SCIENTIFIC COMMITTEE FIFTH REGULAR SESSION 10-21 August 2009 Port Vila, Vanuatu REVISED BIOLOGICAL PARAMETER ESTIMATES FOR APPLICATION IN YELLOWFIN STOCK ASSESSMENT

Previous western and central Pacific Ocean (WCPO) yellowfin stock assessments assume that the reproductive potential is proportion to the spawning stock biomass, with maturity at age the only explicitly reproduction-related factor taken into account. Recruitment to maturity for yellowfin is affected by a number of intrinsic factors, including size-related, age-related, and spatial changes in sex ratio, fecundity per kg, spawning fraction, and egg viability. We provide parameterisations of these factors for consideration in future yellowfin stock assessments. The analysis specifically examines assumptions that sex ratio is constant for all age classes and spatial locations, alternate fecundity and length relationship, and potential spatial variation in maturity at age in the WCPO. We observed differences in the estimates of the maturity at length by longitude, with L 50 for yellowfin sampled from Indonesia smaller than that observed elsewhere in the WCPO. Alternative growth curves influenced maturity at age estimates with the region 3 growth curve resulting in older maturing individuals than that observed when the standard WCPO wide growth curve was applied. This influence was also detected when calculating the maturity schedule for each of the alternative growth curves. Regional differences in sex ratio were detected, with the yellowfin sampled from region 3 more male biased at sizes above 130 cm than was observed in other regions. Natural mortality at age was influenced by the sex ratio data used and by the growth curve applied, with region 3 sex ratio and growth curve lowering the estimate of aggregate natural mortality for age classes between 2.5-5 yrs. Similarly region 3 sex ratio and growth curve influenced the estimate of male bias in comparison to the WCPO-wide parameterisation. Different relationships for fecundity at age and spawning fraction at age for the full model and for region 3 were also observed. These alternative parameterisations of the intrinsic factors that describe reproductive potential for yellowfin in the WCPO indicate that more explicit modelling of these processes and variances may be warranted for future stock assessments of yellowfin. A sensitivity analysis to identify the influence that these alternatives have on the reference points used to assess yellowfin stock status is recommended as a first step. Similarly consideration of alternative or complementary explanations for the observed trend in sex ratio with size, other than increased natural mortality of mature females, is warranted. Models based on such alternative structural assumptions may result in different stock status estimates.