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|Title:||A pre-restoration fishery resource utilization assessment of an estuarine borrow pit in Mobile Bay, Alabama|
|Authors:||United States. Army. Corps of Engineers. Mobile District.|
Dredging Operations and Environmental Research Program (U.S.)
Reine, Kevin J.
Clarke, Douglas G.
Mobile Bay, Alabama
Dredging Operations and Environmental Research Program
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||ERDC TN-DOER ; E37|
Abstract: Many open-water habitats that provide Essential Fish Habitat functions are also thought to be particularly susceptible to dredging project impacts. Evidence exists, however, that placement of dredged material in open-water environments can result in viable — even enhanced — habitat attributes and functions for fish, shellfish and benthic invertebrates. For example, offshore dredged material placement sites are often used extensively as recreational fishing areas. Dredged material can also be used to restore degraded fish habitat, such as to fill artificial pits, holes, and depressions that are scattered throughout a majority of estuaries and coastal embayments. Studies dating back to the 1960s have shown that these artificial pits periodically or chronically have poor water quality conditions and consequently represent degraded fish habitat. Several factors contributing to the degraded habitat within the pits include both low dissolved oxygen and high hydrogen sulfide concentrations, particularly during summer months. A major contributing factor to potentially poor water quality in dredged holes is hypoxia or anoxia resulting from accumulation of organic material, poor tidal flushing, and water column stratification. In 2011, the Engineer Research and Development Center (ERDC) conducted a joint study with the U.S. Army Corps of Engineers, Mobile District to assess habitat quality of two dredged holes, designated Brookley and Airport Holes, located in upper Mobile Bay, Alabama. Seasonal conditions in both borrow pits were assessed in terms of water quality, benthic invertebrate community structure, and fishery resource assemblage composition and borrow pit utilization patterns. Water quality data indicated hypoxic/anoxic conditions were present during spring and summer with dissolved oxygen (DO) concentrations less than 2.5 mg/L (< 30% saturation) at depths of 4 m, falling to near 0.0 mg/L at depths greater than 5 m. During fall, lowest DO concentrations were slightly lower than 3 mg/l at water depths greater than 4.5 m. Haloclines were present during both spring and fall sampling events. Benthic macroinvertebrates were sampled by Ponar grab seasonally to evaluate recruitment and community structure. Thirty-eight taxa were collected over the course of the three sampling events. The average number of animals/sample (abundance) of natural bottoms was generally twice that of Airport Hole and nearly twenty-five times greater than Brookley Hole. Average number of taxa per sample at stations in natural bottoms was two to four times greater than at stations in both Airport and Brookley Holes. Benthic assemblages were dominated by polychaetous annelids. The capitellid polychaetes of the genus Mediomastus was the most abundant taxon comprising 52% of all specimens collected. Fishery hydroacoustic surveys were used to assess temporal and spatial distributions of fishes by site and season. In Brookley Hole, fish densities were greater during summer (mean = 68.5 fish/100 m^3), followed by spring (mean = 11.4 fish/100 m^3) and fall (8.2 fish/100 m^3). At the control site (Airport Hole), fish densities were greater in fall, averaging 29.9 fish/100 m^3, followed by spring (mean = 18.7 fish/100 m^3) and summer (mean = 13.2 fish/100 m^3). Conventional otter trawling was used to determine the composition of the fishery assemblage at each site by season. Spring samples were characterized by juvenile drum (Sciaenidae spp.) and brown shrimp at Airport Hole, and white shrimp (Penaeus setiferus) and juvenile drum at Brookley Hole. Fall samples were characterized by Atlantic croaker (Micropogonias undulatus) in Brookley Hole and bay anchovy (Anchoa mitchilli) at Airport Hole. At both dredged holes, summer samples were characterized by bay anchovy, spot (Leiostomus xanthurus), threadfin shad (Dorosoma petenense), Atlantic croaker and white shrimp. Differences between borrow pits included proportionally smaller numbers of Atlantic croaker and spot and high numbers of white shrimp (numerically dominant species) at Brookley Hole, while at Airport Hole the distribution of these five species was fairly uniform; although bay anchovy was numerically dominant.
|Appears in Collections:||Technical Note|