Deepwater flathead (Neoplatycephalus conatus) is the dominant component of catches caught in the Great Australian Bight Trawl fishery (GABTF), increasing to and accounting for approximately 40-50% of landings. Most of the deepwater flathead catch is taken at depths of 100-200 m, and the majority of fish are between 45 and 60 cm. Very few individuals less than 35 cm TL are caught.
The Central Ageing Facility has been contracted to provide estimates of age for deepwater flathead for the GABTF. The standards/protocols for age estimation have been developed over time by a combination of pattern recognition of the otolith increment structure and the limited biological data available. No validation has been undertaken on the periodicity of increment formation and if they are formed annually. Providing repeatable and accurate age for this species is complicated by the poor increment clarity on the otoliths, uncertainty in the timing of the increment formation and potential temporal differences in otolith growth.
Further complicating the process of age estimation is the protracted spawning period for deepwater flathead. In the Great Australian Bight the spawning period last from October to March. Spawning activity in the western-central Bight peaks in late summer. The protracted and temporal variation in spawning within this fishery is likely to result in variable hatching times and consequently exhibit different patterns of growth on the otolith of juvenile specimens. A hatching event occurring 10 months prior to the formation of the first increment will be evident from the larger distance between the otolith primordium and the first increment than that of an individual hatched 4-5 months later. This difference can bias age estimates, even for an experienced reader. The correct identification of the first increment is particularly important when the catch is made up of younger samples as a bias of 1 year in the age estimation of a 4-year-old has disproportionately greater effects on the population parameter estimates than that of a 30-year-old.
Annual total allowable catches are heavily reliant on age estimates, as other biological data are limited. Unvalidated age estimates have produced a maximum age of 33 years for females and 28 years for males. The majority of the catch is comprised of 4 to 8 year-old-fish and age-at-recruitment is at approximately 3-5 years of age. Therefore accurate and precise (repeatable) age estimates of these younger age classes are essential for the ecologically sustainable development of this fishery. However, it is generally the youngest and the oldest fish which are the most difficult to age accurately and which are most influential in the estimates of growth, mortality and longevity.
Difficulties exist in the stock assessment of deepwater flathead due to the lack of contrast within the CPUE data. A better understanding of CPUE will greatly enhance the confidence in stock status and MSY. However in the absence of strong signal in the CPUE, stock assessment models rely on other information to provide some insight into stock status. The most important information available at the moment is catch at age data to provide some indicator of strength of recruitment into the fishable biomass and the loss of older proportion from the population. Accurate (ie validated) aging is therefore essential.
A simple equilibrium model was run by Brent Wise (BRS) to demonstrate the effect of uncertainty in aging has on MSY using the deepwater flathead biological parameters. This sensitivity analysis was based on the current ageing versus ageing that assumes that fish are in reality 2 years older. The analysis showed that there is a 30% decrease in available biomass if the current ageing is shown to be biased. A bias of 2 or moreyears in the age estimation could be a reality for this species due to the uncertainty in timing and the position of the first zone formation and timing of subsequent zone formations.
In the absence of accurate age data, management may be making decisions based on an estimated fishable biomass 30% higher (assuming 2 year bias) than the true fishable biomass. The effect of this would be unsustainable level of TAC from the key species within the GAB.
The objectives of this project will allow the timing of increment formation to be determined for age classes 4-8 years old and the validation/timing of the first growth increment. Additionally, through the validation of the first increment, information on juvenile growth can be examined. Information on juvenile growth is important because growth rates have been implicated as predictors of annual abundance of young fish. Growth differences may be different between areas and year classes depending on the time of hatching and the timing of first increment formation. A detailed analysis of variation in growth rates within the 0+ cohort will allow for the further refinement of age estimates. This will improve the assessments, which will assist managers in meeting the objectives for the sustainable use of the resource