Modeling Pelagic Ecosystems and Larval Atlantic Bluefin Tuna Dynamics in the Gulf of Mexico under Current and Future Climate Conditions

Atlantic Bluefin tuna (ABFT) (Thunnus thynnus) spawn in the warm oligotrophic regions of the Gulf of Mexico (GoM) where zooplankton biomass is on average low and highly variable. Consequently, their larvae are thought to be highly susceptible to starvation. Mortality during the larval stage of marine fish is considered a key source of variability in recruitment and thus is important to quantify for sustainable fisheries management. However, estimating larval mortality in the field is a major challenge because of the ocean’s moving reference frame. In this work I develop a three-dimensional physical-biogeochemical model (PBM) to estimate prey (zooplankton) biomass for larval ABFT and couple this model with a Lagrangian, individual based, bioenergetics model to evaluate growth and mortality rates of larval ABFT in the GoM during the spawning periods from 1993-2012. Based on mesozooplankton diet estimated by the PBM, microbial and protistan food webs are deemed important for sustaining mesozooplankton biomass in the GoM, which serves as the primary food source for larval tuna. Starvation was found to be the main source of larval mortality during annual spawning periods for ABFT. However, predation was ultimately found to limit survival of older larvae and hence may explain why adults choose to spawn in the open-ocean GoM despite the poor feeding conditions for first feeding larvae. In addition, entrainment of shelf water into the open-ocean GoM by mesoscale eddies is shown to be important for supporting first feeding larvae and may further explain why spawning occurs in the open ocean GoM and not in other regions with similar temperature and zooplankton biomass. Finally, the impact of projected future warming was assessed using downscaled climate simulations for the late 21st century. It is shown that survival is likely to decrease in future climate scenarios, primarily as a result of higher rates of starvation for first feeding larvae, even though growth of older larvae was found to be substantially higher. In addition to studying larval ecology, ocean modeling tools like the ones integrated in this research provide a useful framework for including ecosystem based management into management of important fisheries like ABFT.