CCAMLR

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.

Commercial trawl fisheries in the modern era started within the Heard Island EEZ in 1997. This paper summarises the fishing activities to date and collates the information on the biology of the species gathered from the fishery and from a series of fishery-independent surveys conducted between 1990 and 2000. Icefish abundance varies widely between years and the fishery is dependent on periodic abundant year classes that can be followed through the population before disappearing in their fifth year. The occurrence of abundant cohorts is not synchronised between the Heard Island Plateau, Shell Bank or the Kerguelen Plateau, nor are the respective spawning seasons. This suggests that the populations in the respective areas are mostly independent of one another.