Effects of hypoxia on habitat quality of pelagic planktivorous fishes in the northern Gulf of Mexico

To evaluate the impact of hypoxia (<2 mg O ₂ l ^-1) on habitat quality of pelagic prey fishes in the northern Gulf of Mexico, we used a spatially explicit, bioenergetics-based growth rate potential (GRP) model to develop indices of habitat quality. Our focus was on the pelagic bay anchovy Anchoa mitchilli and Gulf menhaden Brevoortia patronus. Positive GRP was considered high-quality habitat (HQH) and negative GRP was considered low-quality habitat (LQH). Models used water temperature, dissolved oxygen (DO), zooplankton biomass, and phytoplankton concentration collected during the peak periods of hypoxia in 2003, 2004, and 2006 to estimate fish GRP. Results showed that hypoxic areas were always LQH. However, with respect to the entire water column, hypoxia had only a minor impact on overall habitat quality, with habitat quality being driven primarily by prey availability followed by water temperature. These results are in contrast to other ecosystems, such as the Chesapeake Bay, where hypoxia affects a larger fraction of the water column than in the Gulf of Mexico and has a significant impact on overall habitat quality. Differences in the effect of hypoxia on habitat quality between these 2 ecosystems suggest that the vertical extent of hypoxia relative to water column depth (i.e. hypoxic volume) is a fundamental consideration when evaluating the impacts of hypoxia on pelagic fish production.