CCAMLR

Stocks of the mackerel icefish Champsocephalus gunnari in the Southern Scotia arc region of the Southern Ocean (CCAMLR Subarea 48.1 & 48.2) have several different biological characteristics than stocks in the Northern Scotia Arc. The history of commercial C. gunnari exploitation in the Southern Scotia Arc was short lived and intense, and led to the collapse of the stocks. Stocks in this region are presently very low relative to historical levels, particularly around the South Orkney Islands. Aspects of the biology, patterns of distribution, maturity, diet, and abundance based on recent scientific surveys conducted by the U.S. AMLR Program are presented. During the month of March, the species appears to use the shelf areas around Elephant Island and the lower South Shetland Islands differently, with juveniles favoring the western shelf of Elephant Islands and larger, more mature fish favoring areas on western and northwestern parts of the Elephant Island shelf and the lower South Shetland Islands near Livingston Island. The dietary composition of C. gunnari appears to be more diverse in the South Orkney Islands, while their diet in the South Shetland Islands is almost entirely krill (Euphausia superba). The effect of E. superba density on the spatial distributions of C. gunnari abundance, size, and feeding patterns in the South Shetland Islands for the 2001 survey is examined. Preliminary results suggest a complex relationship between spatial distribution, size composition, and feeding intensity of C. gunnari and prey density patterns.

C.gunnari form no aggregations during wintering period and inhabit places that are inaccessible for fishing. Conduction of census surveys during this period is pointless. In spring, during feeding period, they form aggregations near bottom thus being available for harvesting just at the beginning of this period (September). Data on stock assessment collected during accounting survey are good enough, however, this season is far from suitable for accounting surveys because of severe ice conditions. Fish feed intensively during feeding period in summer, performing vertical and horizontal migrations. It is not recommended to make stock assessment of fish yielded with bottom fishing gear, as in this case, fish staying in pelagic zone is missed. In autumn, pre-spawning fish migrate to spawning areas in near-bottom water layers where they are easy of access for fishing. C.gunnari, which do not spawn in the current year, continue feeding, but their feeding intensity considerably decreases and they gradually move to wintering areas. They can also be fished during such migrations.

The applicability of various methods to assess icefish natural mortality is analysed. On the basis of the conclusions made the parameters of Bertalanffy’s growth equation and natural mortality rate are estimated.

The paper contains comparative analysis of mackerel icefish and krill vertical and horizontal distribution based on acoustic measurements and results of bottom and pelagic trawling sampling. Significance of studying the krill and mackerel icefish interaction for understanding the regularity of mackerel icefish distribution is shown.

In 2000, CCAMLR asked the Working Group on Fish Stock Assessment (WG-FSA), whether trawl surveys are the most reliable and appropriate method to survey stocks of mackerel icefish (Champsocephalus gunnari). Recent acoustic data from the South Atlantic suggested that mackerel icefish may not always be distributed closely on the sea floor during the day as conventional wisdom would have it, but that a significant proportion of the stock occurred in midwater. If this is the case, then bottom trawl surveys may underestimate the biomass of mackerel icefish. This paper addresses this question by comparing trawl and acoustic data taken contemporaneously during a survey for mackerel icefish in the Heard Island region (CCAMLR division 58.5.2) in May 2000. The relationship between mackerel icefish catches and the acoustic signal are discussed. The vertical distribution of mackerel icefish in relation to the diel light signal and the potential for biased estimates of abundance from bottom trawl surveys are also investigated.