Trophic Ecology and Structure of Coastal Antarctic Fish Communities

Abstract

The trophic ecology and structure of 5 benthic Antarctic fish communities were studied. Two locations at South Georgia Island, one at the South Sandwich Islands, and two at the South Orkney Islands were sampled using a bottom trawl. Specles diversity was low with 13 or fewer species in each community and a pronounced dominance of 3 or fewer species per community. H' diversity values were < 1.3 in all cases and < 0.8 in 3; e evenness values were < 0.6 in all cases and < 0.3 in 3. The South Sandwich Island community had the lowest species diversity (H' = 0.2), likely due to a very limited prey spectrum, geographic isolation, geologically recent formation, and volcanic origin which formed only a narrow shelf zone where coastal fish communities could develop. The dominant species in all communities were one or more of the nototheniid fishes Notothenia larseni, N. gibberifrons, N. nudifrons, and N. angustifrons. Food resource partitioning was evident within communities at a general taxonomic category level of prey identification. Dominant species within communities generally are morphologically and behaviorally adapted to feed at different levels along a vertical prey-distribution axis. N. larseni feeds above the bottom. N. gibberifrons and N. nudifrons feed on benthos, N. gibberifrons preys more on infauna, and N. nudifrons more on epifauna. N. angustifrons is also a benthos feeder but takes somewhat different prey than the other two species. Low fish species diversity precludes the necessity for fine division of food resources within vertical habitat zones. Feeding niche differences, likely evolved by nototheniids as an adaptation to avoid competition for prey, may contribute to the success and dominance of nototheniid species in these communities. Nearly all cases of high prey overlap were cases involving non-dominant species, and in which krill (Euphausia superba) was important for both species. Krill is a very abundant prey resource in the Antarctic, particularly during austral summer and fall, minimizing competition for this prey among channichthyids and nototheniids. The South Sandwich Island community had a prey spectrum consisting almost entirely of only 3 gammaridean amphipod species. This lowdiversity prey spectrum is probably due to the unstable volcanic rock and cinder bottom type, which does not support a well developed benthic invertebrate fauna. Competition for amphipods likely was great and low fish diversity reflects the low prey resource diversity. Daily rations, calculated in terms of dry prey biomass, for N. angustifrons, N. gibbenfrons, and N. larseni were between 0.24 and 2.03 % dry body weight, depending on mean prey weight in stomachs and gastric evacuation rate for each species. Krill was an important food resource for N. larseni, Champsocephalus gunner], Trernatomus newnesi, and Chaenocephalus aceratus. Based on small sample sizes, T hansoni, Champsocephalus esox, and Pseudochaenichthys georgianus also appear to prey heavily on E. superba. At one South Georgia Island community essentially all fishes preyed substantially on krill, suggesting that even fishes with benthic feeding modes take advantage of abundant krill resources when available close to the bottom.