Stock assessment of the blue grenadier Macruronus novaezelandiae resource off south-eastern Australia

Abstract

The fishery can be divided into two subfisheries (‘spawning’ and ‘non-spawning’). Commercial catch rates for the ‘non-spawning’ subfishery declined from the late 1980s to 1997, whereas those for the ‘spawning’ subfishery exhibit no obvious temporal trend. An ‘Integrated Analysis’ assessment, of the feasibility of reconciling these differing trends, uses catch (landed and discarded), catch rate, length-at-age, and catch-at-age data and estimates of absolute abundance based on the egg-production method. It emphasizes uncertainty due to model assumptions and the data included in the assessment. Use of the discard data allows more precise estimation of the magnitude of recent recruitments. Spawning biomass is estimated to have declined from a peak in 1989–91 to 1999 although fishing mortality has consistently been <6%for each subfishery. One main reason for the reduction in population size is the weakness of year-classes spawned from 1988 to 1993. Differences in catch rates between the two subfisheries can therefore be explained by interactions between the components of the population harvested by the two ‘subfisheries’, and the trends in year-class strength. A risk analysis is used to evaluate the consequences of different future levels of harvest for different assessment assumptions. Overall, the spawning biomass is predicted to increase over the next five to ten years as a result of the strong 1994 and 1995 year-classes, although the extent of this increase remains uncertain.