Spatial distribution of brown shrimp (Farfantepenaeus aztecus) on the northwestern Gulf of Mexico shelf: effects of abundance and hypoxia

Abstract

We used fishery-independent hydrographic and bottom trawl surveys on the northwestern Gulf of Mexico shelf from 1983–2000 to test for density dependence and effects of hypoxia (dissolved oxygen ≤ 2.0 mg·L–1) on the spatial distribution of brown shrimp (Farfantepenaeus aztecus). Spatial distribution of shrimp was positively related to abundance on the Texas shelf but negatively related to abundance on the Louisiana shelf. Density dependence was weak, however, and may have been due to factors other than density-dependent habitat selection. Males were distributed over a broader area and further offshore than were females, though differences in spatial distribution between sexes were not large (~10%–15%). Large-scale hypoxia (up to ~20 000 km2) on the Louisiana shelf occurs in regions of typically high shrimp density and results in substantial habitat loss (up to ~25% of the Louisiana shelf), with shifts in distribution and associated high densities both inshore and offshore of the hypoxic region. We discuss these results in terms of the generality of density-dependent spatial distributions in marine populations and potential consequences of habitat loss and associated shifts in distribution due to low dissolved oxygen.

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

“…Based on models, the authors predicted economical losses from catch declines due to acidification for the sector of mollusc fisheries to reach 87-189 million US$. And we know from the demersal brown shrimp (Farfantepenaeus aztecus) fisheries in the northern Gulf of Mexico, that the spatial extent of hypoxic areas is significantly negatively correlated with the fisheries' landings from 1985 to 2004 (Zimmerman and Nance, 2001;Craig et al, 2005;O'Connor and Whitall, 2007). These numbers may give an idea of the potential economic losses that hypoxia may create.…”

Dynamics and distribution of natural and human-caused hypoxia

“…The reduction in shrimp landings lasted about 25 years, the time it took for increased use of agricultural fertilizer and waste water from dense human populations in the delta region to replace the natural fertility of the Nile River (Nixon, 2003(Nixon, , 2004Oczkowskia et al, 2009). A comparison with the Nile River delta is not appropriate for the Mississippi River, because the increased nutrient load and associated bottom-water hypoxia in the northern Gulf of Mexico have turned the fertile bull's-eye pattern to altered distributions and abundances of demersal species, particularly the brown shrimp (Craig et al, 2005). It is feasible to maintain a high fisheries production with the same nutrient loads of the 1950s, given other factors holding constant, e.g., fishing pressure.…”

Vertical distribution of fish biomass in hypoxic waters on the Gulf of Mexico shelf

“…deep ocean oxygen minimum zones; Rogers 2000, Helly & Levin 2004, hypoxia due to anthropogenic nutrient loading in shallow coastal environments is increasing and can limit the available benthic habitat (Diaz 2001, Rabalais et al 2002. A better understanding of behavioral avoidance responses to hypoxia is needed because indirect effects mediated by these avoidance behaviors and associated shifts in spatial distribution are thought to be a major pathway by which hypoxia can impact mobile species (Craig et al 2001, Breitburg 2002, Craig et al 2005.…”

“…Sensitivity to hypoxia as well as the capacity to avoid it can vary among species (Bell & Eggleston 2005), developmental stages (Breitburg et al 1997), between sexes, with reproductive state (Breitburg 1992), and with body size (Burleson et al 2001). When exposed to hypoxia, organisms may move horizontally to oxygenated refuges that often occur in shallow habitats where mixing is sufficient to aerate the water (Eby & Crowder 2002, Bell & Eggleston 2005, Stierhoff et al 2006), or to deeper offshore habitats beyond the influence of the stratification and nutrients necessary for hypoxia to develop (Craig & Crowder 2005). For example, based on fishery-independent trawl surveys, Craig & Crowder (2005) showed that brown shrimp and Atlantic croaker on the northwestern Gulf of Mexico shelf avoid hypoxic water and occur at high densities in nearby oxygenated edge habitats, with abundance declining rapidly with increasing distance from the edge.…”

“…When exposed to hypoxia, organisms may move horizontally to oxygenated refuges that often occur in shallow habitats where mixing is sufficient to aerate the water (Eby & Crowder 2002, Bell & Eggleston 2005, Stierhoff et al 2006), or to deeper offshore habitats beyond the influence of the stratification and nutrients necessary for hypoxia to develop (Craig & Crowder 2005). For example, based on fishery-independent trawl surveys, Craig & Crowder (2005) showed that brown shrimp and Atlantic croaker on the northwestern Gulf of Mexico shelf avoid hypoxic water and occur at high densities in nearby oxygenated edge habitats, with abundance declining rapidly with increasing distance from the edge. Alternatively, because hypoxia is typically constrained to the bottom portion of the water column by density stratification, organisms may also move vertically to avoid the low-oxygen water near the bottom (Patriquin 1967, Aku et al 1997, Keister et al 2000, Taylor & Rand 2003.…”