Stock assessment of neritic tuna species in Indian Ocean: kawakawa longtail, and narrow-barred Spanish Mackerel tuna using catch-based stock reduction methods

We conduct stock assessments for three Indian Ocean neritic tuna species, Kawakawa and Longtail. We used a newly developed posterior-focused catch-based assessment method, and compared them to the traditional SRA approach developed by Kimura et. al. The method is based on a classical biomass dynamics model, requires only catch history but not fishing effort or CPUE. Known population growth rate will improve the assessment result. In this paper, we assume that both species in the whole Indian Ocean belong to a single stock and the population size in 1950 is the virgin biomass equal to their carrying capacities. We use recently updated catch data in the analysis. The preliminary results show that for Kawakawa the median virgin biomass is about 363-469 thousand tonnes depending on the upper depletion level assumed in 2012. The combination of such carrying capacity and growth rate can support a maximum sustainable yield (MSY) of 127-146 thousand tonnes. This means that catch levels in recent year may have exceeded MSY, or is fully exploited. The situations are similar for Longtail. The median virgin biomass was about 443 to 595 thousand tonnes, and the intrinsic population growth rate is about 0.8–1.11, somewhat less productive than Kawakawa. The entire stock can support a MSY of nearly 106–141 thousand tonnes. Catch levels in recent year may have been too high, and likely overfishing is occurring on the stock. For narrow-barred Spanish Mackerel, the median virsgin biomass is between 380-543 thousand tonnes, and MSY levels are between 112-133 thousand tonnes. Catch levels in recent years indicate that these stocks are also fully exploited. These results are compared across two approaches, and similar results are concluded, i.e. Yield levels are similar across approaches, though tighter precision is observed with the optimal yield and current biomass levels using a newly developed Posterior focussed Catch Reduction Approach (PFCRA) versus the traditional Stock Reduction Approaches (SRA). With respect to fishing based reference points, the results are similar across both approaches. Stock status advice is similar across bothe approaches.