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.

The performance of a size-structured stock assessment method in the face of spatial heterogeneity in growth

“…The result that it is not possible to estimate the initial biomass even with deterministic data suggests that basing management advice on the ratio of the current biomass (or egg production) to that in a virgin (or pre-exploitation) state, although common (e.g. Bentley et al, 2001;Hobday and Punt, 2001), is questionable. Punt et al (2001) reached a similar conclusion for fish species in Australia's South East Fishery.…”

“…The assessments of rock lobster populations off southern Australia have been based on a range of size-class widths, e.g. 2 mm and 5 and 10 mm (Hobday and Punt, 2001), with the width generally being based on computational convenience. The results in Fig.…”

“…This assumption has been made in some recent assessments of Australian rock lobster populations Hobday and Punt, 2001) and is supported by data collected by Treble et al (1998). Inter-annual variability in selectivity is modeled by assuming that it impacts the size at 50% selectivity:…”

“…Management advice for these fisheries is based on stock assessments, which involve fitting size-structured population dynamic models to catch, catch-rate and length-frequency data (e.g. Hobday and Punt, 2001;Breen et al, 2001;Bentley et al, 2001). All of these assessments are spatially structured to some extent.…”

“…The study is based on a simple size-structured assessment framework consistent with the assessments for most rock lobster populations (at least in the southern Hemisphere) (e.g. Johnston, 1998;Hobday and Punt, 2001).…”