Abstract

In the northwest Gulf of Mexico, large energetic eddies spin off the intruding Loop Current and migrate westward along the continental slope region, causing sporadic exchanges of water between the nutrient rich shelf and the more oligotrophic deep basin. These exchanges enrich surface waters over the deep basin, enhancing the spawning grounds of Bluefin Tuna (Thunnus thynnus) and other important big game fish, but may be deleterious for retention of reef fish which spawn and settle as juveniles on the shelf. Many reef fish are broadcast spawners with eggs and larvae susceptible to entrainment in this large-scale, water exchange process. Subsequent dispersal over the deep basin reduces potential larval settlement success for species that have shallow water habitat requirements for post-larval stages, and ultimately may influence recruitment and population abundance. In this study we examine the effect on egg/larval dispersal from the Flower Gardens National Marine Sanctuaries (FGNMS), located toward the outer shelf in an area commonly impacted by the spin-off eddies. During the heavy spawning season (summer) between 2003 and 2014, eddy energy over the upper slope decreased substantially with concomitant egg/larvae retention on the continental shelf and weaker loss to the deep basin. Although retention for settlement in favorable FGNMS habitat (i.e. larvae contributed by local spawners) was increased, ability to broadcast larvae over broad areas was decreased and larval dispersal from dense (western Gulf of Mexico) to depauperate (eastern Gulf of Mexico) red snapper populations was diminished.

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