Preliminary assessment of Champsocephalus gunnari on the Heard Island Plateau (Division 58.5.2) based on a survey in May 2002

A survey of mackerel icefish, Champsocephalus gunnari, was undertaken in Division 58.5.2 in the vicinity of Heard Island in May 2002 to provide the information for an assessment of yield in the 2002/2003 CCAMLR season. This paper provides a preliminary assessment of yield using the assessment methods of CCAMLR and discusses the implications of these results in relation to previous CCAMLR assessments. The estimate of biomass of mackerel icefish for the population at Heard Island was not significantly different from previous years but the one-sided lower 95% confidence bound was greater than in 2001 probably because the population was more uniformly spread in the southeast with relatively high densities in that stratum. The composition of the population was observed to comprise a single large cohort of mostly three and four year old fish in May 2002. Given the size of the fish, the expectation from current estimates of length at age parameters would be for most of the fish to be three year olds. However, given the abundance of fish in the population and the age structure of fish last year, it would be more reasonable to conclude that most of the fish are four years old, as this age class comprised the larger fraction (93%) of the population last year. Two assessments were undertaken as a result of the potentially different interpretations as to the composition of the larger cohort. If the cohort comprised mostly three year old fish then the yield for 2002/03 is estimated to be 1990 tonnes. If it comprises mostly four year olds then the yield would be 1815 tonnes. It is also proposed that the minimum size of capture be increased for 2002/03 to protect the younger unassessed cohort of fish from being exploited as they potentially enter the fishery towards the end of the season.