Indices of Larval Bluefin Tuna, Thunnus Thynnus, Abundance in the Gulf of Mexico; Modelling Variability in Growth, Mortality, and Gear Selectivity

Most indices of stock size for Atlantic bluefin tuna are fishery dependent, and thus do not benefit from statistical design. Alternatively, we derived a fishery independent index of western Atlantic stock size from ichthyoplankton surveys conducted by the National Marine Fisheries Service. The larval abundance indices developed from these surveys have been used to corroborate trends in fishery dependent estimates of stock size, as well as to tune the virtual population analysis. Estimates of average annual larval abundance at first daily increment formation per 100 m² sampled by oblique bongo tows were used to index total annual larval abundance. A model describing the observed mean trend in larva size at otolith daily increment count was developed to estimate a probability of age at length matrix for ageing the captured larvae. Daily loss rates (Z) were estimated through regression analysis of the larval catch curves. Effects of mesh size changes (333 μm or 505 μm) during the time series of sampling were incorporated into the estimates. Uncertainty in the index values for the various components of the estimator is incorporated through the delta method. Zero catch information from the sampling is incorporated through application of Pennington's transform. Finally, we tested several different methods of calculating the index to evaluate the sensitivity of the results to different assumptions.