classified into three 

 groups based on their summer diets. Species that 

 depended heavily on krill comprised the largest group, 

 including Champsocephalus gunnari and Notothenia 

 rossii. The second group was comprised of piscivores. 

 Three of the four members of this group (Dissostichus 

 eleginoides, Chaenocephalus aceratus, and Pseudochae- 

 nichthys georgianus) are commercially valuable. The 

 food of their prey often consists of krill. The third group 

 contained a loose association of species which fed main- 

 ly on benthic organisms. Notothenia gibberifrons and 

 Notothenia squamifrons are the important commercial 

 species in this group. 



Krill is the most important prey species to the fish 

 in the South Georgia system during the austrial sum- 

 mer. It was consumed either directly or indirectly by 

 all of the fish in this study. However, it is unknown 

 whether the krill resource is limiting to these fish at 

 that time or in any other season. More information on 

 the seasonal dynamics and behavior of these fish and 

 their prey is necessary to conclusively determine the 

 role of competition for food in this system. 



The potential for change in this community due to 

 fishing is evident. Selective reduction in populations 

 within the fish community may have widespread reper- 

 cussions. The relatively simple, but highly intercon- 

 nected, food web in the South Georgia system may have 

 a lower potential for fish yield than previously thought 

 (Hemple 1985). 



Acknowledgments 



This work was supported by a grant from the Antarc- 

 tic Marine Living Resources Program of the National 

 Marine Fisheries Service. I am grateful to S. Saila for 

 technical and editorial assistance and S. Pratt for aid 



