Blue and spotted warehou are important species in the South East Fishery. The 2000 actual TACs for blue and spotted warehou were 1907 and 4829 tonnes, respectively. The species, however, exhibit conflicting trends. The blue warehou TAC has been reduced in recent years while that for spotted has been increased. The total blue warehou catch in 1999 was the lowest since 1986 and recent assessments indicate that the "stock" is in decline. The 2001 TAC for blue warehou has been halved to 750 t. For spotted warehou, it was concluded, at a recent stock assessment workshop, that while recruitment is variable trends in standardised CPUE indicate a relatively stable resource.
There have been no studies on the stock structure of these species in Australian waters and for management purposes both species are considered one stock. In this application, the term stock is used to refer to a management unit, that is fishing one unit does not effect another. This may or may not have a genetic significance.
Both species exhibit complex spatial variability, particularly east and west of Bass Strait. The importance of this to assessment is increasingly been realised. The most recent assessment of blue warehou is considerably more uncertain because model fits to the data are very poor, assuming a single population across the fishery, and consequently areas east and west of Bass Strait were modelled separately. However, fits of models themselves are not an adequate base for determining stock structure and there are a number of hypotheses (eg. separate east and west stocks; one stock but the recruitment rates to the east and west differ among years; migrations between east and west) that appear to be consistent with the existing information. Similarly future assessments of spotted warehou will be limited without this information. Clearly, the lack of information on stock structure and spatial dynamics will adversely effect the efficacy and acceptance of stock assessments of both species.
This proposal aims to address this issue by assessing a suite of tools to determine which can provide the most information on stock structure; genetics, morphometrics, otolith morphology and otolith microchemistry. The latter may also provide valuable insights in migratory dynamics. All these techniques can be expensive and sometimes provide ambiguous results. Consequently, the proposal is to undertake a pilot study assessing these approaches to ascertain the most useful method prior to any full study being undertaken. However, although it is a pilot study, it is hoped that the preliminary results will assist BWAG weight the hypotheses used in the modelling and hence reduce uncertainty in the assessment.
