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Assessment of Black marlin and Blue marlin in the Australian fishing zone - Report of the Black and Blue marlin Working Group







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    Stock assessment of three billfish species in Indian Ocean, blue, black and striped marlin using stock reduction methods 2013
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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 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.
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    Stock assessment of blue marlin (Makaira mazara) based on a Bayesian production model 2013
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    Blue marlin is one the bycatch species caught by tu longline and gillnet fleets in the Indic Ocean. Unique stock in the Indic Ocean is assumed to the most probable hypothesis. The status of the blue marlin stock is unknown and the available data is limited to catch and catch rates. Biomass dymic models are one of the altertives to assess the stock status in such poor data scerio. In this paper the blue marlin is assessed by using Bayesian state-space models (Fox and Schaefer types) calculated ba sed on estimated total catches and standardized catch rates of Japan. Informative and non-informative priors were used. Likelihood function was based on log-normal density distributions. Monte Carlo Markov Chains are used to calculate the posterior sample. Three chains starting with different parameters estimations were calculated. The first 30000 samples of each chain were discarded (burnin), and the next 50000 samples were sliced resulting in a fil sample with size equal to 1000. Convergence o f the chains was assessed using Gelman-Rubin diagnostics. Schaeffer type models converged, but all Fox models did not converge. Overall the production models fitted with observatiol error only are biased, while the state-space models are not. Nevertheless, because there are many parameters, and because the data on blue marlin are not that informative, the uncertain on the estimations were very high and the solutions were sensitive to the choices concerning the priors. State-space model needs to be further tested before using it in situations that the data is not informative as is the blue marlin case.

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