Stock structure and spatial dynamics of the warehous: a pilot study
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.
Final report
South East Fishery Industry Development Subprogram: facilitation, administration and promotion
Spawning and reproductive biology of blue grenadier in south-eastern Australia and the winter spawning aggregation off western Tasmania
By mass, the catch of blue grenadier is currently the largest of all of the quota species in the South East Fishery. Blue grenadier is also currently the second most valuable species in the South East Fishery (8.8M during 1998). Industry reports that it anticipates the 10,000t TAC will be taken during 1999 due to the continuing development of the spawning fishery following the introduction of 'factory trawlers' in recent years.
Current assessment of the fishery is undertaken by the The Blue Grenadier Assessment Group (BGAG) which was formed in 1997. BGAG comprises government and industry scientists, industry representatives and AFMA managers. The 1999 assessment of blue grenadier uses an age-structured ‘synthesis’ model (Punt et al in press). The assessment considers fishing during the spawning season off Western Tasmania and that at other times/places separately. The model uses catch (including discards), standardised catch rates and catch-at-age data (retained and discards) as well as estimates of absolute abundance based on the egg production method. The results of the current assessment are generally optimistic for the next five to ten years but are extremely sensitive to biomass estimates from the egg production method.
One key area of uncertainty concerns the proportion of mature fish that spawn each year. It is known from data in New Zealand that not all fish spawn annually. Assessments in New Zealand are based on the assumption that 77% (the average of the range of 68-85% reported by Livingston et al., 1997) of the potential spawners spawn each year. The same assumption is used by BGAG. However, there are no data on the proportion of non-spawners in Australia although there are anecdotal reports of adult fish (non-spawners?) outside spawning areas during the main spawning season. It is important, therefore, that we establish the actual proportion of non-spawning fish as this can have a significant impact on the mature biomass.
Obtaining better information on the spawning behaviour and reproductive biology of blue grenadier to reduce uncertainty about biomass estimates has been accorded a high priority for several years and was stressed again by BGAG in 1999. Information needed includes fecundity estimates, maturity ogives and the proportion of non-spawners.