Incorporating a Habitat Preference Model into the Stock Assessment and Management of Pacific Blue Marlin

Blue marlin is distributed throughout the Pacific Ocean, is sexually dimorphic, prefers certain habitats, and migrates seasonally. However, previous stock assessments for this population have ignored these factors. A spatially-, sex-, and age-structured population dynamics model was therefore constructed and fitted to the data from pelagic longline fisheries, along with information on the relative density of the population over the Pacific Basin derived from a habitat preference model. Results show that blue marlin are more abundant in tropical waters, and females account for most of the biomass. Assessments that take account of environmental variation, movement dynamics, and sexual dimorphism indicate that this population is in a healthy state, with the spawning stock biomass currently at a fairly high fraction of its unfished level and the current fishing mortality less than that needed to achieve maximum sustainable yield. However, stock status is sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship. A risk analysis based on samples from a Bayesian posterior distribution suggests that the status of this population will remain optimistic if exploitation rates are maintained at the current level.