Spawning and reproductive characteristics of Bight redfish and deepwater flathead in the Great Australian Bight trawl fishery
Bight redfish (Centroberyx gerrardi) and deepwater flathead (Neoplatycephalus conatus) are the two principle commercial species captured along the continental shelf of the Commonwealth managed Great Australian Bight Trawl Fishery worth an estimated $7 million per annum.
Current assessments, based on available data, indicate that Bight redfish fishing mortality is rising and likely to reduce the biomass to below the trigger biological reference point. The outcomes of stock assessments are highly sensitive to the shape of the maturity ogive and fecundity-length/age relationships. Given the current high uncertainty about the validity of existing Bight redfish and deepwater maturity and fecundity relationships, GABFAG has recommended that these be determined with high priority. More scientifically defensible maturity ogives and fecundity-length and fecundity-age relationships will markedly reduce uncertainty in the stock assessments.
Final report
This project addressed some of the important information gaps identified during assessment workshops for Bight redfish and deepwater flathead. Improved stock assessments, which incorporate the latest biological parameters (size-at-maturity, fecundity and sex ratio), has lead to the setting of accurate Bight redfish and deepwater flathead TACs. Gear regulations have been proposed that include minimum mesh sizes and T90 extensions, which along with reducing discard species, allow small deepwater flathead to escape capture, and thereby increasing reproductive capacity of the flathead population.
This information has been provided to Great Australian Bight Fisheries Assessment Group (GABFAG) as part of formal and informal reporting arrangements. It has also been distributed to the public via media releases.
Keywords: Bight redfish, Centroberyx gerrardi, deepwater flathead, Neoplatycephalus conatus, eggs per recruit, fecundity, reproduction, size- and age-at-maturity, spawning
Age validation of deepwater flathead from the Great Australian Bight Trawl Fishery
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