Douglas G. Clarke

Biological Effects of Suspended Sediments: A Review of Suspended Sediment Impacts on Fish and Shellfish with Relation to Dredging Activities in Estuaries

Abstract Objective assessment of the effects of increased concentrations of suspended sediment caused by human activities, such as navigation dredging, on estuarine fish and shellfish requires an integration of findings from biological and engineering studies. Knowledge is needed of (1) the suspended sediment characteristics typical of both ambient and dredging-induced conditions, (2) the biological responses of aquatic organisms to these suspended sediment dosages, and (3) the likelihood that organisms of interest will encounter suspended sediment plumes. This paper synthesizes the results of studies that report biological responses to known suspended sediment concentrations and exposure durations and relates these findings to suspended sediment conditions associated with dredging projects. Biological responses of taxonomic groups and life history stages are graphed as a function of concentration and exposure duration. The quality and taxonomic breadth of studies on which resource managers must rely when...

Characterization of underwater sounds produced by hydraulic and mechanical dredging operations

Sound recordings were made of two dredging operations at hydrophone depths of 3 and 9.1 m at distances up to 1.2 km from the source in shallow waters (<15 m) of New York Harbor. Sound sources included rock fracturing by a hydraulic cutterhead dredge and six distinct sources associated with a mechanical backhoe dredging operation during rock excavation. To place sound emitted from these dredges in perspective with other anthropogenic sounds, recordings were also made of several deep-draft commercial vessels. Results are presented as sound pressure levels (SPLs) in one-third octave versus range across the 20 Hz to 20 kHz frequency band. To address concerns for protection of fishery resource occupying the harbor, SPL were examined at frequency bands of 50-1000 Hz and 100-400 Hz, the ranges where the majority of fishes without hearing specializations detect sound and the range of greatest sensitivity, respectively. Source levels (dB re 1 μPa-1 m rms) were back calculated using fitted regression (15LogR). The strongest sound sources (180-188.9 dB) were emitted by commercial shipping. Rock fracturing produced a source level of 175 dB, whereas six distinct sources associated with rock excavation had source levels ranging from 164.2 to 179.4 dB re 1 μPa-1 m (rms).

Fishery resource utilization of a restored estuarine borrow pit:A beneficial use of dredged material case study

Numerous pits in coastal waters are subject to degraded water quality and benthic habitat conditions, resulting in degraded fish habitat. A pit in Barnegat Bay, New Jersey (USA) was partially filled with dredged sediment to increase flushing, alleviate hypoxia, and enhance benthic assemblages. Restoration objectives were assessed in terms of benthic community parameters and fishery resource occupation. Restoration resulted in increased benthic diversity (bottom samples) and the absence of water column stratification. Fisheries resources occupied the entire water column, unlike pre-restoration conditions where finfish tended to avoid the lower water column. The partial restoration option effectively reproduced an existing borrow pit configuration (Hole #5, control), by decreasing total depth from -11 m to -5.5 m, thereby creating a habitat less susceptible to hypoxic/anoxic conditions, while retaining sufficient vertical relief to maintain associations with juvenile weakfish and other forage fishes. Partially filling pits using dredged material represents a viable restoration alternative.

Resident Crab Associations with Sedimentation on Restored Intertidal Oyster Reefs in South Carolina and the Implications for Secondary Consumers

Abstract Benthic–pelagic coupling is an important habitat function provided by oyster reefs that is potentially affected by sedimentation impacts. In this study, the distributions of three resident crab species, the flat mud crab (also known as the flatback mud crab) Eurypanopeus depressus, the common mud crab (also known as the Atlantic mud crab) Panopeus herbstii, and the invasive green porcelain crab Petrolisthes armatus, were examined with respect to sedimentation on six restored intertidal eastern oyster Crassostrea virginica sites in South Carolina. Sedimentation among sites ranged from 4% to 60% cover and was composed of silty clay and sand. Crab associations with sedimentation differed among species. Common mud crabs and green porcelain crab densities were negatively correlated with percent sediment cover, whereas flat mud crab densities were not related to sediment cover. Flat mud crabs occupy the interstices of the reef, and their densities were positively correlated with scorched mussel Brachid...

Distribution of Fishes in the Vicinity of Dredging Operations in the James River, Virginia

Potential effects of dredging-induced underwater noise, suspended sediment plumes, and hydraulic entrainment have been identified as sources of concern for protection of fishery resources. Although such concerns have persisted for decades, conclusive evidence either substantiating or refuting them is lacking. One means of establishing the likelihood of actual impacts to fishery resources is examining the distribution of fishes in relation to the conduct of the dredging project. In the present study, fisheries hydroacoustic techniques were used to examine abundances and spatial and temporal distribution patterns of fishes around a hydraulic cutterhead dredging operation in a tidal reach of the James River, Virginia in October 1999. Hydroacoustic surveys were conducted during various stages of the tide on two spatial scales: several kilometers up and downstream of the dredge, and within several hundred meters of the dredge site or pipeline discharge area. Surveys were completed during periods of active dredging as well as when the dredge was idle. Gill nets were deployed to characterize the local fish assemblage and to identify acoustic targets. White perch ( Morone americana ), spot ( Leiostomus xanthurus ), and gizzard shad ( Dorosoma cepedianum ) accounted for almost half of all fish captures. Estimates of fish density with distance (30 m increments) from the dredge were examined for evidence of fish attraction to, or avoidance of, an operating dredge.

Biological Monitoring of Beach Nourishment Operations in Northern New Jersey, USA: Linkages between Benthic Impacts and Higher Trophic Levels

Between 1993 and 1999 over 19 million m 3 of sand were placed on exposed, high-energy beaches along the northern coast of New Jersey (USA) as an erosion control measure. As part of a multi-faceted biological monitoring program, potential impacts of this large-scale beach nourishment project on benthic infauna and fishes in the intertidal surf zone, nearshore habitat, and offshore borrow areas were examined. Food habits analysis was conducted on northern kingfish ( Menticirrhus saxatilis ), Atlantic silversides ( Menidia menidia ) and rough silversides ( Membras martinica ) collected in the surf zone, and winter flounder ( Pleuronectes americanus ) and summer flounder ( Paralichthys dentatus ) collected at the borrow areas to assess impacts to trophic linkages in the two project areas. Monitoring of benthos and fishery resources was conducted for three years before nourishment, during placement, and up to two years afterwards. The basic sampling design included multiple reference sites and several monitoring components integrated into a cohesive program. In general, impacts on the benthos were found to be short-term (2–6 months) and spatially limited. Stomach contents analyses revealed that the fishes examined have a diverse diet that includes non-benthic prey resources. Prey biomass in kingfish and silversides was usually greater in fishes caught at beach nourishment stations rather than at reference stations when significant differences were detected. Food habits of winter flounder, a benthic omnivore, and summer flounder, an epibenthic feeder, did not change relative to the dredging activities. Winter flounder fed primarily on anemones, which were not common at any of the borrow areas. Results of the present study call into question the need for extensive monitoring of projects where a close match exists between fill and receiving beach sediments. Further studies are definitely needed, however, where fill and receiving beach sediments aredissimilar.