Hypoxia, Fish, and Fisheries in the Northern Gulf of Mexico: An Ecosystem-Based Approach

Similar to many coastal and estuarine systems, there is evidence that the northwestern Gulf of Mexico continental shelf is under increasing stress due in part to anthropogenic activities in its adjoining watershed. Nutrient loading from the Mississippi River system, which drains ~40% of the continental United States, contributes to the formation of one of the largest seasonal hypoxic zones in the northern hemisphere (~22,000 square km during 2002). Despite evidence that hypoxia is becoming more frequent and widespread in coastal and estuarine systems, its effects on fish and fisheries are not well understood. While hypoxia may clearly have a direct mortality effect on mobile species resulting in fish kills, there is growing recognition that the indirect effects of low dissolved oxygen, such as habitat loss, declines in food availability and growth, physiological stress and increased exposure to predators are more likely to have population and community-level consequences. A potentially important, though rarely studied, effect of hypoxia and associated habitat loss is changes in aggregation and spatial overlap with natural (i.e., predators) and anthropogenic (fisheries) sources of mortality.

It is hypothesized that hypoxia and associated habitat loss result in shifts in community structure and the spatial distribution of component species that modify trophic relationships and spatial overlap between prey and predator (including fisheries) species with ultimate consequences for the productive capacity of the Gulf continental shelf ecosystem. The objectives of this project are to: 1) Characterize habitat use of upper trophic levels including commercially, recreationally and ecologically important fishes, bycatch species and species of special concern (sea turtles, marine mammals) relative to fine-scale spatial structure in bottom dissolved oxygen; 2) develop indices of spatial overlap for target and nontarget fishery species, and predator (including shrimpers) and prey species; 3) assess trophic relationships among key components of the upper trophic level community; 4) develop statistical models relating distribution, feeding success, and spatial overlap to important environmental variables including bottom dissolved oxygen; and 5) compare community structure and associated impacts of hypoxia at small and regional spatial scales. Statistical models will be used to infer whether the location of areas of high relative abundance, feeding success and overlap vary as a function of low dissolved oxygen, distance from the edge of the hypoxic zone, or between regions characterized by different oxygen conditions. These empirical models will provide information on ecological responses to hypoxia at the spatial scale at which such responses occur. They will also provide critical information for parameterizing and validating more process-oriented models of food web and fishery interactions, and how they may be impacted by hypoxia.