Hypoxia-induced habitat shifts and energetic consequences in Atlantic croaker and brown shrimp on the Gulf of Mexico shelf

Abstract

Seasonal, summertime hypoxia (dissolved oxygen ≤ 2 mg l -1 ) has occurred over large areas (~1000 to 20 000 km 2 ) of the northwestern Gulf of Mexico shelf during several years since at least the mid-1980s, resulting in habitat loss for demersal species. To evaluate the effects of hypoxia- induced habitat loss on Atlantic croaker Micropogonias undulatus and brown shrimp Farfante- penaeus aztecus, we compared species' spatial distributions and relationships to abiotic factors (tem- perature, dissolved oxygen, salinity) across years differing in the spatial extent of hypoxia. Analysis of 14 yr of fishery-independent research trawl and environmental data (July) indicated that hypoxia- induced shifts in spatial distribution result in considerable shifts in the temperature and oxygen con- ditions that croaker and brown shrimp experience. Croaker, which typically occupy relatively warm, inshore waters, remain in the warmest waters inshore of the hypoxic region but also are displaced to cooler offshore waters. Brown shrimp, which are typically distributed more broadly and further off- shore, shift to relatively warm inshore waters as well as cooler waters near the offshore hypoxic edge. These shifts in the species' spatial distribution are reflected in long-term decreases and increases in the mean temperatures occupied by croaker and brown shrimp, respectively, as well as increases in the variance in occupied temperatures for both species. Despite avoidance of the lowest oxygen waters, high densities of croaker and brown shrimp occur in areas of moderately low oxygen concen- tration (35 to 60% air saturation, 1.6 to 3.7 mg l -1 ) near the offshore hypoxic edge. Because tempera- ture and dissolved oxygen are important abiotic factors that impact metabolic scope, these shifts in spatial distribution during severe hypoxia may impact organism energy budgets. High croaker and shrimp densities near the hypoxic edge likely have implications for trophic interactions as well as the harvest of both target (brown shrimp) and nontarget (croaker) species by the commercial shrimp fishery.