Stock assessment of three billfish species in Indian Ocean, blue, black and striped marlin using stock reduction methods

Stock assessment of three billfish species in Indian Ocean, blue, black and striped marlin using stock reduction methods

Year of publication 2013

Product type Meeting

Edition 1

Synopsis (short abstract) We conduct stock assessments for three Indian Ocean billfish; blue, black and striped marlin. We used a catch-based stock reduction alysis method. The method is based on a classical biomass dymics 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 car rying capacities. We use recently updated catch data in the alysis. The prelimiry 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 combition of such carrying capacity and growth rate can support a maximum sustaible 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 sto ck 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. Filly for striped marlins, the outcome is not optimistic. The geometric mean virgin biomass was about 37.5 to 85.4 tho usand 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.

Statutory body / Meeting title Indian Ocean Tuna Commission (IOTC)

Meeting symbol IOTC-2013-WPB11-28 Rev_1

Meeting Session Number 11

Language English