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We conduct stock assessments for three Indian Ocean billfish; blue, black and striped marlin. We used a catch-based stock reduction analysis 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 all three 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 blue marlin the geometric mean virgin biomass is about 86-432 thousand tonnes using the assumption that depletion in 2011 is between 30% and 70%. The combination of such carrying capacity and growth rate can support a maximum sustainable yield (MSY) of 6-15.1 thousand tonnes. This means that catch levels in recent years may have exceeded MSY. Overfishing maybe occurring on the stock though the stock does not appear to be overfished.
The situations are similar for Black marlin. The geometric mean virgin biomass was about 30.8 to 115 thousand tonnes, and the intrinsic population growth rate is about 0.58 (0.25-1.3 95% CI. The entire stock can support a MSY of nearly 8.6 thousand tonnes. The stock appears to be healthy and approaching optimal fishing levels in recent years.
Finally for striped marlins, the outcome is not optimistic. The geometric mean virgin biomass was about 37.5 to 85.4 thousand tonnes, and the intrinsic population growth rate is about 0.29 (0.18-0.49 95% CI. The entire stock can support a MSY of nearly 4.2 thousand tonnes. Catch levels in recent year may have been too high, and likely overfishing is occurring on the stock.
