Modeling the Effects of Hypoxia on Fish Movement in the Gulf of Mexico Hypoxic Zone

One of the world's largest areas (up to 22,000 square km) of seasonal, coastal hypoxia occurs in the northern Gulf of Mexico off the Louisiana coast. Hypoxia can affect fish through direct mortality, altered movement, increased susceptibility to predation, reduced habitat, changes to food resources, and decreased fecundity. Atlantic croaker (Micropogonias undulatus), a common demersal fish in the Gulf of Mexico, is a good model organism to study the effects of hypoxia. Croaker movement was modeled in 2-D using environmental cues derived from a high-resolution 3-D coupled hydrodynamic-water quality model. Croaker moved either with avoidance behavior in response to low dissolved oxygen (DO) or with default behaviors that acted independently of DO. A restricted-area search algorithm was used for avoidance, in which individual croaker moved towards cells in their local area in order to avoid exposure to low oxygen concentrations (i.e., DO < 2.0 mg/L). Three different default behavioral movement approaches were paired with the avoidance movement: Cauchy correlated random walk, kinesis, and random walk. Areas with DO < 2.0 mg/L had lower cell quality and areas with a temperature of 26°C, the ideal for croaker, had higher cell quality. The position of each individual croaker was simulated every 900 sec or 2 hours (depending on default movement) for seven days in the bottom layer of the 3-D water quality model. Simulated DO concentrations and temperature values were held constant at the values for July 24, 2002. Our initial results suggest that the degree of avoidance assumed in the movement algorithm plays a major role in affecting exposure, regardless of the default algorithm used, and that the time histories of individual-level exposures are complex and difficult to capture with examination of the average fish.