Marine consumers: Fish and squid

Of about 20,000 modern fish species, only about 200 are found south of the Antarctic Convergence. The ichthyofauna is dominated by small species up to 40 cm long. The majority are bottom dwelling forms which belong mostly to the suborder Notothenioidei. Antarctic pelagic fishes are descendants from several faunal groups of different origins: bathypelagic and mesopelagic species, and species originating from demersal families which are secondarily adapted to temporary or permanent midwater life. More than 30 species of squid are so far known from the Southern Ocean. The majority are obviously meso-and bathy-pelagic dwellers. Several adaptations, including stenothermy, freezing resistance, aglomerular kidneys, and neutral buoyancy, enable Antarctic coastal fish to live successfully in the harsh environment. Growth rates of coastal fish are comparable to those found in Newfoundland cod. Most coastal fish do not spawn until they are 5 year old. Three reproductive strategies occur in coastal fish: autumn/early winter spawners with few, but yolky, eggs of 3–6 mm diameter; late winter/spring spawners with many, but smaller, eggs of 1.5–2 mm diameter; and species which guard their nests. Based on their food spectra, fish species can roughly be divided into three groups: fish and krill feeders, plankton feeders, and benthos feeders. Krill consumption by coastal fish around South Georgia and in West Antarctic waters was probably in the order of 5–6 × 106 tonnes before the onset of exploitation. Overall euphausiid consumption may have been about 20 × 106 tonnes. Fish, particularly mesopelagic fish, appear to be more important krill predators than squid. The role of squid as krill predators may have been overestimated in the past. Squid seem to be more important as a food resource for top level predators, particularly sperm whales and birds, than fish appear to be. All estimates of food consumption—krill as well as fish—are still subject to many uncertainties, and so it can be expected that increasing knowledge in the future may alter the given figures quite substantially. Exploitation of Antarctic finfish started at the end of the 1960s. The main fishing grounds are South Georgia and Kerguelen Islands. So far about 2.3 × 106 tonnes have been harvested from the Southern Ocean and the stocks particularly around South Georgia are heavily depleted. Conservation measures adopted by the Commission for the Conservation of Antarctic Marine Living Resources in 1984 and 1985 are probably not sufficient to support the recovery of the stocks

Biology and ecology of mackerel icefish, Champsocephalus gunnari: An Antarctic fish lacking hemoglobin

The mackerel icefish (Champsocephalus gunnari) is a prominent member of the coastal fish fauna of the Seasonal Pack-Ice Zone and the islands north of it. Separated into a number of stocks, its distribution ranges from the Scotia Arc region, namely South Georgia, in the Atlantic Ocean sector to the Kerguelen-Heard Plateau in the Indian Ocean sector. Mackerel icefish have been heavily exploited since the beginning of the 1970s with reported annual catches exceeding 50–100,000 tonnes in some years. C. gunnari has many characteristics typical of Antarctic fish species with respect to life history characteristics, such as egg size, fecundity and growth. These fall well within the range of other sympatric red-blooded notothenioids; eggs are large and yolky. Egg diameter, egg production per gram body weight and growth performance at South Georgia and the Kerguelen Islands was comparable with similar-sized nototheniids and channichthyids. Stocks of C. gunnari have a number of biological characteristics in common, such as feeding mode and food, early life history and growth in the first years of life. However, stocks in sub-Antarctic waters differ in reproductive characteristics, such as length and age at first spawning, natural mortality and life expectancy from those on more southerly grounds. They may grow to more than 60 cm and may become 13–15 yrs old. However, in the vicinity of South Georgia and the Kerguelen Islands, fish apparently do not exhaust this growth potential fully. Their ability to reproduce at an early age coupled with a comparatively high fecundity and growth performance may mean stocks of C. gunnari in sub-antarctic waters have far more resilience and a greater capacity to rebuild than species with the slower reproductive rates common in other notothenioids