Improving biological knowledge of albacore tuna, Thunnus alalunga, in the Indian Ocean: a scoping study

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 indicate the sensitivity of the stock assessment to the range of plausible values that might be estimated for each parameter, 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 parameters was investigated within the framework of the preliminary 2019 Indian Ocean stock assessment model. Estimates of stock status were most strongly influenced by changes related to the assumptions of growth. The 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 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 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 in the South Pacific study (approximately 1000 individuals of each sex) will be sufficient for detecting spatial and temporal trends, provided sampling covers a broad geographical and temporal spread.

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.

The IOTC Scientific Committee is invited to:

• NOTE that the scoping study will be finalised in March 2020;
• NOTE the influence of biological parameters on albacore tuna stock assessments;
• RECOMMEND that biological sampling be carried out for albacore tuna in the Indian Ocean, noting the proposed sampling strategy;
• RECOMMEND that the Commission considers and approves the biological sampling study, and REQUEST that fleets with significant fishing effort in the southern Indian Ocean provide support to ensure the program’s success.