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This report presents results of a scoping study for improving biological knowledge of albacore tuna in the Indian Ocean Tuna Commission (IOTC) Area of Competence. The key objectives of this study were to: 1) undertake sensitivity analyses to evaluate the sensitivity of the stock assessment to the range of plausible values that might be estimated for key biological parameters, and potential structural changes to the assessment that might be informed by biological sampling; 2) undertake power analyses on key biological parameters to determine sampling needs; and 3) using the outcomes of 1 and 2 above, and an understanding of the fisheries, outline the sampling considerations and develop a sampling design required to provide estimates of length- and age-based population parameters to improve stock assessments of albacore tuna in the Indian Ocean (IO).
The sensitivity of the stock assessment results to alternative biological parameter values was investigated within the framework of the preliminary 2019 IO stock assessment model. A range of alternative model options were configured to evaluate the influence of key biological parameters on the stock assessment and status of albacore tuna in the IO, including alternative values for the length-weight relationship, maturity-at-length ogive, variation in length-at-age, growth functions (emphasising growth differences in the younger age classes), and female natural mortality rates. The sensitivity of the model to spatial variation in growth rates was also considered by partitioning the model into two regions (east and west) that was also configured for fish in the eastern region having a higher growth rate than fish in the western region.
Estimates of stock status were most strongly influenced by changes related to the assumptions of growth. The alternate growth function and variation in length-at-age sensitivities both yielded considerably more pessimistic estimates of current stock status. Changes in the maturity ogive and female natural mortality influenced the magnitude of the reference biomass levels, although resulting stock status ratios were not appreciably different from the base model. Partitioning the model into two geographic regions (east and west of 75°E) yielded a considerably more optimistic estimate of current stock status and higher overall yields compared to the base model.
Simulation modelling was carried out to assess sample size requirements for growth and reproductive analyses by bootstrap resampling of the South Pacific Ocean albacore tuna dataset. The ability of the models to characterize the relationship between growth parameters and longitude improved with increasing sample size, but evidence for such a relationship was reliably identified even in the lower range of sample sizes. Similarly, for reproductive parameters, higher sample sizes improved the models’ ability to identify relationships and to estimate parameters. Analyses suggest that sample sizes similar to those used to investigate biology of albacore tuna in the South Pacific Ocean (approximately 1000 individuals of each sex) will be sufficient for detecting spatial and temporal trends in biological parameters, provided sampling covers a broad geographical and temporal spread.
Owing to the biology of albacore tuna, several factors need to be considered when attempting to obtain unbiased estimates of key biological parameters, including the potential for interactive effects regarding annual, seasonal, latitudinal and longitudinal variation in growth, maturity, fecundity and the timing of spawning, as well as the potential for sex-specific differences and the need to ensure adequate numbers of small fish are sampled. In light of these considerations, to undertake a complete study on the biology of albacore tuna in the IO, sampling should ideally cover the full geographic range of the stock in the IO, with good representation across months/seasons, over at least a full year, and ideally multiple years.
The research plan provided here outlines important biological and experimental design considerations, preferred methods and sampling strategies, anticipated timelines and project budget. Sampling of Japanese and Taiwanese longline fleets via observers will be crucial for ensuring a regular and broad geographical and temporal spread of samples, and for ensuring that catch information is available. Two variations regarding time and budget are provided: a five-year study including three continuous years of sampling at a projected cost of approximately $1,305,000 USD, or a four-year study involving two continuous years of sampling at a projected cost of approximately $902,000 USD.
