Stock assessment of two neritic tuna species in Indian Ocean, kawakawa and longtail tuna using catch-based stock reduction methods

We conduct stock assessments for two Indian Ocean neritic tuna species, Kawakawa and Longtail. We used a newly developed posterior-focused catch-based assessment method. 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 358-408 thousand tonnes depending on the upper depletion level assumed in 2011. The combination of such carrying capacity and growth rate can support a maximum sustainable yield (MSY) of 128-151 thousand tonnes. This means that catch levels in recent year may have exceeded MSY. The situations are similar for Longtail. The median virgin biomass was about 380 to 440 thousand tonnes, and the intrinsic population growth rate is about 1.14–1.26, somewhat less productive than Kawakawa. The entire stock can support a MSY of nearly 110–140 thousand tonnes. Catch levels in recent year may have been too high, and likely overfishing is occurring on the stock.