Size-structured population modelling and risk assessment of the Victorian southern rock lobster, Jasus edwardsii, fishery

Abstract

Current annual landings of southern rock lobster, Jasus edwardsii, from southeastern Australia are around 5000 tonnes valued at A$140 million. The Victorian component of this catch during the 1998-99 fishing season was 550 t, valued at A$18 million. During the past 20 years catch rates have declined from 0.8 kg per pot lift to 0.6 and 0.3 kg per pot lift in the western and eastern management zones respectively. The fishery has been managed with input controls during this period, but at the time of writing, the direction of future management is not clear. A size-structured model was developed to assess risk associated with both effort (input) and catch (output) controlled harvest strategies in each zone. The stock-assessment model was fitted to historical catch data (in weight and by number) from 1951, catch rates, and the length-frequency by sex. The uncertainty associated with the estimates of exploitable biomass and egg production was assessed according to Bayesian methods. The output of the assessment formed the basis for projections intended to determine the risk associated with different future levels of effort and catch. Reference points based on estimated biomass and egg production relative to the start of the data series in 1951 were considered.