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.
