Resolution of stock structure of toothfish in Australian and other waters is required. Tagging experiments, while important, cannot by themselves give a complete picture of stock structure. If genetic differences between areas are detected, then the amount of gene flow between areas must be minimal and more than one stock can be assumed. However, if differences are not detected, then the hypothesis of a single genetic stock cannot be rejected. This hypothesis may be rejected if there is no evidence from tagging experiments of fish movement between areas. Therefore, resolution of stock structure is better managed by a combination of tagging and genetic approaches than by either approach alone. Tagging experiments are under way; we propose to carry out the required genetic analyses, and then consider both sets of data jointly to define likely stock structures.

Genetic evidence already collected (based on small sample sizes from two areas of Macquarie), suggests that the amount of movement between fishing locations might be very limited. If this preliminary conclusion is substantiated by larger sample sizes, then careful management of the fishery will be needed, as depletion of one area is unlikely to be quickly replaced by immigration from another area.

The work proposed here will resolve this issue for Macquarie island, and will also determine whether there are separable substocks of toothfish around HIMI, and whether the HIMI stock(s) are separable from the Macquarie island stock(s). Importantly, if William's Ridge fish are identified as part of a straddling stock extending into Australia's EEZ (Heard Island), then they can be managed as though wholly part of Australia's EEZ (recent UNIA agreement).

These issues have to be resolved for effective and sustainable management of toothfish fisheries by Australian management (AFMA). The information gathered will also be relevant to the management authorities of other nations controlling toothfish fisheries.

Note: Recent meetings of CCAMLR (1998, 1999) have recognised the urgent requirement of further work on stock structure in D. eleginoides , particularly as initial genetic studies in other laboratories using isolectric focussing and allozyme analysis have reported inconsistent results (see Box 1 below).

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.