Although bigeye is such an important catch species in the eastern and western AFZ, little is known about its biology. Determining the age composition of bigeye tuna is ET&BFMAC’s priority three in 1999, as it is required in support of stock assessments. The limited tagging data from the Coral Sea (Hampton and Gunn 1998) suggest that bigeye tuna may live significantly longer than previously thought. Validated direct age estimates of bigeye are required to estimate natural mortality rates and productivity; investigate sexual dimorphism in growth rates; develop an age-length key for future age-based assessment models and to determine the mean age at first maturity.
Due to the present uncertainties over the catch-at-age, it has been difficult to estimate natural mortality for bigeye tuna. Estimates of natural mortality rates are vital for stock assessment of the species in the Pacific Ocean. The direct ageing of adult bigeye from a representative sample of the population will help address this issue.
Age-based stock assessments require the breakdown of fish lengths into age classes using a validated length-at-age model. Present attempts to develop VPA models for bigeye in the Pacific are hindered by the lack of catch-at-age data. Mean age at first maturity is also required to assess the size of the spawning stock.
Assessing bigeye tuna stocks in the Indian Ocean is also IOTC's higest priority. Parameters such as growth and age at first maturity are poorly known, and the information provided by this proposal will be particularly valuable in any Indian Ocean-based assessment of bigeye stocks.
We propose to determine the age structure of bigeye tuna caught in the eastern and western AFZ. Development of an age-length key for bigeye tuna from the western AFZ will greatly improve our capacity to assess bigeye tuna stocks in that fishery, and the very large bigeye fishery off NW Australia fished by Indonesians. It will enable us to evaluate the regional variability in growth of bigeye tuna and will complement the ageing programs being carried out in the west, central and east Pacific Ocean.
