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 hypoxiainduced habitat loss on Atlantic croaker Micropogonias undulatus and brown shrimp Farfantepenaeus aztecus, we compared species' spatial distributions and relationships to abiotic factors (temperature, 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 hypoxiainduced shifts in spatial distribution result in considerable shifts in the temperature and oxygen conditions 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 offshore, 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 concentration (35 to 60% air saturation, 1.6 to 3.7 mg l -1 ) near the offshore hypoxic edge. Because temperature 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.

Variability in Fish Tissue Proximate Composition is Consistent with Indirect Effects of Hypoxia in Chesapeake Bay Tributaries

“…The hypothesized outcome-that female Atlantic Croakers from a system characterized by severe hypoxia (Rappahannock River) would exhibit lower mean ovarian lipid content than females from a mildly hypoxic system (York River)-was supported. We also observed that within the Rappahannock River, catches of Atlantic Croakers were greater from normoxic habitats than from hypoxic habitats, indicating that habitat shifts can occur even for a relatively hypoxia-tolerant species (Bell and Eggleston 2005;Craig and Crowder 2005).…”

Spatial-dynamics of Hypoxia and Fisheries: The Case of Gulf of Mexico Brown Shrimp

“…While lab experiments demonstrate that hypoxia slows down shrimp growth and delays molting, and mortality is a limiting case if hypoxia is severe enough, there is a countervailing force at play to limit the effect in the fishery. In particular, marine ecologists have shown that shrimp tend to congregate on the edges of hypoxic areas, making them easier to catch (Craig and Crowder 2005;Craig 2012). These forces raise questions about the net effect of hypoxia on the shrimp fishery.…”

“…Whether there are sufficiently large hypoxic (or anoxic) areas within the spatio-temporally varying Gulf of Mexico Dead Zone to affect shrimp mortality is an open question. The catchability effect follows empirical work showing aggregation of shrimp, other fished species, and fishing vessels along edges of the hypoxic zone in Gulf of Mexico (Craig and Crowder 2005;Craig 2012). The growth effect is consistent with experiments that show slowed lipid uptake and delayed molting in shrimp that are exposed to low oxygen (Craig 2012).…”

Measuring Welfare Losses from Hypoxia: The Case of North Carolina Brown Shrimp

“…Laboratory and field studies in the Neuse River Estuary indicate that brown shrimp have an avoidance threshold of l-2 mg/l dissolved oxygen (Wannamaker and Rice 2000;Eby and Crowder 2002). Furthermore, brown shrimp appear to avoid severely low oxygen levels, but aggregate at high densities in areas of "moderately low dissolved oxygen" (1.6 to 3.7 mg/l) near hypoxic edges (Craig and Crowder 2005;Craig 2012). These results suggest that behavioral changes and resulting shifts in spatial distribution could have energetic consequences and alter trophic interactions that ultimately have consequences for commercial harvest.…”

“…For harvested species, all of these impacts reduce the biomass available for harvest and are expected to be costly to fisheries. Individuals also move to avoid hypoxia, which can lead to aggregations along the edges of hypoxic zones (Craig and Crowder 2005;Craig 2012). Aggregation effects might actually benefit fisheries in the short run by increasing catchability, but movement away from hypoxic zones still requires energy that could otherwise be used for growth and reproduction.…”