Aggregation on the edge: effects of hypoxia avoidance on the spatial distribution of brown shrimp and demersal fishes in the Northern Gulf of Mexico

The northwestern Gulf of Mexico shelf experiences the largest seasonal hypoxic (dissolved oxygen, DO <= 2.0 mg l(-1)) zone in the western hemisphere. This study uses bottom trawl and hydrographic surveys over 3 yr to quantify low DO avoidance thresholds, patterns of aggregation in nearby oxygenated refuge habitats, and spatial overlap of brown shrimp Farfantepenaeus aztecus and several finfishes on the nearshore Louisiana shelf. On average, DO avoidance thresholds were low (1 to 3 mg l(-1)) and near incipient lethal levels for similar species, suggesting organisms avoid the lowest, lethal DO levels on the shelf. Avoidance thresholds varied both within and among years, indicating that behavioral responses to low DO are context-dependent and vary in relation to the severity of hypoxia and possibly other factors. Despite the absence of physical barriers to movement, evading organisms aggregated at short distances (1 to 3 km) just beyond the margins of the hypoxic zone, indicating that sublethal and indirect effects of hypoxia are probably most intense within a relatively narrow region along the hypoxic edge. DO avoidance thresholds and patterns of aggregation were similar between brown shrimp, the primary target of the commercial shrimp trawl fishery, and several juvenile and small adult finfishes that comprise most of the bycatch. In addition, spatial overlap between brown shrimp and finfishes was highest in the years when hypoxia was most severe, and this effect was stronger for benthic fishes than for pelagic fishes. These results suggest the potential for enhanced harvest and bycatch interactions along the margins of the hypoxic zone as an indirect effect of hypoxia-induced shifts in spatial patterns. Such spatially mediated indirect effects are an important means by which hypoxia influences mobile species in the Gulf.