Assessment of the Pacific Sardine Resource in 2019 for U.S. Management in 2019-20

Spawning fraction (S), the proportion of mature female fish that spawn per day, is one of the adult reproductive parameters used in the daily egg production method to estimate the spawning biomass. This parameter is one of the most difficult parameters to estimate with relative large coefficient of variation (CV). Since 2004, number of trawls for Pacific sardine ichthyoplankton-trawl surveys has increased. To fully utilize trawl data from all years, a Bayesian hierarchical model (BHM) was investigated, as recommended by the May 2009 STAR panel. The BHM was used for each of two regions (region1: high density area and region 2: low density area) when data of that region were available. For both regions, the point estimates from the original estimates and the BHM were similar. The CVs of the BHM estimates were lower than those from the original method in most years. In recent year, the CV of estimates from these two methods were similar for region 1, but the CV of BHM estimates were much lower than those of the original estimates for region 2. One of the reasons for the similarities of two estimators in recent years is due to the large sample sizes. We choose to continue using the original method for following reasons: 1). The shrinkage effect is small for future years when sample size is large, thus the gain from the BHM is minimum. 2). In many years (e.g. 1987, 1997, 2001, 2002 and 2004), when trawls were taken only in region 1 but not in region 2, the mean of the posterior predictive distribution for region 2 was used. The BHM is also needed for other adult parameters like fecundity, female weight and sex ratio Extensive computer programming is needed to incorporate the BHM estimates of adult samples and egg samples to compute the spawning biomass. 3). The current practice is for years when no adult samples were available in any one or both regions, the total egg production (TEP) time series was obtained and used in the stock assessment.