Abstract Wells, R. J. D., James H. Cowan Jr, and William F. Patterson III. 2008. Habitat use and the effect of shrimp trawling on fish and invertebrate communities over the northern Gulf of Mexico continental shelf. – ICES Journal of Marine Science, 65: 1610–1619. The goals of this study were to characterize habitat-specific fish and invertebrate community structure over sand, shell-rubble, and natural reef substrata, and to assess the effects of trawling on the sand and shell-rubble habitats and their associated communities during quarterly trawl surveys over a 2-year period. Fish and invertebrate communities differed significantly among habitat types [analysis of similarities (ANOSIM); Global R = 0.436, p < 0.001), and with respect to trawling exposure (ANOSIM; Global R = 0.128, p < 0.001). Habitat characteristics were quantified from video transects sampled with a remotely operated vehicle, and included percentage coverage of tubeworms, bryozoans, anemones, corals, and algae, significantly affecting fish community structure. Diversity indices differed among habitats, with the highest Shannon diversity (H′) and Pielou's evenness (J′) over shell-rubble, specifically non-trawled shell-rubble. In addition, higher values of H′ and J′ were found over trawled sand relative to non-trawled sand habitats. Length frequency distributions of several abundant fish species showed truncated size distributions over trawled and non-trawled habitats and were both habitat- and species-specific. The study describes habitat-specific differences in community structure, highlighting the differences between trawled and non-trawled areas on the northern Gulf of Mexico continental shelf.
During the twentieth century about 25% of the wetlands of the Mississippi delta was lost, partially a result of isolation of the river from the delta. River diversions are being implemented to reintroduce river water to the delta plain. We synthesize here the results of extensive studies on a river diversion at Caernarvon, Louisiana, one of the largest diversions in the delta.
As the Earth's population continues to increase, the projected effects of contaminant loading and human encroachment on biodiversity remain unclear. One area of intense interest is coastal eutrophication and associated hypoxia events (with hypoxia defined as oxygen <2 milligrams per liter = 1.4 milliliters per liter = ˜63 micromoles per cubic decimeter). On average, the Mississippi River discharges to the northern Gulf of Mexico (GOM) 550 cubic kilometers of freshwater and approximately 60–70×10 9 moles of dissolved inorganic nitrogen per year [ Dagg et al., 2004], which contains anthropogenic nitrogen derived primarily from agricultural fertilizer as well as effluent from animal feedlots.
The Freeport Sulphur Mine Artificial Reef (FSMAR) is a decommissioned oil and gas platform and serves as the largest artificial reef complex in the northern Gulf of Mexico (NGOM). Given the increasing numbers of artificial reefs in the NGOM, yet the paucity of information that exists, the goals of this study were to evaluate the biomass, density, and size structure of fishes associated with FSMAR. Mobile acoustic surveys were used to assess both horizontal and vertical distribution and abundance of fishes associated with the shallow water (16 m depth) reef complex and adjacent soft-bottom habitats extending 1 km from the reef complex. Highest acoustic estimates of fish biomass and density were found directly over the reef with a five-fold and 16-fold decrease at 10 and 30 m distances from the structure, respectively. In addition, fish biomass and density were highest in the bottom water column (> 10 m), followed by mid-water (6.1–10 m), and lowest in the upper water column (1.5–6 m). Findings suggest that fish distribution at the FSMAR is much greater than previously surveyed decommissioned oil and gas platforms and natural reef habitats in the NGOM. We consider the potential importance of this unique nearshore complex as an important habitat to fishes utilizing nearby estuarine and inner shelf systems in the NGOM.
A mesocosm system was developed to be deployed from a research vessel and set adrift with its enclosed plankton, including fish eggs, larvae, and gelatinous predators. The system consists of an array of mesocosms that are 1 m diameter, 5 m long, and 3.2 m3 capacity, and which are constructed of 20-μm porosity dacron. Deployment and harvesting procedures are described. The mesocosms "capture" a sample of the water column and provide an assay method to examine planktonic processes in experiments of a few hours' to a few days' duration. Mortality rates of bay anchovy, Anchoa mitchilli, eggs and yolk-sac larvae were estimated from drifting mesocosm experiments in 1989 and 1991. Overall mean instantaneous rates were 0.073 h−1 for eggs and 0.051 h−1 for larvae, indicating that 95% of a cohort had died within 2 days of hatching. Egg and larval mortality rates were variable due to the variability in the initial numbers of captured eggs and larvae and to the complex mix of predator sizes and biovolumes that were enclosed in different deployments. An attempt (only partially successful) was made to relate observed mortalities to the abundance of two enclosed gelatinous predators, a ctenophore Mnemiopsis leidyi and a scyphomedusan Chrysaora quinquecirrha. The mesocosm results are compared with those from an encounter model that predicts predation mortality. The results are also compared with synoptic ichthyoplankton surveys and an analysis of predator gut contents, which provide independent estimates of egg/larvae mortality and their consumption by gelatinous predators. The drifting mesocosm methods holds promise and, with modifications of experimental design, can be a valuable tool to study zooplankton population processes, including those of early life stages of marine estuarine fishes.
Perhaps the most perplexing aspect of the highly engineered and artificial Mississippi River deltaic ecosystem is lack of evidence that fish production has decreased. Louisiana accounts for -75% of the fishery landings in the U.S. Gulf of Mexico and 60%-80% of the nation's total annual coastal wetland loss, the human-caused reasons for which are well documented. Continued alteration, degradation, and loss of Louisiana's estuarine and wetland habitats makes knowledge of the relationship between habitat stability, and its effects on nursery-ground function and fish production, critical. As a result of this issue in Louisiana and elsewhere, concepts of ecosystem management and sustainable development have become part of state, national, and international dialogue about adaptive environmental management. Formulation and implementation of long-term, sustainable coastal policies and integrated management strategies demand a better understanding of (1) habitat and ecological stability and associated functional responses to both episodic and chronic insults, especially given the limited vitality of already-stressed coastal ecosystems, and (2) the compounding and complex effects of multiple impacts superimposed on issues associated with shifting baselines and climate change.
Coastal water hypoxia, where dissolved oxygen is less than 2 milligrams per liter, is a global environmental problem [e.g., Diaz and Rosenberg , 2008]. It is largely associated with eutrophication, whereby nutrient inputs (nitrogen and phosphorous) to coastal waters lead to elevated primary production and accelerated rates of microbial respiration, which results in oxygen depletion. Despite more than 25 years of monitoring [ Rabalais et al. , 2007] (see also Figure S1 in the online supplement to this Eos issue (http://www.agu.org/eos_elec/)), the relative importance of the various processes that control hypoxia in bottom waters of the northern Gulf of Mexico (GOM)–in particular, those beyond the direct influence of river plumes [ Dagg et al. , 2007; Bianchi et al. , 2008, 2010, and references therein]—remains uncertain. For example, a prediction last June pronounced that the 2009 hypoxic area would be the largest on record (∼23,000 square kilo meters; see http://www.gulfhypoxia.net/Research/Shelfwide%20Cruises/2009/ Files/2009
