DEVELOPMENT OF INDICES OF BLUEFIN TUNA (THUNNUS THYNNUS) SPAWNING BIOMASS IN THE GULF OF MEXICO USING DELTA-LOGNORMAL MODELS

SUMMARY Fishery independent indices of spawning biomass of bluefin tuna in western North Atlantic Ocean are presented which utilize NOAA Fisheries ichthyoplankton survey data collected from 1977 through 2005 in the Gulf of Mexico. Indices were developed using similarly standardized data from which previous indices were developed (i.e. abundance of larvae with a first daily otolith increment formed per 100 m 2 of water sampled). An index of larval abundance was developed using a delta-lognormal model, which is a mathematical combination of yearly catch estimates from two distinct generalized linear models: a binomial model which describes the proportion of positive catch values and a lognormal model which describes the variability in nonzero catch data. Covariates, including surface temperature, water depth, time of day, time of month, area sampled and year, were tested for inclusion in both submodels. Due to the large frequency of zero catches during ichthyoplankton surveys, especially in later years, the binomial portion of the delta-lognormal approach was replaced with a zero-inflated binomial model and was considered a more appropriate method to model these data. The results of these two approaches were compared with one another and with other indices of larval abundance from the Gulf of Mexico.