Dredge Tracks Research Articles

Experimental Effects of a Lightweight Mussel Dredge on Benthic Fauna in a Eutrophic MPA

Dredging for wild mussels Mytilus edulis Linnaeus, 1758 takes place in the marine protected areas (Natura 2000 sites) in Denmark. The fishery is strictly regulated to limit the impact of dredging to the benthic environment, and requires the use of modified lightweight mussel dredges. Nevertheless, the depletion of the benthic macrofauna associated with M. edulis beds after impact from such dredges is yet to be quantified. Here, dredging is shown to result in a significant decline in the density, species richness, and biological traits directly in the dredge track but no significant impact on the community composition. Species richness remained significantly affected 4 mo post dredging, although the positive trend after 4 mo may signal recovery. An effect of dredging was also detected in areas adjacent (approximately 5 m) to the dredge tracks, but needs further investigation to understand this impact. The results provide an estimate of benthic macrofaunal depletion in the dredge track as well as of the short-term recovery, which may be of use in the management of the fishery. Finally, it is suggested that species richness may be a more sensitive indicator than density in a eutrophic and species-poor system.

Scientific substantiation of dredging volumes on the Ob river - the most important source of water resources in Western Siberia

The purpose of the work is to substantiate the volume of dredging operations intended to ensure navigation on the Ob River. It is particularly important to find an option to reduce the impact of dredging on the river as a source of water resources. The method for determining the optimal dredging is based on theoretical research in the field of channel formation. A retrospective analysis showed the presence of different intensity dredging stages: superintensive dredging in the 80s, extremely low stage in the 90s, and the optimal one, which began in the 21st century. Limiting riffles were determined on the basis of the channel stability criteria. Dredge tracks were outlined and the volumes of dredging operations were calculated to create and maintain guaranteed track dimensions on all transit river fairways. For each of the riffles, options for the location of capital and operational dredge tracks are proposed. In total, it is necessary to develop 79 capital tracks, 86 obligatory annual and operational tracks, to erect 21 longitudinal dams and 31 series of spur dikes. The volume of capital dredge tracks in their various versions should be about 8 million m3, and the minimum required volume of operational tracks in various versions is about 7.6 million m3. The main dredging operations fall on the sections of the Upper and Middle Ob. The volumes are assessed as very significant, but absolutely necessary to stabilize the route of the fairway.

Persistence of Hydraulic Dredge Tracks Following Surfclam Harvesting in Shallow Water

Hydraulic dredges used for bivalve harvest are known to create immediate biological and physical changes to the substrate, but the rate of recovery is often site specific and habitat specific. Hydraulic dredging for the Atlantic surfclam Spisula solidissima (Dillwyn, 1817) occurred off Provincetown, MA, from November 2014 through April 2015. Acoustic surveys following the dredge activity were conducted in 2015 (June–August) with a phase-measuring sidescan sonar, yielding colocated sidescan backscatter and swath bathymetry. Dredge tracks were documented in 7.9% (402,309 m2) of the 5.1 km2 surveyed. The survey area was divided into 10 × 10 m grid cells, and track density was calculated within each grid by the aggregated count of dredge tracks within each grid cell. The resulting track densities ranged between 0% and 53%. On October 23, 2017, 875 days after the 2015 hydraulic clamming season, a subset (1.05 km2 ∼20%) of the same area was acoustically surveyed. The 2017 survey (1.05 km2) revealed that dredge tracks covered a total area of 9,784 m2 at a density of 0%–24%. Initial areas (2015) with dredge track density greater than 10% had not fully recovered in 2017, and the rate of dredge tracks remaining was proportional to disturbance (linear regression; r2 = 0.95; F = 110.6; df = 1,7; P < 0.001; SE = 0.206). The adjacent dynamic shoreline and the persistence of dredge tracks reinforce the idea that nearshore energy regimes should not be assumed based solely on more visible shoreline processes.

Processes of physical change to the seabed and bivalve recruitment over a 10-year period following experimental hydraulic clam dredging on Banquereau, Scotian Shelf

A previous study on the effects of experimental hydraulic clam dredging on seabed habitat and commercial bivalve populations revealed a lack of recovery after a 3-year post-dredging period (1998–2001) on a deep (65–75m) offshore sandy bank on the Scotian Shelf, Canada. Follow-up sidescan sonar surveys were carried out 5 and 10 years after dredging (2003, 2008) in order to identify long-term processes of seabed recovery. Grab sampling was carried out 10 years after dredging to identify post-dredging commercial bivalve recruitment. Changes in the seafloor, including dredge tracks, were documented with a series of 7 sidescan sonar surveys between 1998 and 2008. A sediment mobility model was constructed based on modeled tidal current and hindcast wave data over this time period to quantify natural seabed disturbance and interpret changes to the dredge tracks mapped by sidescan sonar surveys. The model indicated that tidal currents had minimal effect on sediment mobilization. The main driving force associated with re-working of surficial sediments as evidenced by deterioration of dredge tracks in sonograms was annual fall/winter storms. While the annual frequency of storms and associated wave heights was variable, the observations and sediment mobility calculations suggest that the most influential variable is the magnitude of individual large storms, specifically storms with a significant wave height of ∼11m. These storms are capable of generating mobile sediment layers of 20–30cm thickness, equivalent to the dredge blade cutting depth. It appears that, with minor exceptions, sediment properties have returned to pre-dredging conditions 10 years after dredging in this habitat. Based on known age-length relationships, the four commercial bivalve species showed very low recruitment at the experimental site over the 10-year post-dredging period. However, this is unlikely due to a dredging effect since a similar pattern was observed in non-dredged areas.

Sedimentation patterns caused by scallop dredging in a physically dynamic environment

Scallop dredging grounds in the Firth of Lorn, western Scotland, are juxtaposed with rocky reef habitats raising concerns that reef communities may be impacted by sediment disturbed by nearby scallop dredging. A particle-tracking model of sediment transport and settling is applied at two scales. In the near-field, a suspension of typical sand/gravel-dominated bed sediment is subjected to a steady current across the dredge track. In the far-field, silt particles, which may persist in suspension for multiple tidal cycles, are tracked in the context of a regional model of tidally-driven flow. The principal sedimentary risk to reef habitats is predicted to come from settling sand particles when dredge tracks approach within tens of metres of a reef. The cumulative effect of dredging at the relatively low intensities recorded in this region is not expected to have a significant long-term impact on suspended silt concentrations and settlement in this highly dispersive environment.

Recovery of the seabed following marine aggregate dredging on the Hastings Shingle Bank off the southeast coast of England

The aim of this study was to investigate the effect of dredging intensity on the physical and biological recovery times of the seabed following marine aggregate dredging. Two areas of seabed, previously subject to, respectively, relatively high and lower levels of dredging intensity, were identified on the Hastings Shingle Bank. Two reference areas were also selected for comparative purposes. All four sites were monitored annually over the period 2001–2004, using a combination of acoustic, video and grab sampling techniques. Since the site was last dredged in 1996, this was intended to provide a sequence of data 5–8 years after cessation of dredging. However, an unexpected resumption of dredging within the high intensity site, during 2002 and 2003, allowed an additional assessment of the immediate effects and aftermath of renewed dredging at the seabed. The early stages of recovery could then be assessed after dredging ceased in 2003. Results from both dredged sites provide a useful insight into the early and latter stages of physical and biological recovery. A comparison of recent and historic dredge track features provided evidence of track erosion. However, tracks were still visible 8 years after the cessation of dredging. Within the high dredging intensity site, recolonisation was relatively rapid after the cessation of dredging in 2003. Rather than indicating a full recovery, we suggest that this initial ‘colonization community’ may enter a transition phase before eventually reaching equilibrium. This hypothesis is supported by results from the low intensity site, where biological recovery was judged to have taken 7 years. Further monitoring is needed in order to test this. An alternative explanation is that the rapid recovery may be explained by the settlement of large numbers of Sabellaria spinulosa. As the resumption of dredging within the high intensity site limited our assessment of longer-term recovery it is not yet possible to assume that a 7-year biological recovery period will be applicable to other, more intensively dredged areas at this or more distant locations.

Recovery of substrates and macro-benthos after fishing trials with a new Portuguese clam dredge

The effect of dredging on bottom structure was assessed, to estimate the damage inflicted on the benthic macrofauna left on the dredge path and to evaluate aggregations of scavengers within the track. Sediment suspended during dredging rapidly resettled both on sand and sandy-mud bottoms. Dredge tracks were deeper on sandy-mud sediments and persisted longer than in sandy sediments. The disappearance of tracks in both sediment types depended both on current strength and wave action. Damage and mortality induced by dredging on the macrobenthic animals left on the dredge path was relatively low. Post-fishing SCUBA diver observations showed that damaged animals left on the dredge path rapidly attracted scavengers, mainly Ophiura albida. Immediately after the tow ophiuroids reached densities eight times greater in the track region than in the background. However, they dispersed again rapidly because only few damaged fauna were available on the dredge tracks. Undamaged or slightly damaged shellfish started to rebury immediately after escaping from the dredge.

A novel shear vane used to determine the evolution of hydraulic dredge tracks in sub-tidal marine sediments

A novel shear vane is described which can be used to record the shear strength of discrete depth horizons of a variety of marine sediment types in situ. This vane, or modifications of it, has great potential in allowing measurement of in situ shear strength without requiring any samples to be removed from the sea bed, a process which can often destroy the fine structure of the sediment. The vane was used to monitor the change in sediment shear strength caused by a hydraulic dredge, which was used to fish razor clams ( Ensis arcuatus) in a sheltered bay within the Clyde Sea area, Scotland. Data collected using this apparatus provided valuable and immediate information on the stratification of the sea bed post-dredging and allowed the evolution of the dredge track to be regularly monitored by divers over a period of 100 d.

A comparison of direct macrofaunal mortality using three types of clam dredges

Abstract The white clam Spisula solida is harvested along the entire coast of Portugal using mechanical dredges. In this study, the total direct mortality of the macrobenthic community caused by three types of clam dredges (north dredge—ND, traditional dredge—TD, and the metallic grid dredge—GD) used in the S. solida fishery was determined and compared. The relationship between mortality and catching efficiency for each type of dredge was also assessed. Our results showed significant differences for total direct mortality between the ND and both the GD and TD dredges. This difference was largely attributed to the mortality of animals that died in the dredge track as a direct result of the physical damage inflicted by the dredge passing. It was also found that the damage to uncaught individuals is directly related to gear efficiency. The lower catching efficiency of the ND (64%) led to a higher proportion of damaged individuals being left in the dredge path, when compared with the more efficient GD (98%) and TD (90%) dredges. Short and long-term implications of the impact of dredging on the composition of benthic communities are discussed. From fisheries management and ecological points of view, there are obvious advantages to introduce into the bivalve dredge fisheries more efficient and selective dredges in order to reduce the number of damaged individuals and by-catch, and consequently decreasing the impact on the macrobenthic communities.

An experimental study of the ecological impacts of hydraulic bivalve dredging on maerl

AbstractA short-term experiment to assess the ecological impact of a hydraulic blade dredge on a maerl community was carried out during November 2001 in the Clyde Sea area on the west coast of Scotland. A fluorescent sediment tracer was used to label dead maerl, which was then spread out on the surface of sediment to act as a proxy for living maerl. The fauna collected by the dredge was dominated by the bivalves Dosinia exoleta and Tapes rhomboides, which were found to be intact. The target razor clams Ensis spp. were caught in low numbers, which reflected the low abundance of this genus within the maerl habitat. The hydraulic dredge removed, dispersed and buried the fluorescent maerl at a rate of 5.2 kg m−2 and suspended a large cloud of sediment into the water column, which settled out and blanketed the seabed to a distance of at least 8 m either side of the dredge track. The likely ecological consequences of hydraulic dredging on maerl grounds are discussed, and a case is made for protecting all maerl grounds from hydraulic dredging and establishing them as reservoirs to allow for the recruitment of commercial bivalve populations at adjacent fished sites.

Reburial time and indirect mortality of Spisula solida clams caused by dredging

Clam-dredging results in the exposure of Spisula solida individuals not caught by the dredge. Subsequent survival depends on clam damage, reburial time, and the time needed by predators to reach the impacted area. We analyse these variables and discuss the importance of predation on exposed S. solida caused by dredge fishing. Sampling was performed in July 2000 off the southern coast of Portugal, at Vilamoura, a traditional S. solida sandy fishing ground. We compared the time needed for S. solida individuals to rebury themselves, relative to the abundance of potential predators. Bivalves collected by divers were placed on the seabed, and the times required for reburial were measured. These were compared with the times needed for reburial of the clams exposed by dredge impact. At each of three dredge tracks, we analysed the number of predators that entered three equal quadrats (0.0250 m 2) per minute. These results were compared with a non-affected control area. Impact caused by the fishing dredge significantly increases the number of exposed S. solida clams ( p<0.05) and the abundance of potential predatory species ( p<0.05). The brittle star Ophiura texturata was the most abundant and first species to reach the dredge track (less than 3 min after dredge impact). Other species reaching the dredge track were Pomatochistus spp. (6 min after impact), Diogenes pugilator, and Nassarius reticulatus (both 9 min after impact). Although predators reached the impacted area while S. solida bivalves were still exposed, our results suggest that predation on the non-buried clams in the dredge track is not a major factor for subsequent indirect mortality of S. solida.

Impact of scallop dredging on benthic megafauna: a comparison of damage levels in captured and non-captured organisms

The impact of scallop dredging on benthic megafauna was assessed by direct observation of damage, both in the bycatch and in organisms encountering dredges but not captured. Damage was assessed using a simple 4- point scale adapted for different taxonomic groups. Experi- mental dredging was undertaken on a scallop fishing ground in the north Irish Sea, off the Isle of Man. Divers were deployed immediately after dredges had passed, to record levels of damage to megafauna left in the dredge tracks. Mean damage levels, and the proportions of the 4 damage scores in the bycatch and on the seabed, were the same in most species. Some common species did show differences. The edible crab Cancer pagurus was more severely damaged when not captured, while the starfish Asterias rubens and whelk Neptunea antiqua received greater damage within the bycatch. Capture efficiency for the megafauna was low, rang- ing from 2 to 25% among species. The results indicate that the majority of damage to large benthic invertebrates during scallop dredging occurs unobserved on the seabed, rather than in the bycatch.

The effects of marine gravel extraction on the macrobenthos: Results 2 years post-dredging

An offshore experimental dredging study was initiated off North Norfolk (UK) in 1992 to investigate the impacts of marine gravel extraction on the macrofauna. A dredged ‘treatment’ and a non-dredged ‘reference’ site were selected to evaluate the initial impacts and subsequent processes of recolonization. A survey of the benthos was conducted prior to the removal of 50 000 t of marine aggregate from the treatment site. Thereafter annual monitoring surveys were conducted commencing immediately after the dredging episode. Results indicated that whilst the dominant species recolonized quickly following dredging many rarer species did not. Evidence from sidescan sonar records and underwater cameras indicated a considerable amount of sediment transport during the first two winters following dredging and the once well-defined dredge tracks have now become infilled with sand and gravel. The substantially reduced biomass at the treatment site some 24 months after dredging is thought to be due to a local increase in sediment disturbance caused by tide and wave action over the winter period. Finally, the biological findings of this study are discussed in relation to their wider environmental significance.

Underwater Observations on Tracks of Dredges and Trawls and Some Effects of Dredging on a Scallop Ground

Tracks of three types of fishing gear in bottom sediments were observed from a submersible in Chaleur Bay (Gulf of St. Lawrence). Tracks left by past otter trawling activities covered at least 3% of the bottom by area and were considered to have been made by trawl doors.Shallow tracks made by inshore and offshore scallop dredges during the course of the study could be distinguished from each other and from trawl tracks.Scallop dredging lifts fine sediments into suspension, buries gravel below the sand surface, and overturns large rocks embedded in the sediment, appreciably roughening the bottom. The inshore Alberton dredge is inefficient, dumping its contents back onto bottom at intervals during the tow.Dredging causes appreciable lethal and sublethal damage to scallops left in the track, this damage being greatest on rough bottom. Incidental mortalities to scallops with an offshore dredge of at least 13–17% per tow are of the same order of magnitude as estimates of harvesting efficiency made in earlier studies.Predatory fish and crabs were attracted to the dredge tracks within 1 hr of fishing and were observed in the tracks at densities 3–30 times those observed outside the tracks.