Modeling Fish Movement in 3-D in the Gulf of Mexico Hypoxic Zone

The hypoxic zone on the Louisiana-Texas shelf varies in its area, volume, and vertical distribution. Vertical movement to avoid hypoxia (dissolved oxygen (DO) concentration < 2 mg/L) can be important to understanding fish exposure to low DO. Fish movement was simulated in 2-D (bottom layer) and 3-D with movement algorithms that depended on DO fields generated by a 3-D coupled hydrodynamic-water quality model. Fish exposure to low DO was simulated using two alternative movement algorithm groups, good and bad avoidance competency, and three perception ranges for detecting DO. The algorithm groups (default and avoidance) differed in whether the default movement included a downward bias. Avoidance competencies differed in the degree of random variation imposed on the avoidance movement. Fish exposures to low DO were simulated in 2-D and 3-D for July 24 to August 2 of 2002 during which hypoxia showed both horizontal and vertical variations. The addition of vertical movement (3-D) resulted in mean cumulative hypoxia exposures (days with DO< 2 mg/L) typically about 8 times lower than the 2-D movement results (e.g., ∼0.1 days versus 0.8 days with good avoidance), but vertical avoidance did not decrease the cumulative days of exposure of fish to moderate hypoxia (2–4 mg/L). The differences between the 2-D and 3-D model results, coupled with the limited data on vertical fish movement in response to hypoxia, suggest that 3-D movement could affect exposure to hypoxia and should be considered for spatially complex habitats.