Review of the Ecological Effects of 

Dredging in the Cultivation 

and Harvest of Molluscan Shellfish

US DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Northeast Fisheries Science Center 

Woods Hole, Massachusetts 

December 2011

NOAA Technical Memorandum NMFS-NE-220 
Review of the Ecological Effects of 

Dredging in the Cultivation 

and Harvest of Molluscan Shellfish

Renee Mercaldo-Allen and Ronald Goldberg

NOAA National Marine Fisheries Service, 212 Rogers Avenue, Milford CT 06460 

US DEPARTMENT OF COMMERCE 

John E Bryson, Secretary 

National Oceanic and Atmospheric Administration 

Dr. Jane Lubchenco, Administrator 

National Marine Fisheries Service 

Eric C. Schwaab, Assistant Administrator for Fisheries 

Northeast Fisheries Science Center 

Milford, Connecticut 

December 2011

NOAA Technical Memorandum NMFS-NE-220 

This series represents a secondary level of scientific publishing. All issues employ thorough internal 

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1 

TABLE OF CONTENTS 

Abstract ........................................................................................................................................... 1 

Introduction ..................................................................................................................................... 2 

Evolution of Shellfish Harvest and Cultivation .......................................................................... 3 

Types of Shellfish Dredging Gear ............................................................................................... 4 

Distinguishing Nearshore Shellfish Cultivation from Offshore Harvest Fishing ....................... 7 

Shellfish Dredging Differs from Navigational Dredging ............................................................ 8 

Shellfish Cultivation Similarities to Terrestrial Agriculture ....................................................... 8 

Scale of Disturbance, Faunal and Habitat Sensitivity, and Gear Considerations ....................... 9 

Effects of Shellfish Dredging are Variable ................................................................................. 9 

Experimentally Measuring Ecological Change ......................................................................... 10 

Effects of Shellfish Dredging........................................................................................................ 11 

Physical Effects of Dredging ..................................................................................................... 11 

Biological Effects of Dredging ................................................................................................. 16 

Chemical Effects of Dredging ................................................................................................... 28 

Socioeconomic Effects of Dredging ......................................................................................... 31 

Duration of Dredging Effects .................................................................................................... 31 

Ecological Recovery after Dredging ............................................................................................. 32 

Seasonal Changes Affect Recovery from Dredging ................................................................. 35 

Mitigating Effects of Shellfish Dredging ...................................................................................... 36 

Areas for Further Research and Conclusions................................................................................ 37 

Acknowledgements ....................................................................................................................... 38 

References Cited ........................................................................................................................... 39

Tables .........................................................................................................59 
1 

ABSTRACT 

This document reviews effects of dredging associated with the cultivation and harvest of 

molluscan shellfish, as reported in the literature. Dredges can disturb the structure of the 

substrate, alter the biological community, and modify sediment biogeochemistry. The rate of 

recovery subsequent to dredging varies with habitat and sediment type, composition of the 

resident biological assemblage, and hydrodynamic attributes of the environment. Our goal is to 

evaluate mechanical and hydraulic dredge harvesting of cultivated oysters and clams from 

nearshore, leased grounds located primarily along the Atlantic and Gulf Coasts of the United 

States. In nearshore coastal areas where aquacultural shellfish cultivation is conducted, 

disturbance from natural processes is frequent, and rapidly growing benthic organisms with short 

generation times are common. Typically, the duration and spatial extent of shellfish dredging 

associated with cultivation are limited in scale. These factors, along with the use of sound 

practices, often mitigate impacts and accelerate ecological recovery after shellfish dredging. 

Based on our review of the published literature, the physical, biological, and chemical effects of 

shellfish dredging within the inshore coastal zone are generally short-lived, with the rate of 

recovery varying among studies. 
2 

INTRODUCTION 

Dredging, as used in cultivation and harvest of molluscan shellfish, continues to be a 

topic of discussion among marine resource shareholders. A wealth of information is available 

concerning shellfish dredging, including bibliographies (e.g., Dieter et al. 2003; Dugas et 

al.1991; Redant 1987; Rester 2000; Wion and McConnaughey 2000), historical literature and 

reviews (e.g., Winslow 1882; Nelson et al. 1948; MacKenzie 1997; Wallace and Hoff 2005), and 

environmental impact statements (e.g., Vining 1978; Barnes et al. 1991). Comprehensive review 

articles focus on harvesting practices (e.g., Bigford 1997; Beentjes and Baird 2004; Tarnowski 

2006), generalized effects of mobile fishing gear (e.g., Stevenson et al. 2004; Løkkeborg 2005; 

Steele et al. 2005) and environmental implications of bivalve culture (e.g., Spencer et al. 1996, 

1997,1998, Bartoli et al. 2001; Olin 2002; ENVIRON 2007). Experimental studies have 

investigated the effects of harvesting and fishing-related disturbance (e.g., Rumohr and Kujawski 

2000; Gilkinson et al. 2005; Morello et al. 2005; Fahy and Carroll 2007). This broad body of 

scientific and historical literature indicates a strong and sustained interest in effects of shellfish 

cultivation and harvesting practices on marine environments. Few studies, however, have 

focused on more contemporary shellfish cultivation methods conducted on leased beds using 

intermittent dredging. 

Many research studies have examined the physical, biological and chemical effects of 

mechanical and hydraulic shellfish harvesting on seafloor habitats. Field experiments have 

compared a variety of dredges, locations, substrates, and habitats at differing spatial and 

temporal scales. Understanding the effects of dredging requires knowledge of the gear-specific 

impacts on differing habitat types, the frequency and geographic extent of harvest disturbance, 

and the biological and physical attributes of affected habitats (Steele et al. 2005). Although 

dredging initially disturbs benthic habitat, the rate and extent of ecological recovery vary widely. 

Not all harvesting methods and dredge styles produce identical effects on the seafloor and so 

observed impacts are not always consistent across studies. Early published research on shellfish 

harvesting does not reflect the modern improvements in fishing equipment and techniques 

currently used. The diverse range of human activities occurring in the coastal zone can make it 

difficult to isolate effects of harvesting from industrial, agricultural, and other anthropogenic 

influences (Blaber et al. 2000). Although efficiency of mechanized shellfish dredging has been 

well described, the ecological effects on the benthic community are not as thoroughly understood 

(Thrush and Dayton 2002). Impacts of shellfish dredging can be contradictory when certain 

effects have both beneficial and detrimental consequences within the benthos (Dorsey and 

Pederson 1998). For example, while dredging may initially damage certain organisms, others, 

including scavengers and opportunistic predators, benefit by feeding on exposed prey or by 

colonizing newly exposed bottom surfaces (Rheault 2008). These complex factors have 

contributed to the variety of conflicting viewpoints associated with dredging impacts. 

This review of the literature addresses the ecological effects of dredging for molluscan 

shellfish on marine habitats and benthic communities along the Atlantic and Gulf Coasts of the 

United States. We focus primarily on the nearshore subtidal dredge fisheries for eastern oysters 

(Crassostrea virginica) and hard clams or northern quahogs (Mercenaria mercenaria), which are 

conducted primarily on leased grounds in shallow water (2 – 13m). We have also included 

information on effects of dredge harvesting of softshell clams (Mya arenaria), since this has 

been well studied. Related studies that illustrate harvest impacts of other molluscan species are 

also included. This document does not directly address the offshore harvest fisheries for sea 

scallops (Placopecten magellanicus), Atlantic surfclams (Spisula solidissima), or ocean quahogs 
3 

(Arctica islandica), although they are discussed where relevant. The body of literature available 

concerning shellfish cultivation, dredging, and fishing disturbance is extensive. However, care 

should be used when drawing broad conclusions based on results from one discrete study or gear 

type since research methods and measures used to assess effects of harvesting vary among 

studies. This review presents a selection of the relevant literature and seeks to provide a 

comprehensive overview of the risks and benefits associated with shellfish dredging. 

Evolution of Shellfish Harvest and Cultivation 

Archeological evidence dating as far back as 150,000 years suggests that early 

subsistence harvest of shellfish may have affected the population structure of marine bivalves 

and altered near shore coastal ecosystems (Rick and Erlandson 2009). Native Americans hand 

collected clams and oysters in shallow coastal waters (MacKenzie 1997) and later fished with 

rakes and tongs from canoes and skiffs to access deeper waters. Oyster gathering by Native 

American Indians in precolonial times and the resulting shell middens found throughout New 

England indicate the importance of oysters in the diets of the indigenous people (Ingersoll 1881). 

There is speculation that hunter-gathers may have increased resource productivity by using 

primitive aquaculture to cultivate bivalves (Rick and Erlandson 2009). Shellfish harvesting 

practices have likely modified inshore habitats from their preanthropogenic composition 

(Simenstad and Fresh 1995). Land-based processes, including farming, deforestation, and 

pollution, impacted the marine environment before baseline studies of benthic communities 

structure were conducted (Jones 1992). This implies a longstanding role for humans in habitat 

and resource management and makes it difficult to say with certainty what represents a 

predisturbance community in the coastal zone. 

Urbanization over past centuries has fundamentally changed water quality in many 

estuarine ecosystems and affected the North American oyster fisheries. Growth and expansion of 

human populations along the coastline is directly correlated with the collapse of oyster fisheries 

in the United States (Kirby 2004). The historic declines in oyster landings observed from 1890-

1940 have been attributed to a decreased consumer demand following food-borne illnesses from 

shellfish, downward national economic trends, and direct impacts to oyster habitat such as 

increased siltation and extensive dredge harvesting (MacKenzie 2007). Oyster diseases, such as 

MSX and Dermo, further reduced harvestable natural stocks (Andrews 1984). Extensive oyster 

dredging in the harvest fishery which occurred from 1860-1920 may have destroyed up to 75% 

of Chesapeake Bay’s natural oyster reefs (Paynter 1996). Worldwide, as many as 85% of 

naturally occurring oyster reefs have been lost (Beck et al. 2011). Many factors, including 

channel and harvest dredging, have contributed to the loss of reef complexity and vertical 

structure. Currently, 75% of the remaining global catch of wild native oysters originates in the 

East and Gulf Coast regions of North America. 

Early shellfish harvest methods have been modified over the last two centuries 

(MacKenzie et al. 2002a). European colonists first used treading and short rakes to retrieve clams 

and oysters from shallow waters. From the 1700-1900s, fishermen relied on hand-operated tongs 

and rakes for harvesting hard clams (MacKenzie et al. 1997). Although shellfish dredging with 

gear towed from vessels is the focus of this paper, it should be noted that manual harvest 

methods are still practiced widely in areas where dredging is impractical or prohibited. For 

example, intertidal softshell clam fisheries are collected by hand-raking in Maine, New 

Hampshire, and Massachusetts while bull-raking to harvest hard clams is used by Long Island 

baymen in New York and quahoggers in Rhode Island. Hand-operated patent tongs, still used 
4 

today in Chesapeake Bay (Mann et al. 2004), were mechanized in the 1970s by adding a 

hydraulic piston closing device to increase harvesting efficiency. The history and status of 

fisheries in the eastern United States have been extensively reviewed by Mackenzie (1997) and 

MacKenzie et al. (2002a, 2000b). 

Increasingly, over the last century, the use of aquacultural cultivation for production of 

shellfish in leased areas has replaced the direct harvest of natural stocks. Presently, there are two 

main approaches to commercial molluscan aquaculture in the United States. Some spatiallyintensive, shallow-water operations cultivate hatchery- reared seed by using bags, cages or nets 

to exclude predators. Other more spatially extensive operations rely on natural set or hatchery 

seed that are planted on leased beds, which are eventually dredge harvested. For example, in 

Connecticut oyster seed is dredged from estuaries and then replanted on leased grounds to grow 

until harvest (Getchis et al. 2006). The state also supports a robust hard clam industry, reliant on 

leased beds (Connecticut Department of Agriculture 2011). 

Types of Shellfish Dredging Gear 

Contemporary on-bottom shellfish cultivation uses rake-like dredges to harvest planted 

shellfish seed or to collect naturally recruited stocks from leased beds. Detailed descriptions and 

illustrations of different types of dredges and description of their evolution are provided by 

MacKenzie et al. (2002a). Dredges vary in design, dimensions, and weight depending on target 

species, sediment type, and whether they are harvesting infauna or epifauna. Mechanical or dry 

dredges scrape the seafloor, while hydraulic dredges use pressurized water systems to first loosen 

sediments (Hawkins 2006). Shellfish are collected by towing the dredge slowly, usually in a 

circular progression over the bottom. A conventional mechanical dredge is towed at the end of a 

line and collects the catch in a chain bag. Hydraulic dredges use water jets to loosen sediments 

and then remove clams and oysters into a collecting bag as it is towed. Another type of hydraulic 

dredge uses suction to lift oysters mechanically off the beds (Nelson et al. 1948). Dredge style 

and fishing methods vary considerably along the eastern seaboard and Gulf Coast corresponding 

to local harvesting traditions. 

Oysters 

Mechanical oyster dredges generally consist of a steel frame, a blade with teeth, a tow 

chain with wire, and a bag attached to the frame which collects the catch (Nelson 1927; NRC 

2002; Stevenson et al. 2004). Dredge teeth penetrate soft bottom to capture partially buried 

epifauna and remove oysters and shell directly from the substrate surface. To perform efficiently, 

oyster dredges have considerable weight, proper tooth angle, and are dragged slowly over the 

bottom by the vessel with the proper amount of scope (line length relative to water depth). 

Oyster dredge efficiency can vary from 7.6-85% depending on substrate type and the size of 

harvested oysters (Powell et al. 2002). In the collection of seed oysters, the speed and duration of 

dredge tows are often modified to improve harvest efficiency on differing bottom types (Getchis 

et al. 2006). Overfilling of the dredge and damage to the bottom can be avoided with a proper 

awareness of bottom type, water depth, and tidal cycle (Nelson 1927). To improve catch rates 

and loosen bottom sediments, dredgers often pass over the same area multiple times (Krauter 

pers. commun.). Oyster dredging represented an improvement over hand harvest since it allows 

for efficient collection and offloading of the catch, reduced manpower and equipment wear, 

minimized damage to the oyster crop, and effective control of oyster pests (Nelson et al. 1948). 
5 

Suction dredges have been used to collect juvenile oysters and cultch and can be more 

efficient than traditional oyster dredges. These dredges are used to transplant oysters for 

growout, to relocate shell and cultch material, and to clean leased grounds of predators (Ismail 

1985). Acting as a large vacuum cleaner, hydraulic suction dredges pump water from the sea 

bottom into a hose and efficiently lift shellfish (Powell and Ashton-Alcox 2004). In the process, 

surface material is vacuumed up, washed onto a conveyor, screened to remove mud and sand, 

and then oysters deposited on the boat deck. 

Clams 

The mechanical (dry), “rocking chair” dredge, was developed to harvest hard clams in the 

mid-1940s (MacKenzie et al. 2002a) but was eventually replaced by more efficient hydraulic 

dredges beginning in the late 1950s. High pressure water jets loosen clams from sediments which 

are collected in a chain mesh bag as the dredge bar passes beneath the sediment surface. Water 

pressure must be sufficient to loosen clams from the substrate, but not so great that clams are 

damaged (Jolley 1972). The performance of hydraulic dredges can vary with sediment type, 

exhibiting greater efficiency when used on sand as compared to mud bottom (Jolley 1972). The 

manner of fishing, along with the design of the dredge, can determine harvesting efficiency and 

the degree of seafloor impact (Williams pers. comm. 2009). In Connecticut, typical hydraulic 

dredges weigh approximately 200 - 400 kg (450 – 900 lbs) and are about 0.6 – 1.0 m (24 – 40 in) 

wide. 

The hydraulic escalator dredge has a conveyer situated alongside the vessel with a sled 

connected at the front (North Carolina Department of Marine Fisheries 2001), so that as the 

escalator is lowered to the bottom, the sled can travel across the seafloor. A blade attached to the 

sled penetrates several centimeters into the sediments and collects the clams as they are 

dislodged by water pressure. Hydraulic water jets liquefy the sediments and clams are carried by 

the conveyer belt to the surface for culling, while undersized clams and debris are returned to the 

water (Adkins et al. 1983). The escalator dredge has a highly efficient collection rate, covers the 

ground rapidly, and can harvest continuously with little to no damage to market sized animals 

(Manning 1960; MacPhail 1961ab; Coen 1995). Efficiency of the hydraulic escalator shellfish 

harvester is affected by towing speed, sediment type and compactness, and hydraulic nozzle 

angle and height. 

Although the hydraulic escalator dredge can be used to harvest oysters and other species, 

it was first used in Maryland during 1954 for harvesting populations of softshell clams 

(MacKenzie et al. 2002a) and modified in 1955 to harvest softshell clams in shallow water on 

Canadian intertidal flats (MacPhail 1961ab). A similar hydraulic clam rake also developed in 

Canada was used to harvest clams and could be hand operated by fishermen standing in shallow 

intertidal flats (MacPhail and Medcof 1962). Presently, softshell clam populations in the 

Chesapeake Bay are at remnant levels and are no longer commercially viable (Homer et al. 

2009). 

Shallow water “clam-kicking” unique to North Carolina, is used to harvest natural 

populations of hard clams (Guthrie and Lewis 1982). Hard clams are blown from the bottom by 

wash from the boat propeller and collected in a towed, heavily chained trawl or dredge. Turbidity 

and habitat damage associated with clam-kicking is of environmental concern, especially for 

submerged aquatic vegetation (Peterson et al. 1987; Street et al. 2005). 
6 

Wild harvested shellfish 

Much larger and heavier hydraulic dredges are used in offshore fisheries to harvest 

commercially valuable clam species including surfclams and ocean quahogs (Stevenson et al. 

2004). For harvesting sea scallops in the offshore waters of George’s Bank and the Mid-Atlantic, 

New Bedford style dredges are commonly used. These dredges are approximately 4.3m (14 ft) in 

width, can weigh as much as 1 MT (2,200 lbs), and are sometimes fished in pairs (Stevenson et 

al. 2004). Scallop dredges are steel-framed structures with a cutting bar on the leading edge 

which rides above the surface of the substrate, kicking up sea scallops and collecting them into 

an attached bag, commonly made of steel rings (DeAlteris et al. 2000). Scallop dredges operate 

most efficiently on soft, flat, muddy sediments and are least efficient on firm, uneven, sandy 

sediments (Currie and Parry 1999). Dredge size, towing speed, and length of tow vary with water 

depth and scallop density (Stevenson et al. 2004). 

Small, light, mechanical dredges have also been used to harvest bay scallops (Argopecten 

irradians) in shallow waters of the northeastern United States, as well as in North Carolina and 

Florida (MacKenzie 2008). These dredges are designed to remove adult scallops from areas often 

containing dense submerged aquatic vegetation. Dramatic decreases in bay scallop populations 

along the East Coast since the mid-1980s have limited commercial dredge harvest. 

Historical and Anecdotal Observations on Dredge Harvesting 

Historically, opinions on the best methods for shellfish dredging have varied widely. 

Fishermen who used hand tongs or rakes often considered dredging detrimental to shellfish, 

while those who operated dredges believed that dredging of the seafloor enhanced the 

environment for clam and oyster recruitment (Glude and Landers 1953; Manning and 

Dunnington 1955; Visel 1990b). Rake and tong fishermen believed specifically that dredging 

activities could injure, smother, or kill shellfish; increase exposure to predation; interfere with 

successful recruitment; destroy eelgrass; or damage the seafloor (Glude and Landers 1953; 

MacKenzie et al. 2002b). In contrast, dredge operators felt that bottom cultivation improved 

seafloor conditions, kept sediment from becoming too compact for habitation by clams, and 

enhanced bivalve recruitment and growth rates. Anecdotal observations by fishermen have 

suggested that dredging may be more efficient than hand tonging or raking and results in less 

shell breakage, mortality, and unintentional burial than manual techniques (Kyte and Chew 1975; 

Coen 1995). Further benefits of dredge harvesting may include reduced mortality among target 

species, decreased impact on the benthos, increased catch rates, and reduced labor as compared 

to nonmechanical collection methods (Coen 1995). 

Observations made by shellfish growers can provide valuable data on harvesting impacts 

and recovery. Harvesters sample the seafloor each time their gear is deployed (Cranfield et al. 

1999), a level of effort which exceeds sampling regimes associated with most experimental 

studies (Dorsey and Pederson 1998). Access to local knowledge held by fishermen allows for a 

better understanding of habitats when selecting areas for study (Peterson et al. 1995). Frank 

Dolan, a Connecticut clammer for over fifty years, associated poor clamming and high clam 

mortality with acidic sediment conditions. He monitored acidity by rubbing clam shells together 

in a bucket of seawater; formation of a white cloud indicated “sour” or low pH substrate, while 

shells from healthy beds produced no cloud when rubbed together (Visel 1990a). Dolan indicated 

that spreading a light coating of shell to remediate or “sweeten” sour bottom could improve 

setting of shellfish. These anecdotal observations suggest that dredging and adding shell may 

actively increase pH levels. Dolan also believed that differences in sediment pH can impact shell 
7 

morphology and observed that clams grown in “sweet” areas exhibited sharp edged shell margins 

compared to slower growing clams with blunt shell edges found in “sour” areas of little or no 

shell cover. He found that rotation of shellfish beds, light shelling, and allowing a 5-7 year 

waiting period before harvest were key elements to good shellfish production (Visel 1990a). He 

avoided detrimental practices such as early harvest, which exposes small clams to predation, 

planting in water deeper than 30 ft and cultivating in areas with too much clay. Dolan found 

cultivation of oysters to be associated with improved growth and enhanced production of hard 

clams. Interviews conducted with long time clammers and oystermen provide a rich history of 

observations concerning habitat and resources and suggest that shellfish cultivation may improve 

bottom environments and increase clam and oyster abundance (MacKenzie 1979; Rice et al. 

1989). 

Anecdotal evidence suggests that disturbance from large-scale coastal storms is often 

followed by large natural clam sets, likely because of removal of surface detritus to reveal clean 

sandy bottom (Visel 2008, 2009). Storms and hurricanes in the marine environment act much 

like forest fires in terrestrial habitats by facilitating succession from one type of community and 

habitat to another through wave action and sediment transport. Natural storm disturbance, which 

washes silt and organics off the seabed, mimics the action of hydraulic dredging (Visel 2008, 

2009). Extended periods without benthic disturbance can reduce sediment quality with a 

corresponding decline in clam populations. The concept of “marine soil cultivation” using 

dredges, rakes, and tongs has long been advocated by the shellfish industry to loosen and 

oxygenate sediments and to remediate unoxygenated and heavily silted bottom devoid of clams. 

A lack of shellfish cultivation in certain coastal areas of Long Island Sound may have resulted in 

reduced shellfish sets and a loss of potentially harvestable clams and oysters (Visel 1990b). 

Cultivation efforts which remove silt from cultch may create good settlement habitat for oyster 

spat and may benefit clam recruitment by increasing sediment pore size to improve water 

circulation (Visel 2006). Many fishermen believe mud bottoms are detrimental to shellfish and 

that turning over of oyster shell matrix improves clam and oyster production (Lenihan and 

Micheli 2000) by promoting settlement of larval shellfish (Coen 1995) and keeping the bottom 

free of silt and organic matter (Ingersoll 1881; Visel 2006). 

Distinguishing Nearshore Shellfish Cultivation from Offshore 

Harvest Fishing 

In the offshore finfish trawling and molluscan dredge fisheries, shared resources are 

harvested continually from common grounds over a broad geographic area (Northeast Region 

EFHSC 2002; Cashin Associates 2008a, 2008b). Offshore dredges, which are larger and heavier 

than inshore hydraulic dredges, affect a greater portion of the seafloor (Hawkins 2006). 

Harvesting of nearshore cultivated beds with high shellfish densities is generally intermittent and 

of short duration, in contrast to the wild harvest of shellfish at low densities, which often 

necessitates a more intensive fishing effort. Biological impacts resulting from finfish otter 

trawling and scallop dredging are often more evident because these activities are conducted in 

lower energy deeper water environments where sessile emergent animals dominate (Hill et al. 

1999; Kaiser et al. 2000). Deepwater hydraulic dredging for ocean quahogs and surfclams on 

sand bottom may pose fewer environmental concerns than when conducted on other sediment 

types (MacKenzie 1982; Wallace and Hoff 2005). For these reasons, the analogy that mobile 

fishing gear disturbance of the seabed during wild harvest is equivalent to forest clear cutting 

(Watling and Norse 1998; Baulch 1999; Watling 2005) is not directly applicable to impacts of 
8 

small-scale nearshore shellfish cultivation, which is typically conducted on leased grounds, 

where access is limited to the leaseholder and grounds remain undisturbed until shellfish reach 

market size. Following commercial harvest, replacement of cultch or seed oysters and natural 

reseeding of clam beds can restore physical structure, enhance habitat productivity, and promote 

resource sustainability. 

Shellfish Dredging Differs from Navigational Dredging 

Dredging used to cultivate and harvest shellfish is sometimes confused with harbor or 

channel dredging projects. These practices are very different and are conducted at vastly 

different spatial scales. Hydraulic shellfish dredging superficially penetrates the sediment while 

navigational dredging projects deepen channels and harbors by removing significant quantities of 

dredge spoils from the seafloor to barges for disposal (Tarnowski 2006; Visel 2008). Shellfish 

dredging in a given area is usually conducted over hours or days whereas navigational dredging 

may continue for weeks or months. Short term mechanical shellfish harvesting takes place during 

daylight hours (Kennedy and Breisch 1973) but navigational dredging can occur around the 

clock at high levels of intensity. Deep channel dredging is often conducted in highly 

industrialized zones where sediments are likely to be contaminated. Therefore, harbor dredging 

and channel deepening projects may potentially result in a greater environmental impact than do 

smaller scale shellfish dredging and cultivation. 

Shellfish Cultivation Similarities to Terrestrial Agriculture 

Shellfish dredging can break up hard-packed sediments, just as farmers till fields to turn 

over and aerate soil (Nelson 1927). Following harvest of clams or oysters, beds are generally left 

undisturbed for several years or reseeded and not dredged again until the young reach 

commercial size. This provides much the same benefit as when upland fields are allowed to rest 

between planting of crops (Ingersoll 1881). When shellfish beds are allowed to remain 

undisturbed, the temporary alterations in benthic community structure caused by dredging revert 

to preharvest conditions (Olin 2002).The ecological communities associated with shellfish 

cultivation may differ somewhat from the initial benthic fauna, just as occurs in traditional 

agriculture when fields are cleared for replanting (Watling 2005). Although frequent harvest can 

prevent benthic succession to climax communities (Watling and Norse 1998), the intention of 

both terrestrial and shellfish farming is to cultivate a particular crop rather than to allow the site 

to reach a climax community. 

Historically, productive shellfish beds are known to support an abundant benthic biota 

(MacKenzie et al. 1997). Surveys of benthic communities have shown high invertebrate 

populations and diverse species composition in areas where shellfishing has occurred for more 

than 50 years (Bigford 1997). Northern quahog clam populations have shown high resilience 

following harvest-related reductions, attributable to large breeding stocks, successful larval 

settling, good survival of seed, and low harvesting mortality among juvenile clams (Vining 

1978). Shellfish farmers must manage leased bottom responsibly since successful clam farming 

depends on sustainable harvesting of product (Vining 1978) and healthy seafloor environments. 
9 

Scale of Disturbance, Faunal and Habitat Sensitivity, and 

Gear Considerations 

Spatial and temporal scales of shellfish dredging activity are important considerations 

when assessing the potential impact on habitat. Shellfish cultivation generally occurs on small 

spatial scales, relative to a much larger surrounding coastal marine ecosystem. Leased shellfish 

beds typically make up a very small percentage of the total available seafloor of the coastal zone 

in the United States. For example, in Connecticut, only 9% of the seafloor (77,000 acres) is 

under lease within the 844,800 acre Long Island Sound. At any given time, much of this leased 

acreage is not actively fished, reducing the dredging impact further. For these reasons, the shortterm, immediate effects of dredging and subsequent ecological recovery represent a series of 

relatively brief events with impacts limited to discrete portions of the coastal zone. In a review of 

experimental studies on shellfish dredging, Tarnowski (2006) compares the extent and duration 

of impacts associated with anthropogenic and natural disturbances to illustrate the wide range of 

spatial and temporal scales. 

Evaluating ecological responses to harvesting disturbance requires an understanding of 

the sensitivity (the ability of an organism to withstand physical damage) and susceptibility (the 

probability that an organism will be exposed to damage) of resident organisms as well as an 

assessment of the intensity (frequency and magnitude) and severity (nature of the impact on the 

biotic community) of the event (NRC 2002; Tarnowski 2006). Susceptibility of benthic habitats 

to damage, recovery rate among species, substrate type, time scale, resident biological 

community, water depth, and presence or absence of vegetation all contribute to the level of 

dredging impact sustained by the environment (DeAlteris et al. 2000; Barnette 2001; Beentjes 

and Baird 2004; Cashin Associates 2008a, 2008b; Constantino et al. 2009). Severity is 

influenced by substrate characteristics, seafloor response to disturbance, faunal and floral 

sensitivity and distribution, frequency of fishing, gear attributes, and life history characteristics 

(Kaiser et al. 1989; Langton and Auster 1999; Olin 2002; Rheault 2008; Wilber et al. 2008). 

Magnitude of physical disruption to the seafloor is contingent on the type of mechanized 

harvester as well as depth of gear penetration; frequency of fishing; towing speed; and how the 

equipment is rigged, modified, and fished (Meyer et al. 1981; Gaspar et al. 1998; DeAlteris et al. 

2000; NRC 2002). For example, when gear was properly rigged and catch efficiency was high, 

clam dredging caused only minor damage and mortality to macrobenthic organisms within a 

dredge track (Gaspar et al. 2003). Dredge efficiency, which is determined by towing speed, depth 

of the cutting blade, and hydraulic flow rate, varies with location, bottom type, shellfish density, 

gear design, weather and operator skill (Jolley 1972; Smith and LeBlanc 1976; Smolowitz and 

Nulk 1982; Lambert and Goudreau 1996; Hawkins 2006). Pump performance can influence 

harvesting efficiency since insufficient water pressure fails to effectively remove clams, while 

too much pressure can be damaging (Jolley 1972). Contemporary hydraulic dredges are highly 

efficient, collecting up to 90% of clam biomass in a single tow when fishing is optimized 

(Hauton et al. 2007). The first tow of a dredge is expected to have greater impacts than 

subsequent passes (Northeast Region EFHSC 2002); however, the effects of shellfish dredging 

are generally cumulative (NRC 2002). 

Effects of Shellfish Dredging Are Variable 

Variability in harvesting practices, local environmental conditions, and differing benthic 

community characteristics explain the broad range of physical, biological, and chemical impacts 
10 

documented in the literature. Harvesting devices vary in their design and implementation, and 

seafloor characteristics differ among habitats. Disturbance is seldom consistent across a 

harvested area since portions of dredged bottom can remain unharvested while others may be 

worked multiple times (Vining 1978). Systemic dredging of seafloor is difficult because it is 

impossible to physically mark the exact path travelled by the dredge (Vining 1978). Evidence of 

mechanical harvest can vary with some areas appearing undisturbed while others are crisscrossed 

with dredge tracks and depressions (Klemanowicz 1985; Manzi et al. 1985). This may account 

for differences in the diversity, distribution, and abundance of benthic organisms sometimes 

observed across dredged zones (Chícharo et al. 2002a) and can complicate assessment of 

dredging impacts. Variability in performance of mechanized gear during towing may also result 

in changes to macrofaunal communities that are not uniform along dredge tracks (Chicharo et al. 

2002a). An understanding of gear specific effects on various habitat types (experimental studies), 

frequency and geographic distribution of tows (fishing effort), and physical and biological 

attributes of the benthos (seafloor mapping) is necessary when evaluating ecosystem effects of 

dredging (NRC 2002). 

Experimentally Measuring Ecological Change 

Designing a field experiment to evaluate impacts of dredging or fishing is challenging, 

since the goal is to detect change in a dynamic system. Impacts are often assessed by conducting 

experimental dredging and measuring the benthic community response. Data are compared 

before and after disturbance, against an undisturbed control site, or by comparing experimental 

results to historical data for that area (Watling and Norse 1998; Johnson 2002; Løkkeborg 2005). 

Preliminary “before” sampling is used to identify environmental or biological gradients that 

could complicate analysis and to determine the sampling effort required to detect a given level of 

change for a variable (Kaiser 2005). Community response is often mediated by the source of 

disturbance and site-specific factors, such as site history and the inherent ecological plasticity of 

many benthic species, and these factors can make detection of subtle ecological change difficult 

(Whomersley et al. 2010). Sampling methods, such as BACI (Before-After and Control-Impact), 

are used to distinguish natural change from that associated with human activities by statistically 

assessing and determining the ecological significance of a given impact (Hewitt et al. 2001). The 

principal assumption of BACI experiments is that anthropogenic disturbance alters an 

environment so that it differs both from its original state prior to fishing and from concurrent 

natural changes occurring in control sites (Underwood 1992). When experimentally dredged 

areas are compared with nearby undredged sites, it is assumed that control and experimental sites 

are equivalent (NRC 2002). Water depth, substrate type, and benthic assemblages in the 

experimental site should be similar to those of control areas (Collie 1998). Few areas within the 

coastal zone remain unaffected by anthropogenic disturbance or fishing activity, and this lack of 

undredged areas complicates interpretation of harvesting impacts on benthic habitat and marine 

resources (Kaiser et al. 1998; NRC 2002). Dredge types and designs, disturbance regimes, 

bottom substrates, natural disturbance levels and biological communities are seldom identical 

(Løkkeborg 2005), and because each experimental protocol is unique, these differences can 

complicate comparisons across studies. 

Further, the complex nature of field studies can result in experimental design weaknesses. 

Differences in fishing intensity across a study area can introduce variability, and lack of 

appropriate experimental replicates can overestimate impacts (Collie 1998). The short time frame 

of some studies may limit data on recovery, and it can be difficult to relate experimental plots to 
11 

the spatial and temporal scale of actual dredging disturbance (Hall and Harding 1997; Langton 

and Auster 1999; NRC 2002; Gilkinson et al. 2003; Løkkeborg 2005). Studies of fishing 

disturbance and benthic communities should be conducted at a scale relevant to the commercial 

fisheries (Queiŕos et al. 2006). In studies where recent experimental fishing is compared to 

historical data, availability of archival information on species abundance and composition prior 

to disturbance may be limited (NRC 2002). Although time series analysis can assess changes in 

community structure, distinguishing fishing effects from other environmental or anthropogenic 

perturbations is not always possible (Kaiser et al. 1998). Natural changes in control or unfished 

sites can interfere with comparisons against fished areas, and since large temporal and spatial 

variations occur in nature, anthropogenic disturbances may be masked by more dominant natural 

processes (Løkkeborg 2005). Benthic community structure maintains some fluidity from year to 

year because of changes in local conditions which influence reproduction, survival, and 

recruitment processes, so it may be hard to differentiate these natural variations from dredgeinduced effects (Gaspar and Chícharo 2007). Most studies of fishing effects have been conducted 

in shallow water on small spatial scales, study acute rather than chronic disturbance, look at short 

term response, and focus on animal communities rather than ecosystem level processes (NRC 

2002). The interrelationships among fishing intensity and frequency, fishing methods and gear, 

seafloor structure, productivity, abundance of economically important species, and community 

diversity are not easily resolved (Gilkinson et al. 2003). Even when fishing effects are not 

detected, it cannot always be concluded that no impact has occurred (Løkkeborg 2005). 

EFFECTS OF SHELLFISH DREDGING 

In the following sections, we review effects as reported in the literature. Dredging effects 

are divided into broad physical, biological, and chemical categories. Impacts reported here have 

been compiled from studies conducted under different geographic and environmental conditions, 

spatial scales, with many types of dredge equipment. For this reason, it is important to consider 

the specific details and context of each study when drawing general conclusions concerning the 

effects of shellfish harvest dredging. Some of the relevant research studies examining dredging 

effects on specific shellfish species and habitats are summarized in an appendix to this document 

(Tables 1-4). Target species; habitat and sediment type; harvesting equipment used; study 

location; biological, physical, and chemical impacts; recovery; and author citations are provided 

for each study. Tables 1-4 provide an ecological and geographic context for research and 

experimental studies investigating the effects of shellfish dredging. Genus and species names 

listed in the tables reflect current accepted nomenclature and may differ from earlier versions of 

scientific names which appear in the original cited literature. 

Physical Effects of Dredging 

The action of mechanical and hydraulic shellfish dredges physically disrupts benthic 

substrate. Harvesting may suspend sediment, increase turbidity, alter substrate composition, and 

cause sediment plumes. Dredges may leave behind tracks of varying depths and widths along the 

bottom that persist until sediments are restored by natural processes. In the following section we 

review studies that report specific gear effects and information on the time required for recovery 

of the physical structure of habitat. 
12 

Suspension of Sediment, Increased Turbidity, and Sediment Plumes 

Shellfish harvesting can have a direct effect on water quality by suspending sediments 

and increasing water column turbidity (Dayton et al. 1995; Johnson 2002: Morgan and 

Chuepagdee 2003; Rheault 2008). Hydraulic dredges use water jets to liquefy the substrate, 

dispersing fine silt and clay into the water column, elevating turbidity, and creating a sediment 

plume downstream of operations (Kyte and Chew 1975; Kyte et al. 1976; Coen 1995; Barnette 

2001). Sediment disturbance appears greatest near the conveyer belt and water jets where 

substrate material is returned to the bottom (Kyte and Chew 1975). Elevated turbidity and 

sediment plumes may extend 75-100 ft beyond the dredge zone (Manning 1957; Haven 1979; 

Manzi et al. 1985; Maier et al. 1998), transporting and redepositing sediment into adjacent areas 

(Vining 1978). In most cases, suspended solids returned quickly to low values moving away 

from dredge activity (Kyte et al. 1976; Maier et al. 1998) with 98% resettlement occurring within 

50 ft (15m) (Rheault pers. commun. 2010). Suspended sediment dispersed during suction 

dredging returned to background levels within 40 m of harvest activity (Spencer 1997). Effects 

of sediment dispersal, turbidity, and plume formation appear to be transitory as most plumes 

dissipate within a few hours of dredging (Maier et al. 1998). Occasionally, sediment may remain 

unsettled for a more extended period after cessation of dredging (Ruffin 1995; Tarnowski 2006). 

Particles suspended in plumes dissipate exponentially over time, rapidly at first as heavy 

coarse sediments (pebbles, shell fragments, coarse sand) settle out, followed by progressively 

smaller particles (medium to fine sand) and lastly silt and clay (Goodwin and Shaul 1980; Ruffin 

1995). Larger sand particles are redeposited near the dredge while measurable amounts of fine 

silt and clay particles remain in suspension and may be carried away by currents (Godcharles 

1971; Tuck et al. 2000). Sediment grain size, wave activity, current levels, and water column 

depth all determine the initial turbidity, light attenuation, size, and dissolution of dredge plumes 

(Ruffin 1995; Tarnowski 2006). Substrate type can determine the amount of suspended solids in 

a plume and how long it persists, while the distance and direction of the plume is primarily 

controlled by water currents (Tarnowski 2006). The volume of displaced sediment determines 

the concentration of suspended particles. Mechanized harvesting can increase siltation at lower 

depths (Rothschild et al. 1994; Breitburg et al. 2000; Street et al. 2005) by winnowing of fine 

sediments by strong currents and wave action (Cranfield et al. 1999). The largest plumes, highest 

turbidity, greatest light attenuation levels, and slowest plume decay rates are generally produced 

in shallow water environments containing high silt and clay content (Ruffin 1995; Tarnowski 

2006). 

Suspension of sediment by dredging can affect many aspects of the benthic environment. 

Suspended sediment and turbidity plumes temporarily degrade water quality and intensify 

bottom siltation (Cashin Associates 2008a, 2008b). Turbidity transports and redistributes 

substrate material (Kyte and Chew 1975; Manzi et al. 1985) resulting in reduced water clarity 

(Watling and Norse 1998). Dispersed sediments may take 30 min-24 h to resettle (Lambert and 

Goudreau 1996; Northeast Region EFHSC 2002). Compared to long-term, natural wind-induced 

suspension of sediments and nutrient loading from land runoff, release of suspended sediments 

during dredging can be relatively minor (Auster and Langton 1999). Localized effects can, 

however, be more significant where low dissolved oxygen levels occur and reduced substances 

become resuspended. Natural turbidity levels generated by wind and tides can produce particle 

loads equaling or exceeding that of dredging disturbance (Tarnowski 2006) which are generally 

tolerated by organisms inhabiting dynamic inshore environments. In the Thames River of 

Connecticut, sediment suspended during storm events exceeded that of navigational dredging 
13 

projects by an order of magnitude (Bohlen et al. 1979). The impact of smaller scale shellfish 

dredging would be even less significant, as bottom disruption is limited to a more discrete area 

than that impacted by either coastal storms (Wilber and Clarke 2001) or navigational dredging. 

Disruption of the Sediment Surface

Movement of dredges across substrate or through the sediments has the potential to 

reduce bottom roughness, smooth surface topography, damage sedimentary bedforms, scar the 

seabed, and modify the physical environment (Pranovi and Giovanardi 1994; Dorsey and 

Pederson 1998; Langton and Auster 1999; DeAlteris et al. 2000; Emmett 2002; NRC 2002; 

Morgan and Chuepagdee 2003; Willner 2006). Dredging can erase structural features from the 

seafloor and remove burrows, tubes, and shells through destruction, burial, or sedimentation 

(Gilkinson et al. 2003; Hauton et al. 2003a). Mobile gear which scrapes the seafloor can reduce 

habitat complexity by diminishing vertical and three-dimensional structures (Watling and Norse 

1998; NRC 2002; Morgan and Chuepagdee 2003; Rheault 2008). Initial harvesting effects can be 

significant when the upper layers of the substratum and overlaying fauna are removed and 

seafloor topography becomes physically altered by the dredge (Kaiser et al. 1989). Substantive 

changes to physical structure can include smoothing of sand ripples, waves, and ridges 

(Northeast Region EFHSC 2002). Hydraulic harvesting equipment can resuspend the top 

sediment layers and bring up deep anoxic material from the bottom (Badino et al. 2004). Water 

jets from hydraulic dredges penetrate into the sediment releasing trapped gases, resorting 

sediments, and creating tracks along the bottom (Bigford 1997). The large volume of water 

channeled through hydraulic dredges to fluidize the seabed can disrupt the physical nature of the 

seafloor (Hauton et al. 2003a), create linear tracts of fluidized sand, suspend silt into the water 

column, and can vertically homogenize the seabed to a depth of 20 cm (Hauton and Paterson 

2003). 

Dredging activities can cause sediment mobility and instability, creation of spatial 

patchiness, and increased sediment load (DeGrave and Whitaker 1999). Sediment loosening, 

emulsification, and loss of vertical stratification can cause displacement of substrate in harvested 

clam beds (Goodwin and Shaul 1980). Seafloor sediments can experience increased sediment 

porosity and reduced compaction for up to a year after dredging (Pfitzenmeyer 1972a, 1972b). 

Dredging may create transient sand bars which decrease substrate compactness and stability on 

intertidal flats (Goodwin and Shaul 1980) and in deeper offshore waters (Smolowitz and Nulk 

1982). Dredging may change the appearance and texture of bottom sediments with a reduction in 

the silt-clay component (Haven 1979) or may cause a shift in sediment type (Hauton et al. 

2003b). Hydraulic suction dredging has been shown to alter sediment composition in muddy 

sand locations (Ismail 1985). Suspension of fine sediment results in qualitative as well as 

quantitative changes to the bottom with a net loss or redistribution of organic matter (Kyte and 

Chew 1975). Extended intensive harvesting may result in a long term shift in bottom 

composition (Tarnowski 2006) by increasing grain size and sorting coefficients (Fahy and 

Carroll 2007). Although horizontal movement of particles can result in their permanent 

relocation away from the point of origin (Dorsey and Pederson 1998), most material displaced by 

dredging resettles within 50 ft of the harvest zone (Rheault pers. Commun. 2010). In cases where 

medium and fine sand dominated, no differences in grain size were observed after harvest 

(Pfitzenmeyer 1972a). 
14 

Changes to Sediment Composition 

Shellfish harvesting can potentially alter the morphology and texture of seafloor 

sediments (Constantino et al. 2009). Dredging causes visible changes to sediment particle size, 

type, structure, stability, and processing (Kaiser et al. 1989; Pranovi and Giovanardi 1994; 

Dayton et al. 1995; Simenstad and Fresh 1995; Emmett 2002; NRC 2002; Willner 2006). 

Impacts of mechanized harvest are highly dependent on sediment composition since 

sedimentation rates vary in relation to many factors, including particle size. Resuspension and 

dispersal of fine particles can cause long term effects on sieve fractions (Pranovi and Giovanardi 

1994) and decrease the clay portion of the sediment (Maier et al. 1998). Hydraulic escalator 

dredging of some sand flats can suspend and disperse finer sediment fractions from worked areas 

(Kyte and Chew 1975), leading to a relative increase in larger grain sizes and a change in the 

sorting coefficients (Pranovi and Giovanardi 1994; Pranovi et al. 1998; Fahy and Carroll 2007). 

Hydraulic dredging in Chesapeake Bay resuspended and dispersed fine particles but had no 

major effect on sediment grain size or structure in areas where medium and fine sand dominated 

(Pfitzenmeyer 1972a, 1972b). Clam dredging has been shown to completely disrupt sediments by 

changing them from rich organic sand to coarse sand with broken shell (Moore and Orth 1997). 

Larger sediment particle size may be more typical of the predisturbance benthos prior to the 

addition of fine organic silt input from anthropogenic sources such as deforestation, agriculture, 

and sewage input (Rheault pers. commun 2010). Dredging can also modify hydrodynamics, 

changing erosional and depositional processes at the sediment water interface (Thrush and 

Dayton 2002), affecting patterns of water flow and altering environmental parameters such as 

temperature and salinity regime (Simenstad and Fresh 1995; Emmett 2002). High frequency of 

clam fishing by mechanical harvest in the Venetian lagoon in Italy was shown to prevent 

consolidation of sediments (Aspden et al. 2004). Conversely, in Narragansett Bay, RI, one time 

experimental hydraulic disturbance of the bottom did not significantly alter grain size 

characteristics or water content of sediments (Sparsis et al. 1993). 

Creation of Trenches and Dredge Tracks 

Hydraulic dredging can cause visible trenches along the seafloor; redistribution of 

dredged material; altered substrate composition; and creation of deep, wide and curvilinear 

furrows while the magnitude of these changes is largely determined by sediment type (Kyte and 

Chew 1975; Gilkinson et al. 2003). In southern Portugal, passage of a clam dredge over the 

bottom can produce a depression 30 cm wide and 10 cm deep (Constantino et al. 2009). The 

seafloor may appear undulating and wavy across the directional axis of dredge tracks with the 

formation of troughs and ridges (Drobeck and Johnston 1982). Dredge furrows within the tow 

path are often accompanied by sediment slumping along the sides of depressions (Falcão et al. 

2003). Dredge tracks and trenches are sometimes hard and sharply defined with spoil heaps and 

clay chunks (Kyte et al. 1976; Pranovi and Giovanardi 1994) or may have sharply angled walls 

and a flat floor (Meyer et al. 1981). Crumbling of track shoulders and erosion by tidal currents 

can extend track widths. Escalator dredge harvest can dig deep trenches that crisscross the 

surface, leaving deep holes, mounds composed of side castings, and scattered empty shell along 

the trenches (Adkins et al. 1983). Poorly operated clam dredges may act like a snow plow 

pushing clams and sediment to the sides (Meyer et al. 1981) so that windrows of sediment and 

organisms form along either edge of the trenches (Northeast Region EFHSC 2002; Wallace and 

Hoff 2005). Formation of a sand buffer at the mouth of certain dredges can push sediment 

sideways affecting macrofauna differently over the dredge track (Chícharo et al. 2002a). Material 
15 

removed from the bottom by the dredge and redeposited as spoils in and around the trenches can 

remain softer than before dredging and may be less firm than bottom in nearby undredged areas 

(Kyte et al. 1976). Sediment within trenches and/or tracks may be softer, less compact, and 

vertically stratified for months after dredging (Vining 1978). Sediment in dredge tracks can 

remain fluidized for an extended period even after the trenches start to refill (Tuck et al. 2000). 

Persistence of Dredge Tracks 

Studies show variability in the time required for dredge tracks to resolve but in most case 

trenches and tracks created by harvesting dissipate quickly. Trenches are partially backfilled 

when heavier clumps of material and resuspended sediment fall off the back of the dredge during 

harvesting operations (Tarnowski 2006). Physical recovery from dredging tends to be very site 

specific and tied closely to sediment composition. For example, dredge tracks on sandy bottom 

can diminish within 24-h (Gaspar et al. 1998, 2003). Dredge tracks may persist as a series of 

shallow depressions (Meyer et al. 1981) or leave deeper marks which remain for days (Gaspar et 

al. 2003) or weeks (Manning and Dunnington 1955). Erosion and wave action can reduce the 

depth and width of dredge tracks within a month of dredging (Hauton et al. 2003a). Escalator 

dredging in Virginia created trenches 6-8 in (15.4-20.3 cm) deep which resolved in a 1-2 m 

period (Haven 1979). Ten months after hydraulic escalator harvest in Maine, a complete stratum 

had reformed, trenches had refilled, and a fine layer of soft sediments had settled in the tracks 

(Kyte et al. 1976). Following escalator clam harvest in Florida, tracks remained visible up to 86 

days (Godcharles 1971). Also, there was no evidence of permanent relocation of suspended 

sediment or changes to sediment grain size. Physical evidence of hydraulic harvest dissipated 

within 40 days of dredging in Scotland (Hall et al. 1990). Trenches began refilling within 5 days 

of hydraulic dredging, and by 11 weeks they were no longer obvious at a site in England (Tuck 

et al. 2000). Seven months after suction dredging at another English site, sediment structure was 

nearly restored via natural sedimentation processes (Kaiser et al. 1996). Also in the United 

Kingdom, trenches created by suction dredging dissipated naturally within a 3-4 m period 

(Spencer 1997). Dissipation of dredge tracks can also be depth dependent. Clam dredge tracks in 

a Portuguese study were no longer distinguishable after 24-h at 6 m depth but remained visible 

for 13 days at a depth of 18 m (Constantino et al. 2009). 

The time required for a trench to return to preharvesting conditions depends on natural 

sedimentary processes (Vining 1978). Hydrodynamics and substrate characteristics of the 

environment influence the persistence of dredge tracks. Sediment type, grain size, water depth, 

wind, presence of algae or seagrass, wave action, currents, tides, storm events, and location in 

the subtidal or intertidal zone all influence how long tracks remain visible (Manning and 

Dunnington 1955; Godcharles 1971; Northeast Region EFHSC 2002; Hauton and Paterson 2003; 

Gaspar et al. 2003; Morello et al. 2006; Tarnowski 2006; Thorarinsdóttir et al. 2008). Trenches 

can persist from hours to years as a function of the erosional characteristics of a site (Northeast 

Region EFHSC 2002; Tarnowski 2006). The temporal scale of effects is also influenced by the 

background energy of the environment so that recovery may take days in high energy 

environments and months in low energy areas (Northeast Region EFHSC 2002; Wallace and 

Hoff 2005). Dredge tracks remain for longer periods where there is reduced potential for erosion 

because of lower currents and less wave energy. 

In some cases, tracks and trenches created by dredging alter the sea bottom for a longer 

period (Kyte and Chew 1975). Depressions left behind by dredging in estuaries may sometimes 

take up to a year to refill (May 1973). Although no longer visible in video images, hydraulic 
16 

clam dredge furrows on the deep offshore Atlantic Scotian shelf were still detectable by sidescan 

sonar one year after dredging, showing a gradual degradation of furrow margins through 

slumping, active transport, and bioturbation (Gilkinson et al. 2003). According to acoustic 

reflective sonar, comparisons between dredge furrows and the surrounding seabed indicated a 

long term change to sediment structure. One year after escalator harvest in Maryland, the 

seafloor exhibited less compaction of sediments (Pfitzenmeyer 1972a), increased porosity, and 

softer substrates (Pfitzenmeyer 1972b). Another study of hydraulic escalator harvesting 

conducted in Florida found differences in sediments between dredged and adjacent areas were no 

longer detectable after 1y (Godcharles 1971). Sediment has been shown to return to predredging 

substratum within a year of oyster dredge harvest in Florida (Simon and Connor 1977). In a 

review of environmental effects of bivalve aquaculture, Olin (2002) suggests that changes in 

sediment structure associated with shellfish dredging are reversible and readily dissipate over 

time. 

Effects of Dredging on Adjacent Undredged Areas 

Suspension and redistribution of sediments from dredging can impact organisms living 

some distance away from the harvested area (Kyte and Chew 1975). In one study of the offshore 

clam fishery on the Atlantic Scotian Shelf, burrow structures made by benthic organisms 

declined in both hydraulically dredged and nearby unworked areas (Gilkinson et al. 2003). 

However, many nearshore studies have shown no impact of dredging outside the dredged area. 

Shellfish dredging in Virginia and Washington State did not impact adjacent unharvested 

eelgrass beds negatively (Moore and Orth 1997; Goodwin and Shaul 1980). Downstream from 

escalator harvesting activity in Maryland, the impact zone appeared limited, and oysters and 

cultch material remained undisturbed (Drobeck and Johnston 1982). No detrimental effects of 

siltation were observed on live oyster beds located adjacent to shellfish dredging during studies 

in Florida and Maryland (Ingle 1952; Manning 1957). Uniform redistribution of sediments 

caused by dredging did not appear to smother nearby benthic organisms during studies 

conducted in England and Florida (Schroeder 1924; Spencer et al. 1998). These studies suggest 

that in general, harvesting impacts are limited to the directly disturbed area. 

Harvest of Fossil Shell for Use as Cultch Material 

Dredge harvesting is sometimes used to uncover buried fossil shell and bring it to the 

sediment surface where it can be redistributed as cultch to increase bottom complexity and 

provide settlement substrate for larval oysters (Manning 1960; Haven 1979; Goodwin and Shaul 

1980). Hydraulic dredging may be among the least harmful ways of recovering and processing 

natural cultch shell since the process produces only minor and transitory changes to the physical 

and biological environment (May 1973). Mining of fossil oyster shell and effective redeployment 

can improve spat recruitment and has been considered useful in shellfish restoration efforts 

(Rothschild et al. 1994). 

Biological Effects of Dredging 

Many research and experimental studies have been conducted to better understand the 

biological effects of dredging. Collie et al. (2000) used a meta-analysis of 56 previous 

experimental studies to quantitatively compare relative levels of impact on benthic organisms 

across gear types, period of disturbance, scale, region of the study, depth, and habitat type. 

Among these factors, gear type, region, and taxonomic class had the greatest influence on effects 
17 

to the benthic biota. The impact of shellfish dredging on benthic organisms is species-specific 

and largely a function of their particular biological characteristics and the physical habitat. Level 

of impact resulting from hydraulic or mechanical dredges is determined by an organism’s 

relationship to the substrate (i.e., infauna or epifauna,), its activity level (i.e., mobile or sessile), 

and inherent structural morphology (i.e., soft-bodied or hard-shelled). Hard clams and oysters 

that can close their shells tightly are less vulnerable to dredging than are other mollusks with 

shells that gape. Damage to bivalves often varies depending on shell thickness and burrowing 

depth (Hauton et al. 2003a). In the following section we review effects of shellfish dredging on 

marine fauna and flora as reported in the literature. 

Mortality and Damage in Benthic Organisms 

When dredges move along the seafloor or liquefy sediments, surface dwelling organisms 

can be removed, crushed, buried, or exposed (Dayton et al. 1995; Watling and Norse 1998). 

Tubes and burrows may be scraped away and erect and sessile organisms removed from the 

substrate surface (Dayton et al. 1995; Watling and Norse 1998; NRC 2002). Direct burial and/or 

smothering of infaunal and epifaunal organisms is possible because of increased sedimentation 

rates (Coen 1995; Barnette 2001; Morgan and Chuepagdee 2003). Clam and oyster harvesting 

can result in direct mortality when shellfish pass through or under the dredge, and indirect 

mortality can occur when shellfish are caught in the dredge, washed out, or discarded (Beentjes 

and Baird 2004). Mortality of target shellfish can result from gear impact, pressure inside the net 

bags, surface anoxia, or temperature effects (Alves et al. 2003). Dredge induced mortality can 

include clams cut or crushed by the dredge blade or assembly (Meyer et al. 1981). Higher 

postdredging mortality has been associated with low catch efficiency (Gaspar et al. 1998). 

Increasing dredge efficiency by extending tooth length has reduced the number of injured clams, 

while prolonged tow length has a higher risk of damaging animals. During the 1960s in Long 

Island Sound, a minor modification of the tooth angle of oyster dredges from perpendicular to a 

15o angle minimized damage and mortality to shellfish seed (MacKenzie 1970). 

The number of clams injured by hydraulic dredge harvesting is proportional to increasing 

water pressure (Moschino et al. 2002). Sorting processes and water pressure can significantly 

elevate shell damage and stress among clams, especially in commercial sized animals (Moschino 

et al. 2002, 2003). Levels of biological damage correspond to sediment type and are often greater 

in fine sand areas as compared to coarser sand sites (Moschino et al. 2002). Shellfish have 

demonstrated resilience to dredge induced shell damage. Oysters show a capacity for rapid repair 

of minor shell chipping or abrasions from dredging (Powell et al. 2001), while clams have been 

shown to experience low mortality and minimal injury after harvest collection (Glude and 

Landers 1953; Smolowitz and Nulk 1982; Coen 1995). 

Effects of Suspended Sediment on Marine Organisms 

When sediments are disturbed, turbidity increases as fine-grained materials are 

resuspended into the water column (Maier et al. 1998). If large amounts of sediment are 

resuspended at high concentrations and exposure is chronic, impacts can be severe. Under 

extreme conditions of turbidity, visual feeders and photosynthesizing plankton may be disturbed 

(Watling and Norse 1998). Discolored water can reduce primary productivity thereby limiting 

numbers of macroscopic benthic organisms (May 1973). In some cases, turbidity can interfere 

with normal respiratory and feeding functions in benthic dwellers and result in hypoxia or anoxia 

(Morgan and Chuenpagdee 2003). Sediment resuspension can potentially reduce the survival of 
18 

bivalves and fish from gill clogging or by inhibiting their movements and/or burrowing activity 

(Dorsey and Pederson 1998). Small organisms and immobile species caught in suspended 

sediment can potentially smother as a result of sedimentation (Manning 1957). Redistributed 

sediments can rebury shell and cultch material making it unavailable to other organisms for 

settlement or refuge (Tarnowski 2006). However, most species found in shallow coastal waters 

where harvesting is conducted can be expected to tolerate extended periods of turbidity (Gaspar 

and Chícharo 2007). Studies of mechanical clam harvesting in South Carolina creeks showed no 

significant effect of short term elevated turbidity on bottom dwelling invertebrate and finfish 

assemblages (Maier et al. 1998). 

A review by Wilber and Clarke (2001) identifies the acute effects of suspended sediment 

exposure on fish and shellfish associated with navigational dredging. Laboratory bioassays 

exposed target organisms to suspended sediment at increasing concentrations and exposure 

durations until responses were measured. Survival of fish and shellfish varied with dosage from 

no effect to 100% mortality. For example, in a 2-day exposure at a relatively high sediment level 

of about 10,000 mg/l, the effect on larval bivalves ranged from sublethal to 75% mortality. Since 

the degree of sediment resuspension is proportional to the duration and magnitude of dredging, 

short term intermittent shellfish harvesting would likely have less of an effect than navigational 

dredging. 

Immediate Initial Reduction in Benthic Organisms 

Mechanical harvest may remove or spatially redistribute target and nontarget meiobenthic 

and macrobenthic organisms resulting in decreased abundance, number of taxa, evenness, 

biomass and/or diversity, and possibly altered community structure (Alves et al. 2003; 

Constantino et al. 2009). After hydraulic clam dredging, a short term nonselective reduction in 

species number, biomass, and abundance of infaunal organisms has been observed, with the 

greatest declines occurring inside dredge tracks (Simon and Conner 1977; Connor and Simon 

1979; Ismail 1985; Hall et al. 1990; Provani and Giovanardi 1994; Tuck et al. 2000; Gilkinson et 

al. 2005). Organisms may be captured directly in dredges or resuspended in the sediment plume 

and transported away by water currents (Pranovi et al. 1998). The physical action of the dredge 

removes macrofauna and smaller organisms such as polychaetes and amphipods (Kyte and Chew 

1975) by resuspension, and/or advection (Gilkinson et al. 2005). 

Some studies which documented an initial decline in species abundance and biomass, 

also observed rapid benthic community recovery. For example, declines in benthic organisms 

immediately after dredging were of short duration on subtidal clam beds in Washington State 

(Goodwin and Shaul 1980). Harvesting of manila clams (Tapes philippinarum) by hand raking 

and suction dredging caused a 50 and 90% initial reduction, respectively, in species diversity and 

abundance within an aquaculture site located in the River Exe, in Devon, United Kingdom, with 

recovery of the invertebrate community within 8 m of harvesting (Spencer 1997). Experimental 

water jet dredging in Scotland caused an immediate decline in species number, biomass, and 

number of individuals within the dredge track but this was no longer significant after 5 days 

(Tuck et al. 2000). 

Effects Vary with Faunal Composition 

Dredging and other types of seafloor disruption can alter the variety and abundance of 

organisms. Responses of the biological community to shellfish dredging are species-specific 

(Kyte et al. 1976) and can be difficult to quantify depending on whether affected organisms are 
19 

hard bodied like clams or soft bodied like polychaetes (Wallace and Hoff 2005). Dredging 

removes large bivalves and epibenthic organisms from dredge tracks, leaving smaller thickshelled animals intact while damaging fragile thin-shelled bivalves and polychaetes (Hauton et 

al. 2003a). Epifauna inhabiting the substrate surface experience greater disruption from dredging 

than do infaunal organisms which dwell within sediments (Drobeck and Johnston 1982). Some 

studies have found soft bodied, deposit-feeding crustaceans, polychaetes, and ophiuroids to be 

the most affected by dredging activities (Constantino et al. 2009). For example, in Scotland, 

water jet dredging decreased polychaete populations while amphipod numbers increased (Tuck 

et al. 2000). In a Maine study, numbers of amphipods and polychaetes were immediately reduced 

by dredging (Kyte et al. 1976). Amphipods were least affected, bivalves most affected, and 

polychaetes and ophiuroids moderately affected in a Florida dredging study (Simon and Connor 

1977; Connor and Simon 1979). The greater mobility of amphipods and the attraction of 

predators and scavenging species to the disturbed benthos inhabited by polychaetes may account 

for observed differences in dredging impacts among these species (Tuck et al. 2000). 

Dredging can impact benthic community structure (Smolowitz and Nulk 1982; Dayton et 

al. 1995; Pranovi and Giovanardi 1994; DeAlteris et al. 2000; Emmett 2002; Johnson 2002; NRC 

2002; Willner 2006) and population dynamics (Cranford et al. 2006) by increasing abundance, 

diversity, and biomass of some benthic invertebrates and by decreasing biomass and species 

richness among others (Dorsey and Pederson 1998; Morgan and Chuepagdee 2003; Rheault 

2008). Benthic community structure changes when fishing activities disrupt physical habitat, add 

food or nutrients to the benthos (in the form of discarded organisms), or reduce or remove 

populations at certain trophic levels (Blaber et al. 2000). In an Italian coastal lagoon, bottom 

sediments modified by hydraulic dredging activity showed altered community composition 

(Pranovi and Giovanardi 1994). Dredging resulted in a shift among the dominant organisms 

within a muddy marl substrate in Ireland (DeGrave and Whitaker 1999) where 6 m after 

dredging, omnivorous crustaceans became more plentiful in the dredged area while filter feeding 

bivalves were more abundant in the control site. High intensity hydraulic dredging in the 

Adriatic Sea resulted in dominance of a few organisms, but the number of species began to 

increase as fishing intensity declined (Morello et al. 2006). Changes to the physical environment 

can reduce the value of benthic habitat for certain species (Langton and Auster 1999) and force 

some species into suboptimal environments (NRC 2002) while other species may benefit. For 

example, dredge-induced resuspension of organic matter increased numbers of clams in affected 

areas of Italy (Pranovi et al. 2003). Hydraulic clam dredging can alter ambient community 

structure when target species decline and are replaced by opportunistic colonists (Fahy and 

Carroll 2007). Lack of sediment consolidation after mechanical clam harvest may inhibit 

biological community succession and establishment of stable benthic assemblages (Aspden et al. 

2004). Many benthic animals will not reestablish in a dredged area until sediment stability has 

returned (Goodwin and Shaul 1980). However, shellfish dredging does not always measurably 

alter density or species composition among small benthic macroinvertebrates because of their 

short life spans and capacity for rapid recolonization (Peterson et al. 1987). 

Shell Damage to Bivalves Caused by Dredging 

Since mechanical harvesting requires physical collection and processing of shellfish, a 

potential for handling damage exists (Nelson et al. 1948). When hydraulic dredging brings large 

numbers of clams to the surface, shells may be chipped or broken (Gilkinson et al. 2005). Razor 

clams collected by suction dredge from Mallow Bank in the Bay of Ireland contained sand grains 
20 

embedded within clefts in the shell matrix, presumably sustained during previous dredging 

events (after which they had been returned to the bottom) or while escaping dredge capture 

(Robinson and Richardson 1998). Injury to clams appears inversely proportional to catch 

efficiency or the ratio of number of clams entering the dredge to the number found along the 

dredge path (Gaspar et al. 1998). Damage to surfclams from hydraulic dredging is considered a 

function of water pressure, blade height, and towing speed (Lambert and Goudreau 1996). When 

mechanical harvesters are operated improperly or at low efficiency, some mortality of target 

bivalve species and other shellfish has been observed (Nelson 1927; NRC 2002; Morgan and 

Chuepagdee 2003). 

Conversely, some studies of hydraulic dredging have shown little breakage or mortality 

among juvenile and adult clams as a result of burial, failure to reburrow, or predation (Manning 

and Dunnington 1955; Manning 1957; Vining 1978). Only a small fraction of softshell clams 

dredge harvested in Maine were found to experience injury (Kyte et al. 1976). Less than 1% of 

clams dug with a mechanical shellfish-digger (early hydraulic conveyer dredge) were damaged 

in Canada (Dickie and MacPhail 1957). Contemporary mechanical harvesting in South Carolina 

documented minor damage to the shell matrix among 5% of harvested oysters (Collier and 

McLaughlin 1984). Repeated dredging of oysters in Delaware Bay resulted in only minor 

chipping and abrasion of shell (Powell et al. 2001). 

Dredging Effects Vary with Clam Size 

Generally, smaller clams demonstrate a greater resilience to water pressure and sorting 

associated with hydraulic shellfish harvesting than do commercial-sized animals (Pfitzenmeyer 

1972a). No significant reduction was observed in populations of clams < 35 mm shell length 

after commercial harvesting in the Adriatic Sea (Moschino et al. 2002, 2003). Ninety percent of 

small softshell clams < 2 in, collected by Maryland escalator dredge, fell through the conveyer 

mesh and were returned to the bottom within 100 ft (30.48 m) of the point of entry (Medcof 

1961). Most clams reburied quickly with fewer than 10% experiencing damage. Hydraulic 

escalator dredging of oysters showed no measurable detrimental effect on juvenile softshell clam 

populations in Maryland (Drobeck and Johnston 1982). 

Temporary Decline in Commercially-Sized Shellfish 

Shellfish dredges are typically designed to harvest bivalves of a particular size for 

market. Harvesting of commercial-sized shellfish temporarily reduces the number and biomass 

of adults of the targeted bivalve species (Rice et al. 1989; Gilkinson et al. 2005). Hydraulic 

escalator dredging for softshell clams in Maine resulted in an immediate decline in commercial 

sized clams within the harvested area (Kyte et al. 1976). Shellfish dredging in the United 

Kingdom reduced overall densities and mean length of remaining clams through preferential 

removal of commercial-sized animals (Robinson and Richardson 1998), although such 

population reductions are generally short term (Morgan and Chuepagdee 2003). Fishing 

activities decrease the abundance, change the age structure, and modify the size composition of 

the target species (Blaber et al. 2000). Although harvesting in Washington State significantly 

reduced numbers of commercial clams, abundance of smaller seed clams remained unaffected 

(Goodwin and Shaul 1978). 
21 

Sediment Type Affects Biological Effects 

The magnitude of dredging impact on benthic organisms is often a function of sediment 

type. Clams in fine grain sand generally experience more damage than those on coarser sand 

(Moschino et al. 2003). Fauna in stable gravel, mud, and biogenic habitats are more adversely 

impacted by fishing activities than those in less consolidated coarse sediments (Collie et al. 

2000). Mortality of clams in Canada was greater in compact clay sediment than in clay silt 

(Medcof and MacPhail 1964), while clams living in fine sand showed higher damage levels and 

more impaired individuals than those within coarse sand areas of the Adriatic Sea (Moschino et 

al. 2003). High energy sandy habitats with low to moderate complexity can be resistant to 

disturbance and show fewer changes in abundance and biomass of organisms following trawling 

and dredging activities (Link et al. 2005). Infauna which inhabit sandy and shallow areas are 

morphologically and behaviorally adapted to dynamic environments and show little long term 

impact from dredging, aside from an initial dispersal of organisms (Tuck et al. 2000). 

Experimental studies of disturbance in shallow sandy environments indicate that changes in 

community response are generally short term (Kaiser et al. 1998). 

Alterations in particle size and texture may change the type of organisms residing in 

benthic communities (Pranovi and Giovanardi 1994; Skilleter et al. 2006), since sand and mud 

bottoms support very different species assemblages. Species distribution and abundance of 

benthic biota are strongly associated with the suitability of the substrate type. For example, clam 

density is often inversely related to the amount of clay contained in bottom sediments (Nickerson 

and Brown 1979). Changes to sediment grain size may make a habitat more or less suitable for 

resident organisms. Hydraulic escalator harvesting can alter sediments and habitat can be either 

improved or degraded (Kyte and Chew 1975). Often topography and original sediment 

characteristics must be restored before the biological community returns to its predisturbance 

composition (Boyd et al. 2005). 

Effects of Dredging on Oyster Reef Habitat 

Over time, shellfish dredging has modified traditional oyster reef habitat from thick beds 

of oysters to thinner layers distributed over fine sediments. Long term harvesting of oysters in 

Chesapeake Bay, first by hand and later with dredges, leveled the profile of oyster bars, 

contributing to a reduction in oyster productivity (Rothschild et al. 1994). Dredging of historic 

oyster beds lowered reef heights, elevated sedimentation rates, increased mortality, damaged 

shells, and expanded reef diameter by spreading shell (Lenihan and Peterson 1996, 2004). 

Reduction in the height of oyster reefs can alter hydrodynamics and impact oyster recruitment, 

growth, and survival (Lenihan and Peterson 1996). Leveling of shellfish beds can result in 

broken valves and mortality of oysters (Winslow 1882; MacKenzie 2007). Removal of substrate 

by oyster dredging can scatter shells and oysters into less suitable locations, reduce the number 

of spawning adults, eliminate settlement area for spat, lower disease resistance, and reduce 

substrate complexity of established reefs (Breitburg et al. 2000; Barnette 2001; Street et al. 

2005). The morphology of oyster reefs can be altered by improper or excessive dredge 

harvesting if dredging occurs too frequently or if equipment is too heavy, resulting in oysters that 

may become weak and thin, close up, stop feeding, and lose weight (Nelson 1927). Removal of 

shell or substrate and/or heavy siltation can create a suboptimal environment for oyster growth 

and survival (Rothschild et al. 1994; Powell et al. 2001). Large amounts of suspended sediment 

can overwhelm the filter feeding capacity of oysters, stressing or smothering them (Barnette 

2001). When hydraulic suction dredging preferentially removes juvenile oysters and cultch 
22 

material, a long term decline in live oyster abundance and shell coverage can occur (Powell and 

Ashton-Alcox 2004). The likelihood for dredging damage to cultured oysters is greatly reduced 

in contemporary farming on leased beds, since oysters are spread evenly across the bottom and 

vertical reef structure does not form. 

While extensive dredging can clearly damage established oyster reefs, other studies have 

shown harvest dredging to have minimal or positive effects. Manzi et al. (1985) observed little 

damage to oyster communities associated with mechanical harvesting. Shellfish cultivation 

practices can enhance oyster settlement and increase overall productivity (MacKenzie 1996). 

Oysters were shown to fatten quickly after feeding on fine particulates, organic detritus, and 

materials suspended by dredging and adjacent oyster beds showed no detrimental effects from 

sedimentation (Ingle 1952). Undisturbed oyster beds that become thickly covered with young 

oysters can experience abnormal shell growth and smothering of crowded animals. Harvesting 

effectively remediates this concern by breaking up crowded clumps of oysters, scattering the 

shells to enlarge shellfish bed, and giving the oysters more room to grow (Brooks 1905). The 

increased efficiency of capture associated with repeated dredging may reduce overall impacts of 

fishing on oyster beds. Transplantation of oysters by hydraulic suction dredge or traditional 

oyster dredge within Delaware Bay showed no deleterious effects on bottom complexity; cultch 

availability; or growth, mortality, and health of oyster populations (Powell and Ashton-Alcox 

2004). Repeated dredging showed no significant impact on disease pressure, growth, mortality, 

or recruitment within this Delaware Bay oyster reef (Powell et al. 2001). 

Increased Potential for Predation and Damage to By-catch 

Shellfish and other benthic organisms dislodged but not retained by the dredge are 

returned to the seafloor. Nontarget infauna, attached epifauna, soft-bodied organisms, and other 

by-catch can be removed, damaged, or reduced through the action of mechanical harvesting 

(Vining 1978; Kaiser et al. 1989; Hall and Harding 1997; Auster and Langton 1999; Coen 1995; 

Barnette 2001; Dorsey and Pederson 1998; DeAlteris et al. 2000; NRC 2002; Morgan and 

Chuepagdee 2003) and can sometimes result in direct mortality (Emmett 2002). Reduction in bycatch may also result when organisms are redistributed from dredged to adjacent undredged 

areas (Hiddink et al. 2003). Larger motile organisms may be disturbed by dredging related 

turbidity, noise, and habitat disruption; removal of emergent growth; disturbance of sediments; 

and changes to food sources (Maier et al. 1998). Nontarget bivalves may represent 10-15 % of 

the harvested by-catch (Gaspar et al. 1998). Juvenile organisms which use the inshore zone as a 

nursery environment may also be impacted by fishing activities (Blaber et al. 2000). Effects of 

dredging on by-catch are determined by gear type and tow length. Composition of the by-catch 

can indicate the extent of disturbance experienced by benthic communities after fishing activities 

(Dorsey and Pederson 1998). 

Scavenging and predatory organisms gain an enhanced food source when thin-shelled 

mollusks and annelids are damaged and left behind in dredge tracks, (Ferns et al. 2000; Wallace 

and Hoff 2005; Tarnowski 2006). Numbers of predators and scavenging organisms in tracks 

peak just after the dredge passes (Klemanowicz 1985; Hauton et al. 2003a; Morello et al. 2005). 

Shell breakage during harvesting operations can increase vulnerability of clams to predation 

(Thorarinsdóttir et al. 2008), although undamaged clams are also sometimes consumed (Meyer et 

al. 1981). Increased numbers of predatory flatfish and a corresponding decline in macrofaunal 

organisms were observed for 2 days after hydraulic clam dredging in Canada (Gilkinson et al. 

2005). Predatory fish and crustaceans have been shown to increase in numbers in the vicinity of 
23 

clam dredges (Manning 1960). Arrival and survival of early benthic stage fish can be 

compromised if fishing coincides with settlement of sessile benthic invertebrates (Langton and 

Auster 1999). 

Many nontarget organisms survive dredge harvesting intact or sustain only minor short 

term effects (Maier et al. 1998). Many organisms are too small to be caught in the dredge and 

escape without damage (Alves et al. 2003). Very few collected mollusks and polychaetes were 

broken by hydraulic clam dredging in a Florida study (Godcharles 1971). Upon return to the 

bottom, nontarget by-catch species frequently reburrow with little harvest mortality or long term 

habitat alteration (Bigford 1997). Hydraulic escalator harvesting is generally less destructive to 

shellfish and by-catch than nonconveyer dredges since the conveyer acts to minimize damage 

(Tarnowski 2006). Hydraulic dredging in the offshore surfclam and ocean quahog fishery has 

been shown to produce a low by-catch of nontarget species (Wallace and Hoff 2005). 

Most studies indicate low numbers of mobile by-catch, since active swimmers, fish and 

decapod crustaceans generally avoid capture (Jolley 1972; Hauton et al. 2003a, 2003b; Hawkins 

2006). Shellfish harvesting gear is towed slowly for short periods of time in shallow waters, and 

as a result, finfish are collected infrequently (Rheault 2008). Numbers of young-of-the-year 

yellowtail flounder (Limanda ferruginea) showed no immediate decline after one time passage of 

a scallop dredge over an area in the New York Bight (Sullivan et al. 2003). A survey of 

recreational finfish in South Carolina indicated no differences in abundance related to harvesting 

(Maier et al. 1998). Modification of dredge design is commonly used to mitigate adverse 

ecological effects and to reduce by-catch of nontarget species (Gaspar and Chícharo 2007). 

Clam Reburrowing After Dredging 

Survival of undersized clams may be compromised by the harvest process which includes 

removal from the sediment, airlifting to the surface, on-deck sorting, followed by a return to the 

water (Robinson and Richardson 1998). Undersized clams returned to the bottom reburrow into 

the sediment at a rate related to their size, substrate type, and water temperature (Pfitzenmeyer 

and Drobeck 1967). Smaller clams reburrow more quickly than do larger clams, and reburrowing 

capability decreases with increasing particle size. Large adult clams (>86 mm) were shown to be 

least able to reburrow successfully (Rice et al.1989). Survival of sublegal clams was reduced 

when harvesters redeposited them in inverted positions, from which they were slow to rebury 

(Glude 1954). 

Some clams returned to the water following dredge harvest may be unable to 

immediately rebury in the sediment. Razor clams returned to the bottom after dredging showed a 

slow initiation of “escape digging” (Robinson and Richardson 1998). Repeated mechanical stress 

has been shown to reduce the number of successfully reburrowing clams (Chamelea gallina) in 

the Adriatic Sea (DaRos et al. 2003). In a laboratory experiment, clams exposed to simulated 

harvesting disturbance experienced a reduction in burrowing speed (Marin et al. 2005). 

Failure to reburrow promptly may expose shellfish to elevated predation risk (Medcof 

1961; Meyer et al. 1981; Robinson and Richardson 1998; DaRos et al. 2003). Disturbed bottom 

attracts predatory fish and crustaceans which can feed on reburrowing or quiescent clams and 

other exposed benthic invertebrates (Pfitzenmeyer and Drobeck 1967). Although dredging may 

increase the time required for exposed clams to rebury and elevate predator abundance, it does 

not necessarily increase the predation rate (Chícharo et al. 2002b). An eightfold increase in 

predator abundance occurred just after dredging off Portugal, but mortality of clams was low, 
24 

and predators dissipated quickly due to prompt reburial of exposed razor clams (Gaspar et al. 

2003). 

Reburial time varies with clam species. After collection by hydraulic dredging off 

Scotland, up to 85% of returned razor clams reburied rapidly (Hauton et al. 2003b). Undersized 

razor clams reburied quickly within 30 m of returning to the bottom (Hauton et al. 2003b), while 

small surfclams remained immobile for 1-24 h, possibly because of some form of dredgeinduced shock (Meyer et al. 1981). 

Dredging can loosen compacted sediments, making it easier for clams and other bivalves 

to reburrow successfully (Manning and Dunnington 1955). Fluidized sand offers less physical 

resistance to entry by the pedal muscle and shell of razor clams (Hauton et al. 2003b). The ability 

of clams to reburrow can be affected by the presence of vegetation, occurrence of other benthic 

species, water depth and currents, and predation rates as well as by substrate consistency, 

contour, and composition (Pfitzenmeyer and Drobeck 1967; DaRos et al. 2003). Reburial can 

also be impacted by environmental factors, such as temperature and dissolved oxygen levels and 

changes to sediment chemistry. Depletion of the oxygen layer observed after experimental 

dredging may present an obstacle to the reburrowing of displaced fauna (Badino et al. 2004). 

Indirect Effects of Dredging Related to Habitat Disruption 

Shellfish beds provide physical structure and therefore habitat for marine animals. Finfish 

abundance has been observed to be greater on natural oyster shell reefs than on adjacent 

featureless mud bottom sites (Plunket and La Peyre 2003). When dredging disrupts habitat, 

finfish and crustaceans may be impacted (Coen 1995). When habitat structure is altered, biomass 

may be reduced (Smolowitz and Nulk 1982; Coen 1995; Watling and Norse 1998; Emmett 2002; 

NRC 2002). Although Ingle (1952) reported that oyster dredging resulted in no damage to motile 

fish and crustaceans in Florida, a New Zealand study found that dredging reduced fish 

abundance in oyster habitat, although the population rebounded once the dredged bottom 

stabilized (Carbines 2005). Similarly, demersal fish in New Zealand were less abundant in one 

area 4 m after oyster dredging, compared to a fallow area, and this change was attributed to a 

reduction in habitat complexity (Carbines and Cole 2009). 

Effects of Dredging on Biodiversity 

Species diversity may be impacted when fishing activities remove or damage certain 

species and attract scavengers (DeAlteris et al. 2000; Barnette 2001). Removal of bioturbator 

organisms from an ecosystem by disturbance can have an indirect ecological effect on stability 

and maintenance of biodiversity (Widdicombe et al. 2004). Loss or alteration of habitat 

complexity, especially in low structure soft sediments, will reduce biodiversity (Thrush et al. 

2001; Ferreira et al. 2007). In South Carolina, a diverse assemblage of species was observed in 

the high intertidal following dredging, while the opposite was true in the low intertidal zone 

(Klemanowicz 1985; Manzi et al 1985). A temporary reduction in macrobenthic diversity was 

associated with dredge harvesting in the waters off Portugal (Constantino et al. 2009). Diversity 

declined immediately following hydraulic suction dredging in Delaware Bay (Ismail 1985) while 

as many as 90% of species were removed by harvesting in the United Kingdom (Spencer 1997). 

When disturbance occurs too frequently for recovery, susceptible species are eliminated and 

biodiversity is reduced (Dorsey and Pederson 1998). Once hydraulic dredging has significantly 

reduced diversity and evenness within a biological community (Morello et al. 2006), it may take 

years for species diversity to return to predredging levels (May 1973). Replanting of shell and 
25 

shellfish seed following dredging has the capacity to restore benthic biodiversity to its previous 

state. Habitats accustomed to disturbance may experience no change in diversity after dredging. 

In shallow, sandy, subtidal habitat near Scotland, mechanical harvesting of razor clams resulted 

in no apparent impact on species diversity (Tuck et al. 2000). 

Shellfish culture may enhance benthic species diversity when harvested shellfish are 

replaced by other opportunistic organisms (Fahy and Carroll 2007). Oyster cultivation has been 

associated with increased species density and richer diversity (Hosack et al. 2006). Oyster beds 

support a greater biomass and diversity of benthic invertebrates than similar bottom without 

oysters (Bigford 1997). In Lousiana, species diversity on natural shell and artificial clam shell 

reefs was twice that observed on mud bottom (Plunket et al. 2003). Enhanced diversity in 

dredged areas may be due to mobilization of resources and the creation of spatial patchiness, 

factors which may promote recolonization by a new suite of species (DeGrave and Whitaker 

1999). Small scale disturbances caused by dredging can create a spatial and temporal mosaic of 

successional states which can enhance community diversity (Tarnowski 2006). Dredge 

disturbance can prevent any one species from becoming dominant, resulting in a more diverse 

biological community (Collie et al. 2005). Shellfish cultivation and aquaculture promote 

biodiversity by increasing nutrient inputs which extend food webs (Ferreira et al. 2007). Careful 

selection of shellfish cultivation sites promotes biodiversity by reducing predation pressure on 

harvested species, increasing biological productivity, and enhancing substrate heterogeneity 

(Ferreira et al. 2007). 

Alterations of the Biochemistry and Physiology of Shellfish after Dredging 

Many experimental studies demonstrate short-term, stress-related physiological responses 

of bivalves to harvest processes. Hydraulic dredge harvesting can temporarily affect the 

physiological and biochemical metabolism of shellfish (Moschino et al. 2008). Laboratory 

simulations of cumulative dredging found a sublethal impact on noncommercial sized clams, 

Spisula solida (Chícharo et al. 2003). Results showed a decrease in condition as indicated by 

reduced biochemical indices (RNA/DNA and lipid ratios). Clams, Chamelea gallina, exposed to 

simulated mechanical sorting experienced reduced clearance and respiration rates (Moschino et 

al. 2008). Following simulated dredging, survival of clams exposed to air declined significantly. 

In field trials, high water pressure and mechanized sorting reduced scope for growth 

measurements, clearance rates, and survival time in air-exposed dredged clams. Shellfish 

exposed to actual hydraulic water jet harvesting showed a reduction in physiological well-being. 

Clams exposed to high pressure dredging had reduced adenylate energy charge levels and 

reburied less frequently than did clams in a low pressure treatment. Both of these responses are 

indicative of acute stress (Da Ros et al. 2003). High mechanical stress caused by dredging 

decreased clam filtration rates and acid phosphatase activity and increased respiration and βglucuronidase activity (Marin et al. 2003). High water pressure reduced scope for growth 

measurements, increased shell damage, and affected immune response through depressed 

hematocrit and phagocytic index measurements. Exposure to air reduced median survival times 

in severely stressed clams. Observed physiological responses to high water pressure dredging 

suggest an acute response, although clams typically recover rapidly (Marin et al. 2003). When 

clams were repeatedly shaken to simulate mechanical harvesting effects (Marin et al. 2005), 

physiological parameters revealed a slight but statistically nonsignificant decline in their wellbeing, indicated by decreased filtration rates, reduced scope for growth, and increased respiration 

rates. When clams were stressed daily for 3 days, decreased survival out of the water and 
26 

reduced burrowing speed were observed. Organisms exposed to dredging may also experience 

enhanced susceptibility to other stressors (NRC 2002). Physiological and biochemical effects of 

dredging can increase vulnerability of clams to predation, pathogens, or environmental stressors, 

and sensitivity to dredging may be further enhanced by factors such as season, seawater 

temperature, and reproductive state (Moschino et al. 2008). 

Effects on Submerged Aquatic Vegetation (SAV) from Dredging 

 Use of dredges in seagrass habitats is currently restricted by state and federal law, but 

many previous studies have reported the effects of dredging on submerged aquatic vegetation. 

The impacts of fishing gear on seagrass systems are similar to those of plowing on terrestrial 

plants and vary in severity with the type of harvester (Peterson et al. 1983; Blaber et al. 2000). 

Lighter implements, which target shallow depths, have less impact on vegetation while heavier 

equipment, which sinks more deeply into sediments, is more destructive. Submerged aquatic 

vegetation can clog dredges, impair proper operation, and reduce catch efficiency disrupting 

performance (Manning 1960). Experimental studies have shown that dredging in seagrass habitat 

can remove, smother, or destroy SAV and algae along the dredge path (Vining 1978; Dayton et 

al. 1995; Barnette 2001; Morgan and Chuepagdee 2003). Hydraulic clam dredging can remove 

all vegetation within an eelgrass bed (Moore and Orth 1997) and prevent recolonization and 

diffusion (Pranovi and Giovanardi 1994). High water pressure produced by hydraulic jets can dig 

up aquatic vegetation, uproot plants, and suppress seed germination (Manning 1957; Godcharles 

1971; Barnette 2001; Street et al. 2005; Tarnowski 2006). Plants can be defoliated, removed, or 

buried beneath sediment resulting in depressed growth and survival (Street et al. 2005). Other 

negative impacts of dredging on seagrass beds include burial of plants, disruption of root and 

rhizome systems, diminished plant growth, and leaf dysfunction (Jolley 1972; Tarnowski 2006). 

Elevated turbidity can reduce or eliminate sea grass habitat (Dayton et al. 1995). Sediments 

suspended by dredging can temporarily reduce ambient light and inhibit photosynthetic activity 

(Barnette 2001). Suction sampling can completely remove rooted vegetation from dredge tracks 

(Kyte et al. 1976). Trenches created by dredging in seagrass habitat may remain visible longer 

than in unvegetated areas (Godcharles 1971) since seagrasses create energy dampening and 

sediment stabilizing effects (Tarnowski 2006). Clam dredging in Chincoteague Bay, Virginia, 

effectively removed all vegetation in harvested areas unlike adjacent undredged bottom where 

vegetation remained dense and healthy (Moore and Orth 1997). 

In Willapa Bay, Washington in the Pacific Northwest, oyster cultivation and dredge 

harvesting are successfully conducted in eelgrass habitat. Although dredging was shown to 

reduce eelgrass growth and biomass in oyster culture areas (Wisehart et al. 2004), higher rates of 

eelgrass seedling density, production, and germination have been observed after dredge harvest 

when compared to longline culture or hand harvest areas (Wisehart et al. 2006, 2007). In a 

similar study, dredged oyster beds showed lower eelgrass densities, plant size, and production 

than did hand picked and long line cultured beds (Tallis et al. 2006). Eelgrass density declined 

with increasing oyster density, likely because of competition for space or harvest damage to 

shoots or rhizomes (Tallis et al. 2006, 2009). Eelgrass shoots are uncommon where the seafloor 

is covered by oysters or shell, and bivalves can potentially damage eelgrass through breakage or 

wear (Tallis et al. 2009). Removal of adult eelgrass plants from the benthos allowed enhanced 

germination of eelgrass seedlings (Wisehart et al. 2007). Density of new eelgrass seedlings was 

highest in dredged oyster beds where the quantity of adult eelgrass was low. Removal of 

neighboring adult plants, modification of the physical environment, and the physical presence of 
27 

oysters may alter the nutrient or light environment, enhancing eelgrass seed production, 

germination, and/or survival. Oyster dredging may promote recovery of eelgrass by liberating 

sources of eelgrass seed. Elevated growth rates in dredged zones may result from lower eelgrass 

densities, reduced intraspecific competition, or increased nutrients from oyster waste products 

(Tallis et al. 2006). Minimizing impacts to eelgrass is a management goal that can allow oyster 

culture and seagrass beds to coexist (Tallis et al. 2009). 

Other types of marine vegetation can also be impacted by dredging. Macroalgal holdfast 

attachments may be dislodged by dredging (Vining 1978). Hydraulic harvesting of clams has 

been shown to reduce biomass of kelp beds (Goodwin and Shaul 1978). Damage to benthic 

microalgae resulting from harvest can reduce primary productivity (Auster and Langton 1999). 

Loss or damage to marine plants from mechanical action of the dredge can result in long term 

changes to co-occurring ecological communities (Pranovi and Giovanardi 1994). 

It should be noted that shellfish cultivation can provide benefits to SAV. Shellfish culture 

can enhance seagrass productivity and/or recovery, provide an attachment site for algae, and 

prevent it from being transported away by currents (Brumbaugh et al. 2006; Hosack et al. 2006; 

Rice 2008). When feeding, oysters and other bivalves remove suspended particles from the 

water, increasing light at the sediment surface, and enhancing growth of SAV and benthic 

microalgae (Newell and Koch 2004; Newell et al. 2005; Street et al. 2005). 

Evidence of Enhanced Shellfish Recruitment from Dredging 

A number of studies have observed increased oyster recruitment following dredging. 

Enhanced settlement of oyster spat was reported after hydraulic dredge collection of softshell 

clams on beds in South Carolina (Burrell 1975a). Harvesting redistributes oysters from isolated 

clumps to expanded beds, creating more surface area for spat settlement (Kennedy and Breisch 

1973). Dredging of oyster shell to collect cultch resulted in increased spat settlement on 

remaining shell in an Alabama study (Eckmayer 1977). Distribution of cultch on oyster beds 

following tong fishing was shown to increase harvests in Florida (Allen and Turner 1989). 

Removal of algae and silt by dredging and laying down of clean shell was shown to improve 

oyster recruitment in Alabama (Engle 1935). Dredge tracks may act as sinks where broken shell 

is trapped and retained, providing cultch for shellfish spat (Gilkinson et al. 2003). The consistent 

attraction of oyster spat to harvested bottom suggests that dredging does not necessarily 

negatively impact oyster reef development (Klemanowicz 1985; Manzi et al. 1985). 

Similarly, clam recruitment has been reported to increase after beds have been dredged. 

Hard clams continue to settle and survive after removal of adults (Rice et al. 1989), and 

spreading of clean shell may contribute to increased settling densities (MacKenzie et al. 2002a, 

2002b). Cultivation methods which turn over bottom sediments appear to create a more favorable 

environment for clam settlement (Visel 1990a, 1990b; Kyte and Chew 1975). Greater numbers 

of seed clams were associated with harvesting activities in Washington State (Goodwin and 

Shaul 1980). Recruitment of young softshell clams in Maryland increased in plots where 

hydraulic escalator harvesting reduced numbers of adult clams (Pfitzenmeyer 1972a, 1972b). 

Tilling of sediment by harvesters brings buried shell material to the surface, increases the 

quantity of shell hash, and significantly elevates calcium carbonate concentrations in harvested 

areas (Maier et al. 1998). Abundance of hard clams has been shown to increase in the presence 

of shell (Pratt 1953). Survivorship of juvenile hard clams in North Carolina was higher in oyster 

shell hash areas versus vegetated and unvegetated sand and mud/sand bottom (Peterson et al. 

1995). Areas of high surfclam abundance in the Great South Bay of New York were associated 
28 

with shells or shell fragments in or on the sediment surface and with the presence of relic oyster 

beds (Kassner et al. 1990). High shell density increased hard clam settlement in New Jersey with 

areas of little or no shell showing low or no recruitment (Kraeuter et al. 2003). Shell cultch may 

enhance clam recruitment and abundance by confusing predators, stabilizing surface sediments, 

or accumulating settling clams through active or passive mechanisms (Kassner et al. 1990). Shell 

pieces or large particles of gravel provide natural protection to small shellfish (Peterson et al. 

1995), reduce current speeds to promote settlement of spat (Pratt 1953), and provide a point of 

attachment for the byssal threads of young clams (Wells 1957; Kraeuter et al. 2003). 

Evidence of Reduced Shellfish Recruitment or No Observed Effects from 

Dredging 

Some evidence suggests that cultivation may have a negative effect on settlement or may 

not result in enhanced recruitment. Dredging may disrupt or partially bury oyster spawning 

stock, removing potential settling surfaces and decreasing numbers of oyster spat (Drobeck and 

Johnston 1982; Kaiser et al. 1989; Breitburg et al. 2000). At one time, oyster harvesting was so 

intensive that few oysters and little shell remained to provide cultch material for ready-to-settle 

oysters (MacKenzie 2007), reducing recruitment potential. Removal of shell can reduce 

buffering capacity and lower pH, causing shell dissolution and mortality of juvenile shellfish 

(Green et al. 2009). Total oyster spatfall on a harvested reef was lower than that found in a 

nearby control area in South Carolina (Manzi et al. 1985). Other bivalve species have also 

demonstrated a decline in recruitment following shellfish harvest. Suction dredging for cockles 

reduced settlement substrate for the blue mussel (Mytilus edulis) (Hiddink et al. 2003) and had 

long lasting negative effects on recruitment of bivalves in sandy areas (Piersma et al. 2001). 

Disturbance from dredging and subsequent loss of fine grained sediment may account for 

declines in shellfish recruitment. 

Dredge harvesting may have no impact on shellfish recruitment. No increase in hard clam 

recruitment was observed following escalator dredge harvesting in either Florida (Godcharles 

1971) or Virginia (Haven 1979). Softshell clam spat was equally abundant in hydraulically 

harvested areas and unharvested locations in South Carolina (Burrell 1975b). Plots dredged on a 

daily or weekly basis showed similar recruitment of soft-shell clams in Maryland (Pfitzenmeyer 

1972a, 1972b). Artificial shelling of bottom in the Great South Bay of New York showed no 

increase in surfclam recruitment one year later, despite observations of elevated clam abundance 

in adjacent areas with high natural shell (Kassner et al. 1990). 

Chemical Effects of Dredging 

The chemistry of bottom sediments may be altered when the benthos are disturbed by 

dredge activity. Modification of the benthic community as a result of dredging can impact the 

exchange of chemicals and nutrients between the sediments and water column and affect system 

dynamics. Cultivation of bivalve shellfish can also improve water quality and redistribute 

chemical constituents within sediments. Dredging may modify the biogeochemistry of sediments 

to favor settlement and recruitment of shellfish. In the following section we report the effects of 

shellfish dredging on sediment chemistry as described in the literature. Although many 

experimental studies have directly addressed the physical and biological effects of dredging, 

information on chemical effects is more limited and derived from the basic understanding of 

benthic processes. 
29 

Sediment Biogeochemistry 

The biochemistry of marine sediments is largely influenced by bioturbation and 

burrowing activities of benthic organisms. The mechanisms by which benthic invertebrates 

obtain food produce the chemical transformations that regulate carbon, nitrogen, and sulfur 

cycling; water column processes; distribution and fate of pollutants; secondary production; and 

transport and stability of sediments (Dame, 1996; Snelgrove 1997; Widdicombe et al. 2004). 

This interaction is particularly true for deposit feeders which dominate depositional areas of 

silt/clay fine sediments that result from limited water movements (Wildish and Kristmanson 

1997). Bottom dwelling organisms stabilize sediments through tube construction and mucous 

binding of particles and contribute to sediment destabilization via bioturbation processes 

(Snelgrove 1997). Since benthic organisms involved in sequestering and recycling processes are 

essential to ecosystem function (Thrush and Dayton 2002) their removal or disruption by 

dredging can alter sediment biogeochemistry. Removal of large benthic bioturbators through 

dredging can have long term effects on sediment nutrient fluxes and influence whether the 

seafloor acts as a source or sink for nutrients (Olsgard et al. 2008). 

Dredging disturbance can alter the biogeochemical composition of sediments. Physical 

changes in sediment compaction from dredging can alter the magnitude and rate of 

biogeochemical cycling between the sediment and overlying water (Badino et al. 2004). 

Disruption of the substrate by harvesting can change the rate of resuspension and enhance the 

upward flux of nutrients in pore water from periodic release to pulse (Auster and Langton 1999; 

Thrush and Dayton 2002). In one study conducted in Portugal, ammonium, nitrates, organic 

nitrogen, phosphates, and silicates decreased in porewater of reworked sediment immediately 

after dredging with a simultaneous increase in nutrient levels in near bottom water, but these 

chemical characteristics returned to previous levels within hours (Falcão et al. 2003). Chemistry 

of sediments within dredge furrows may differ from that measured along the outside ridges 

where substrate material was reworked by harvesting (Falcão et al. 2003). In Washington State, 

removal of substrate material by dredging caused little or no reduction in chemical parameters 

such as biological oxygen demand, nitrogen, sulfides, and phosphates within harvested areas 

(Goodwin and Shaul 1978). Experimental hydraulic dredging in Narragansett Bay had no 

measurable impact on total organic carbon of sediments (Sparsis et al. 1993). In many cases, 

chemical impacts of seabed disturbance are both localized and temporary in nature (DeAlteris et 

al. 2000). 

Dredging can enhance system metabolism, impact water column processes, and influence 

trophic transfer. Suspension feeding bivalves serve an important chemical role in coastal 

ecosystems by transferring nitrogen and phosphorus from the water column to the sediments 

(Newell 1988; Newell and Koch 2004; Newell et al. 2005). Bivalves ingest and process 

resuspended organic matter, which is returned to the environment as biodeposits (feces and 

pseudofeces) (Snelgrove 1997; Pranovi et al. 2003). Biodeposition, accumulation, and 

remineralization of organic matter from these shellfish waste products affect the biogeochemical 

properties of sediments which drive biological community structure (Dame 1996; Cranford et al. 

2006). Increased availability of nutrients modifies the lower food chain by reducing microbial 

activity (Morgan and Chuepagdee 2003) or increasing the role of smaller organisms and bacteria 

(Dorsey and Pederson 1998). The metabolic activities of bivalves can accelerate the movement 

of carbon, nitrogen, and phosphorus between sediments and the water column (Dame 1996). 

Benthic macrofauna regulate chemical cycles, irrigate and mix the sediment, redistribute 

oxygen, and enhance pore water exchange with the overlying water column (Pranovi et al. 2003). 
30 

Organisms that construct multidimensional burrows and tubes provide conduits for pulling 

oxygenated water into the sediments, and some organisms, which oxygenate sediments to even 

greater depths, occur in deeper anoxic sediment (Watling 2005). Shellfish cultivation activities 

may promote short-term redistribution and increased oxygen and minerals within bottom 

sediments. Substrate overturned by dredging can enhance oxygen penetration into the upper 

sediment layers (Falcão et al. 2003). Organic carbon in the first inch (25.4 mm) of undisturbed 

sediment may be redistributed within substrates by dredging (Pfitzenmeyer 1972a). 

Disruption of surface sediments by dredging, however, can sometimes create short term 

and localized oxygen deficits in the water column (Bartoli et al. 2001), decreasing dissolved 

oxygen concentrations. Resuspension of sediments and subsequent release of previously buried 

organic material into the water column may create an oxygen deficit (Aspden et al. 2004). When 

dissolved oxygen is reduced by disturbance of the seafloor environment, the risk of hypoxia 

affecting the associated biological community is elevated (NRC 2002; Rheault 2008). Stress 

from dredging can increase biological oxygen demand among benthic organisms (Coen 1995). 

Depletion of the oxidized layer can inhibit reburrowing of infauna displaced during harvesting 

operations and reduce faunal activity postharvest (Badino et al. 2004). Stimulation of oxygen and 

carbon dioxide fluxes by clam culture can affect nutrient regeneration rates across the watersediment interface (Bartoli et al. 2001). Low dissolved oxygen and slightly reduced pH values 

associated with dredging indicated that reduced sediments were resuspended by dredging and 

mixed back into the water column (Kyte et al. 1976). The associated decline in oxygen and pH 

appeared short term, as sediments oxidized rapidly. Tilling of sediments brings buried shell and 

shell hash to the surface and can increase calcium carbonate concentrations in harvested areas 

(Maier et al. 1998). There is evidence that recruitment among sediment dwelling bivalves can be 

enhanced by distributing shell hash to increase pore water calcium carbonate saturation levels 

(Green et al. 2009) and pH concentrations, a practice commonly used by shellfish farmers to 

boost clam recruitment. In some cases, however, removal of extensive quantities of old shell 

during harvest may lower sediment pH, potentially impacting larval settlement (Green et al. 

2009). 

When large shellfish are dredge harvested, minerals and nutrients are removed from the 

ecosystem (NRC 2002; Cranford et al. 2006). Shellfish cultivation and production sequesters 

nitrogen in the form of protein in meat and shell and stabilizes phytoplankton growth dynamics 

through modulation of ammonia cycling in the water column (Rice 2008). With each harvested 

oyster (7.6 cm length), 0.52 g nitrogen and 0.16 g of phosphorus are eliminated from the marine 

environment (Newell and Koch 2004; Rheault 2006). Actively growing shellfish can remove as 

much as 16.8 g of nitrogen from an estuary for every kg of meat harvested (Rice 2001). 

Dredging may also act to liberate nutrients, metals, or contaminants from anaerobic bottom 

sediments (Coen 1995; Watling and Norse 1998; Barnette 2001; Northeast Region EFHSC 

2002). Although bivalves can tolerate high concentrations of pollutants such as heavy metals 

(Stiles et al. 1991), release of significant quantities of toxicants seems unlikely since harvestable 

bivalves are presumably associated with cleaner substrates (Kennedy and Breisch 1973). The 

likelihood of pollutant impact is relatively minor since bivalve shellfish occur in moderate to 

high energy environments where substantial current velocity provides constant flushing and good 

circulation, contributing to high water quality (Vining 1978). 
31 

Socioeconomic Effects of Dredging 

Small scale shellfish harvesting can provide benefits to coastal community shareholders. 

In Connecticut, oyster farming produces 300 jobs, $15 million in revenue, and an annual oyster 

harvest of 450,000 bushels with operations conducted within a small area relative to the broader 

Long Island Sound ecosystem (Connecticut Department of Agriculture 2011). Cultivation can 

expand the marine economy through promotion of private investment, creation of shellfish farms 

and jobs, reestablishment of traditional shellfish aquaculture, generation of income for state and 

local governments via the licensing and leasing process, initiation of employment opportunities 

for coastal communities, development of shoreline processing facilities, and finally, production 

of high-quality locally-farmed shellfish for consumer markets and restaurants (Vining 1978; 

Ferreira et al. 2007; Cashin Associates 2008a, 2008b). Contemporary cultivation methods 

enhance spawning potential of native shellfish, improve utilization of growing areas; reduce 

harvesting labor; increase landings; and allow more efficient movement of shellfish to depuration 

areas, better access to underwater land for raising shellfish, effective transportation of seed and 

increased economic gain for shellfish growers (Vining 1978; Manzi et al. 1985; Cashin 

Associates 2008a, 2008b). Superior water quality and stewardship of marine resources are 

associated with active shellfish culture and result in a positive relationship between ecological 

health and users of the coastal zone (Cashin Associates 2008a, 2008b). Shellfish aquaculture can 

meet objectives for environmental and social sustainability, help meet the world’s needs for 

aquatic foods, and contribute to food security, and economic development (World Wildlife Fund 

2009). 

Dredging may also impact coastal communities. Harvesting can disrupt marine 

recreational activities, create hazards to navigation, detract from the visual aesthetic, increase 

onshore noise levels, and result in a loss of some maritime traditions (Vining 1978; Barnes et al. 

1991; Rice 2008; Cashin Associates 2008b). Shellfish cultivation on leased beds could 

potentially result in inadvertent harvesting activity on unauthorized bottom impacting private 

property owners (Vining 1978). Actions such as muffling sound from vessels, restricting 

working hours, and marking leased bottom to prevent infringement on unleased areas all provide 

possible solutions to these issues. The short term nature and limited spatial scale of most 

shellfish dredging on leased grounds could largely mitigate many concerns. Harvesting a 

resource that would otherwise be lost to natural perturbations may outweigh potential short term 

effects (Kaiser et al. 1998). 

Duration of Dredging Effects 

In many studies, the impacts of shellfish dredging on aquatic resources, benthic food 

webs, or marine ecosystems were of short term duration (Stokes et al. 1968; Vining 1978; Kaiser 

et al. 1996; Marsh 2004). Hydraulic dredging over a 30 y period in the Adriatic Sea showed no 

lasting effect on benthic fauna such as polychaetes, crustaceans, detritus, and suspension feeders 

(Morello et al. 2005). Hydraulic escalator harvesting of softshell clams in Maine showed few 

significant changes in biological, geological, and hydrographic systems at the harvest site (Kyte 

et al. 1976). In South Carolina, escalator harvesting caused no long-term chronic impacts, and 

effects were indistinguishable from ambient levels of natural variability (Coen 1995). Studies in 

Maryland and South Carolina showed minimal and temporary impacts of dredging on bottom 

dwelling invertebrates, finfish, and infaunal benthic communities (Drobeck and Johnston 1982; 

Maier et al.1998). 
32 

Studies have shown that, in many cases, clam dredging has no long term negative impact 

on the benthic community. Hydraulic dredging for surfclams had no harmful effects on resident 

benthic organisms on either a long or short term basis (Lambert and Goudreau 1996). In 

Narragansett Bay, one time disturbance of sediment by a hydraulic bullrake during clam 

cultivation resulted in no significant differences among infaunal communities when compared 

with an untreated control. Further, the scale of observed seafloor effects was much lower than 

the observed natural spatial and seasonal variability (Sparsis et al. 1993). Experimental short 

raking of quahogs from sand sediments in Raritan Bay, New Jersey had no measurable effects on 

the number of benthic invertebrates (MacKenzie and Pikanowski 2004). Clam (quahog) 

harvesting in North Carolina had no effect on density or species composition of small benthic 

macroinvertebrates; these findings were attributed to their rapid recolonization and short life 

spans (Peterson et al. 1987). Suction dredging for Manila clams in England left no long term 

effect on the environment or benthic community (Kaiser et al. 1996). Effects of dredging on 

softshell clam abundance in Maryland were largely considered to be short term (Pfitzenmeyer 

1972a, 1972b), since heavy annual sets, rapid growth rates, and the ability to reburrow fostered 

rapid recovery rates among populations (Pfitzenmeyer 1972b; Drobeck and Johnston 1982). 

Communities thriving in hydrodynamic areas, well-adapted to natural physical stress, do not 

appear greatly affected by clam dredging (Constantino et al. 2009). 

Mechanical harvest of oysters has also been shown to have minor effects on the benthos. 

Sampling conducted prior to and just after hydraulic escalator oyster harvesting in South 

Carolina showed no negative effects on benthic organisms (Burrell 1975a). Mechanical 

harvesting in South Carolina caused little or no damage to oysters (Collier and McLaughlin 

1984). At present levels of exploitation, oyster dredging in Delaware Bay is thought to have a 

benign impact on the health and productivity of oyster populations (Powell et al. 2001). 

Practices associated with shellfish harvesting do not appear to inhibit shellfish 

reproduction, interfere with replacement of harvested stocks (Manning 1960), or consistently 

impair shellfish recruitment processes (Tarnowski 2006). Detrimental effects of fishing 

disturbance are generally short term because shellfish and other organisms removed by dredging 

are replaced through natural recruitment processes (Bigford 1997) or by replanting of shell and 

seed as part of the cultivation process. Dredging does not collect every marketable clam, so those 

that remain serve as broodstock and can repopulate harvested areas (Manning and Dunnington 

1955) through the seasonal arrival of new recruits (Gilkinson et al. 2003). 

ECOLOGICAL RECOVERY AFTER DREDGING 

Experimental studies, conducted at differing spatial scales, show that reestablishment of 

benthic communities and recovery of infaunal abundance, taxonomic richness and biomass 

following dredging occurs within a matter of hours, days, weeks (Godcharles 1971; Hall et al. 

1990; Hall and Harding 1997; Ferns et al. 2000), or months (Ismail 1985; Kaiser et al. 1996; 

Spencer 1997) with recovery often complete within one year (Simon and Connor 1977; Connor 

and Simon 1979). The hydrodynamic energy within an environment determines recovery rate 

which can range from days in high energy areas to months in low energy habitats (Wallace and 

Hoff 2005). On intertidal flats the ecological recovery rate among benthic communities varies 

but generally is rapid following hydraulic clam harvest (Goodwin and Shaul 1980) or suction 

dredging (Spencer et al. 1998). Tarnowski (2006) provides a table summarizing recovery times 

for coastal and estuarine benthic fauna exposed to different kinds of disturbance, including 

shellfish dredging. He reports that benthic equilibrium following disturbance generally returns 
33 

within a 12 m period. Biomass in particular can be slow to recover since it responds to growth 

rates of newly settled organisms and immigration into the area (Tarnowski 2006). Recovery from 

dredging reflects the ability of damaged organisms to repair, regenerate, and withstand 

conditions within disturbed habitat, as does successful larval production, settlement and 

recruitment to the adult population (MacDonald et al. 1996). Recovery times vary with the 

intensity, frequency and spatial scale of disturbance, the physical characteristics of the habitat, 

and the life history attributes of resident species (Northeast Region EFHSC 2002). 

Recolonization after dredging is an ongoing process, dependent on natural recruitment. 

For example, in Canada, abundance of polychaetes and amphipods continued to increase for over 

a year after harvesting (Gilkinson et al. 2005). Organisms within the same community may 

recover at differing rates based on their individual level of sensitivity to disturbance (Beukema 

1995). A meta-analysis of general effects from different types of fishing gear found the most 

rapid recovery in less physically stable habitats where opportunistic species occur (Collie et al. 

2000). Recovery occurs most quickly in less-consolidated sediments where resident organisms 

are adapted to disturbance by anatomy, behavior, or life history characteristics (Dorsey and 

Pederson 1998; NRC 2002). Areas prone to frequent disruption select for species which tolerate 

instability and favor communities dominated by juvenile stages, mobile species, and rapid 

colonists (Dayton et al. 1995; Thrush and Dayton 2002). For resident organisms to survive, they 

must adapt to the dredged environment while new colonizers may be attracted to altered seafloor 

(Kyte and Chew 1975). Most species living in frequently disturbed areas have evolved 

characteristics that allow them to reside or repopulate these areas (Watling 2005). Rapid 

recovery following clam cultivation in Narragansett Bay, Rhode Island was attributed to the 

short life spans of the dominant organisms, amphipods, and polychaetes (Sparsis et al. 1993). 

Settling of lighter organisms following the resettlement of suspended sediment might account for 

the minimal damage to infauna observed. Despite the creation of 1 m deep grooves during this 

experimental study of hydraulic dredging effects, recovery of the sediments and biota was almost 

immediate. Effects of shellfish harvesting are short term where the infaunal benthic community 

expresses an opportunistic recolonization response (Drobeck and Johnston 1982). 

The type and spatial extent of habitat can influence initial mortality rate, benthic 

community recovery and species composition following disturbance (Adkins et al. 1983; Link et 

al. 2005). Shellfish cultivation is conducted in environments coinhabited by short-lived, 

disturbance-tolerant, opportunistic organisms (NRC 2002) which appear less affected by fishing 

activities than are stable deepwater communities (Auster and Langton 1999; Barnette 2001). 

Species residing in coastal waters and accustomed to rapidly changing habitats and 

environmental conditions are resilient and recover quickly from disturbance events (Taranowski 

2006). Rigorous and dynamic estuarine environments, characterized by large predictable and 

unpredictable fluctuations in environmental variables, are subject to almost constant disturbance 

(Turner et al. 1995). Effects of dredging appear low and benthic community recovery rapid in 

shallow hydrodynamic environments (Constantino et al. 2009). This observation is in contrast to 

stable biogenic gravel and mud seabeds, inhabited by long-lived benthic organisms, which 

experience a greater loss of flora and fauna and recover more slowly after disturbance than do 

sandy sediment communities (Dorsey and Pederson 1998; Auster and Langton 1999). 

The nature, magnitude, and frequency of impact, recovery time of the habitat, and scale 

of cultivation determine the duration of harvest impact (Kaiser et al. 1998; Johnson 2002). 

Recolonization rate and restoration of sediment structure vary with local hydrology, wave action, 

currents, sediment stability, natural disturbance, recruitment rates, and presence of SAV 
34 

(Godcharles 1971; Adkins et al. 1983; Spencer et al.1998). The ability of a habitat to recover 

from fishing disturbance is often a function of wind, wave, and tidal action (Stevenson et al. 

2004). Infaunal communities recover faster than do emergent epifauna (Northeast Region 

EFHSC 2002), and recovery rate of benthic fauna is faster at shallow water depths (Constantino 

et al. 2009). Use of the same dredge in several areas can result in different recovery and 

recolonization patterns depending on previous exposure of the seafloor to disturbance (Pranovi et 

al. 1998). Recovery can be quicker in previously dredged versus newly dredged areas because of 

greater adaptation to stress by organisms already present in the disturbed site (Pranovi and 

Giovanardi 1994).The recovery rate of habitat depends on whether disturbance is a one time 

instance or persistent multiple events with cumulative effects (Northeast Region EFHSC 2002; 

Wilber et al. 2008). Frequent dredging of sheltered areas may have a significant impact on 

biological recruitment, productivity, and benthic community structure (Hauton and Paterson 

2003). Repetitive disturbance may impede sediment consolidation, disrupt succession of 

biological communities, prevent establishment of stable benthic assemblages (Aspden et al. 

2004), and may interfere with recruitment processes (Watling and Norse 1998). The interval 

between disturbances is critical for the ability of benthic organisms to repopulate because time 

between events can be short relative to the life span of certain species (Watling and Norse 1998; 

Watling 2005). 

Ecosystems respond to disturbance with changes to species composition, abundance, or 

biomass (Dorsey and Pederson 1998). Invertebrates undergo a natural pattern of community 

development after dredging (Klemanowicz 1985). In some cases, organisms rely on disturbance 

to create conditions favorable for recruitment, growth, and reproduction (Sousa 1984; Dernie et 

al. 2003a, 2003b). Physical changes to topography, sediment, or removal of organisms can alter 

ecological community structure and modify the environment so it is more or less suitable for the 

existing community (Langton and Auster 1999; Tarnowski 2006). This promotes establishment 

of an alternative bottom community with different species composition, functions, and ability to 

provide ecological services (Zajac et al. 1998). Where fishing is frequent, few long term effects 

are observed because the resident biological population has been selected for adaptation to 

disturbance (Watling and Norse 1998). Communities can return to their previous state when 

disturbance is not too intense, the time interval between disturbances is sufficient, the system has 

high resilience, and/or harvesting is discontinued (NRC 2002). Benthic community structure may 

not completely return to its previous state until bottom substrate stabilizes (Goodwin and Shaul 

1980). Resilient communities can maintain structure and function even in the presence of 

frequent disturbance (Zajac et al. 1998). 

Chronic disturbance from fishing activity may prevent benthic succession from 

proceeding to climax communities (Watling and Norse 1998). Reduction or replacement of 

organisms caused by mobile gear may prevent an ecosystem from returning to its original state, 

even in the absence of fishing (Northeast Region EFHSC 2002). There may be a threshold of 

intensity or a cumulative effect beyond which persistent changes to the ecosystem occur (NRC 

2002) or a level of anthropogenic impact beyond which even populations resistant to disturbance 

collapse (Pranovi et al. 1998). Marine communities are threatened when fishing-related changes 

occur faster than nature can respond (Thrush and Dayton 2002). If disturbance exceeds recovery 

time, susceptible species will be eliminated and biodiversity reduced (Dorsey and Pederson 

1998). It is not known whether benthic communities with a prolonged history of disturbance will 

return to their original state after disturbance ends or if the ecosystem would now be too altered 

to recover (Zajac et al. 1998). In areas subject to repeated-dredging disturbance, a return to 
35 

preharvest ecological conditions may be an unrealistic expectation (Wilber et al. 2008). Some 

studies which followed dredged areas for an extended period detected no or incomplete recovery 

of benthic communities (Gilkinson et al. 2003, 2005). Studies of fishing disturbance are often 

short term, and the period required for complete recovery may exceed the duration of 

experimental monitoring (Tarnowski 2006). Longer term studies might allow a better 

understanding of natural cycles of postharvest recovery and recruitment within the context of 

larger ecosystem level influences including global warming, ocean acidification, and other 

anthropogenic factors (Gaspar and Chícharo 2007). 

Seasonal Changes Affect Recovery from Dredging 

Recovery from dredging occurs against a background of natural seasonal changes (Hall 

and Harding 1997) and benthic recovery rate is impacted by the time of year when harvesting 

occurs (Pfitzenmeyer 1972a, 1972b; Langton and Auster 1999). Recovery happens quickly when 

seasonal dredging avoids sensitive periods of larval recruitment (Ismail 1985). Repopulation is 

mitigated through active migration and passive transport processes, and recovery is closely tied 

to the reproductive cycle of constituent species (Hall et al. 1990; Tarnowski 2006). Immigrationbased recovery begins just after the disturbance ends, while recruitment may have a strong 

seasonal or annual signal corresponding to reproductive cycles (Langton and Auster 1999). 

Numbers of commercial-sized, softshell clams in dredged and undredged areas of Maryland 

returned to similar levels 4 m after spring harvest but differed for up to a year following late 

summer dredging because of seasonal differences in recovery (Pfitzenmeyer 1972a, 1972b). The 

time frame necessary for restoration is influenced by timing of harvest with regard to recruitment 

of both target and nontarget organisms (Kaiser et al. 1989; Spencer et al. 1998). 

Marine ecosystems are subject to many weather-related seasonal changes (e.g., storms, 

salinity fluctuations, and icing) which can make it difficult to distinguish harvesting effects from 

disturbances caused by natural events. Impacts of mobile fishing gear can mimic benthic changes 

caused by natural disturbance (Street et al. 2005). In many cases, changes to the seafloor induced 

by wind, waves, currents, tide, and storms may exceed those caused by shellfish harvesting 

(Drobeck and Johnston 1982; Bigford 1997; Alves et al. 2003; Rheault 2008; Constantino et al. 

2009). Infrequent and low intensity harvesting have impacts within the range of naturally 

occurring habitat perturbations (Barnette 2001). Experimental studies have shown that faunal 

changes are sometimes more strongly linked to natural seasonal variability than to dredging 

effects (Godcharles 1971). In a study of hydraulic harvesting of hard clams in Narragansett Bay, 

natural spatial and seasonal variability exceeded the effects of bottom cultivation (Sparsis et al. 

1993). This finding suggests that shellfish dredging may represent a small-scale and short-term 

impact relative to other natural environmental processes (Alves et al. 2003). 

The relative significance of dredging disturbance should be evaluated in the context of 

how magnitude and frequency of natural disturbance impacts the dredged environment 

(DeAlteris et al. 1999; Constantino et al. 2009). Effects of mechanical shellfish harvesting must 

be detectable above natural and temporal variability in dynamic study areas (Gilkinson et al. 

2003; Morello et al. 2006). Large spatial effects of mobile fishing gear can be masked by cooccurring natural disturbances, while effects from small scale disturbances may not be apparent 

where recolonization occurs rapidly (Kaiser et al. 1998). Knowledge of seasonal, annual, and 

spatial variations in benthic assemblages should be considered when assessing the impacts of 

disturbance caused by mobile fishing gear (Løkkeborg 2005). 
36 

MITIGATING EFFECTS OF SHELLFISH DREDGING 

The practice of sustainable shellfish cultivation means avoiding, remediating, or 

mitigating any adverse effects of harvesting on the aquatic environment (Beentjes and Baird 

2004). The minimal, short-lived, and localized nature of shellfish harvesting practices can be 

further reduced through responsible program modifications (Barnes et al. 1991). Long term 

disruption of the seafloor environment by dredging can be controlled through a combination of 

selective harvesting practices, careful site selection, rotational seeding, cultivation, creation of 

fallow areas, and seasonal harvesting to avoid disrupting larval shellfish recruitment and to allow 

time for recovery (Kaiser et al. 1989; Spencer et al. 1998). Management of renewable shellfish 

resources includes habitat preservation and limiting harvest to surplus populations (Hargis and 

Haven 1999). Effective management of shellfish populations includes creation of habitat for a 

variety of marine species, water and sediment quality enhancement, stabilization of critical 

shoreline habitat, and maximizing harvested product (Opton-Himmel and Whelchel 2010). 

Restoration of shell material to dredged bottoms following harvest could mitigate any negative 

effects associated with cultivation by providing fresh cultch for settling larvae (Manzi et al. 

1985). Creation of unharvested refuge areas can protect populations of broodstock oysters to help 

ensure adequate larval supply and recruitment (Breitburg et al. 2000). Regulatory restrictions can 

prevent harvesting during periods of peak biological activity or reproduction and can create 

broodstock and seagrass sanctuaries (Tarnowski 2006). Potential adverse effects of dredging on 

essential fish habitat can be mitigated by fishing equipment restrictions, area closures, and 

harvest limits (Wallace and Hoff 2005). Modifications to harvesting equipment can reduce 

certain impacts but may require trade-offs. While trials of a new hydraulic clam dredge with 

vibrating bottom grid improved size selectivity, allowed escape of undersized clams, reduced 

nontarget by-catch, and enhanced the quality of commercial catch, this dredge also increased 

slightly the number of shells damaged during harvesting (Rambaldi et al. 2001). 

Ecological effects of hydraulic escalator dredging are often mitigated by the attributes of 

the target species and the physical dynamics of inshore coastal ecosystems where harvesting 

occurs. In most cases, hard-shelled bivalve species show high resilience to and recovery from 

harvesting impacts. Benthic communities established in response to historical shellfishing 

practices demonstrate rapid recovery to disruption (Tarnowski 2006). Since high-energy 

environments with resilient biological communities recover quickly from disturbance, the 

physical and biological attributes inherent to these ecosystems help offset the effects of dredging 

and may account for the rapid elimination of morphological and sedimentary impacts of harvest 

(Constantino et al. 2009). 

Shellfish cultivation may improve habitat quality. Some natural shellfish beds lack the 

physical or biological factors necessary for successful growth, and this condition can limit 

shellfish abundance (MacKenzie 1989). Hydraulic shellfish harvesters have been used as a 

remediation tool to treat marginal clam habitat and enhance the substrate for improved survival 

of transplanted clams (Nickerson and Brown 1979). Cultivation of oysters can prevent the 

mortality or malformation that occurs in natural undredged oyster beds, when animals growing 

side by side in clusters become too crowded for normal healthy growth (Brooks 1905). Hydraulic 

dredging can decrease the quantity of anoxic sediments and enhance shellfish production (Visel 

2008). Increased clam abundance is associated with relocation of shellfish to cleaner areas for 

depuration, periodic cleaning of cultch to remove silt, cultivation of bottom sediments, and 

addition of shell to reduce acidity (Visel 2008). Periodic resuspension of fine sediments and 

organics by dredging can improve sediment quality (Emmett 2002). Hydraulic harvesting 
37 

removes old shell cover from the bottom, loosening sediment, and improving grain size 

(MacKenzie 1979). Shellfish cultivation may increase pore area within sediments, improve 

circulation, and elevate pH levels which can positively impact clam populations (Visel 1990a). 

Dredging stirs up organic detritus which may enhance shellfish production by increasing food 

availability for filter feeding organisms (Pranovi et al. 2003). Release of nutrients from 

sediments into the water column can create a food source, and temporary turbidity can create a 

predation refuge for benthic fish and invertebrates (Dorsey and Pederson 1998; Auster and 

Langton 1999). 

AREAS FOR FURTHER RESEARCH AND CONCLUSIONS 

Understanding all potential effects of shellfish dredging on physical habitat, biological 

communities, and biogeochemistry of sediments is challenging. Designing a statistically 

defensible experimental study that comprehensively evaluates harvesting effects requires direct 

comparison of dredged areas to undredged areas in close proximity that have similar habitat, 

environmental, and hydrodynamic characteristics. Sampling treatment areas prior to 

experimental dredging may help account for seasonal effects. In an ideal study, effects of 

dredging in different seasons would be compared at different durations and intensities and over a 

wide range of spatial scales. Optimally, physical changes to the bottom would be documented by 

using photography, video, or sonar. Impacts to the biological community would be determined 

through intensive sampling of benthic infauna and epifauna. Analyses of sediment at different 

depth strata would be used to determine the magnitude and duration of biogeochemical changes. 

Data that define habitat, sediment type, and grain size would be important, as well as a detailed 

description of the dredge gear, including its dimensions and degree of impact. Further, spatially 

extensive, long-term monitoring would be valuable to give an ecosystem-level context to the 

study. Unfortunately, many of the research studies we report in this document lack key elements 

in their experimental design or methods. Differences in spatial scale, geography, dredging 

equipment, habitat and sediment type, and experimental design make it difficult to directly 

compare results across studies and draw universal conclusions concerning dredging impacts. 

Despite an abundance of literature concerning shellfish dredging, there are no definitive 

“one size fits all” answers to questions concerning the impact of shellfish harvesting. From 

experimental studies, it is clear that the observed physical, biological, and chemical effects of 

dredging are highly variable. The level of impact resulting from shellfish dredging corresponds 

to the spatial and temporal scale, sediment type, harvesting equipment, biological community, 

environmental attributes of the ecosystem, and the assessment metrics used to evaluate effects 

over time. Physical alteration of the seafloor resulting from dredging usually reverts back to its 

prior state through the natural physical processes of tides, currents, and storms. Ecological 

recovery of benthic biota following dredging is linked to natural and ongoing seasonal 

recruitment processes. In dynamic, shallow-water, coastal areas that regularly experience natural 

disturbance and where the duration and spatial scale of dredging is limited, ecological recovery 

is usually rapid. Common, fast-growing benthic organisms with short generation times quickly 

recolonize benthic habitats following dredge disturbance. Bioturbation and other natural 

processes can restore sediment biogeochemistry. When considered in this context, the impacts of 

shellfish dredging are minor relative to the many natural disturbances that occur within the 

coastal marine environment. The effects of dredging in shellfish cultivation are further mitigated 

by its generally limited spatial and temporal scale and the use of sound harvesting practices on 

leased grounds. 
38 

ACKNOWLEDGEMENTS 

The authors express their appreciation to NEFSC librarians Angela Cook and Claire 

Steimle and to Catherine Kuropat, Paul Clark, Jose Pereira, and James Reidy for additional 

support. We also thank Christopher Brown, John Kraeuter, Clyde MacKenzie, and Robert 

Rheault for reviewing this document and Jarita Davis for editorial review. 
39 

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North Amer. J. Fish. Manage. 21:855-875. 
58 

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59 

Table 1a. Dredging effects reported from research studies on the northern quahog, Mercenaria mercenaria. N/A indHabitat Sediment 

Harvest Gear 

Study Location Biological Effects Physical Effects Chemical 

Effects 

Tidal river bottom 

Hydraulic escalator dredge 

Santee River, South Carolina 

No harmful effects on biota, more live oysters found after harvest because of early spat settlement, fewer found where harvesters removed commercial sized animals 

N/A N/A 

Subtidal mixed sediment 

Hydraulic escalator dredge 

Santee River, South Carolina 

Amount of spat in water column similar between harvested and unharvested areas 

0.1 M tow depth N/A 

Firm sandy mud 

8 and 12-toothed clam dredges 

Narragansett Bay, Rhode Island 

1% gear-related breakage of commercial sized clams, low breakage or smothering of undersized clams, removed clams > 60 mm, decreased tube worms 

Mixing of sandy-mud and clay layer more pronounced than in control area, sediment was also softer 

N/A 

Mud and sand 

Hydraulic escalator dredge 

Rappahannock, James and York rivers, Virginia 

Dredging did not increase hard or soft clam set, oysters 75-100 ft from dredged area were unaffected, dredging uprooted eelgrass and removed invertebrate tubes 

Dredging changed appearance, composition and texture of seafloor, created trenches 6-8 inches deep, reduced siltclay fraction, moved buried shell to surface, effects within 75 ft of dredge 

N/A 

Assorted sediments sand, silt, clay, CaCO3,

Hydraulic escalator harvester 

South Carolina Tidal creeks: Back Creek, Hamlin Creek, Summerhouse Creek, 2 creeks near Isle of Palms 

After harvest polychaetes decreased, amphipods increased, diversity and abundance increased, no difference in mobile fish and invertebrates 

Elevated turbidity in vicinity of harvest, plumes extended < 2 km, highest < 1 km away, several hours duration, increased CaCO3 and 

decreased clay following harvest 

N/A 
60 

Table 1a, continued. Dredging effects reported from research studies on the hard clam, Mercenaria mercenaria. N/

SAV beds Clam 

dredge 

Chincoteague Bay, Virginia 

SAV absent from dredge circles, vegetation was dense and healthy just outside of dredge zone 

Dredging disrupted sediments from rich organic sand to coarse sand and broken shell 

N/A 

SAV beds Clam 

dredge 

Chincoteague Bay, Virginia 

15% less vegetation than found at scar edge, low cover in scar area except at center 

Increased bottom depth in scars, large holes up to 1 meter diameter and 30-40 cm deep 

N/A 

Seagrass bed and sand flat 

Clam “kicking” and raking 

Back Sound, North Carolina 

Raking and light clam kicking: seagrass declined 25% below controls; intense clam kicking: 65% decline, clam harvest had no effect on density or species composition, harvest did not boost clam recruitment 

N/A N/A 
61 

Table 1b. Dredging effects reported from research studies on the softshell clam, Mya arenaria. N/A indicates no daHabitatSediment Harvest Gear Study Location Biological Effects Physical Effects Chemical Effects Recovery Intertidal flats 

 Hydraulic escalator dredge 

Nova Scotia, Canada 

Removed 90% of small clams and 50% of market sized, 1% shell breakage 

Dug to 14 in depth, high efficiency 

N/A N/A 

Mud and sand substrate 

Hydraulic escalator dredge 

Chesapeake Bay, Virginia 

Dredging did not increase hard or soft-shell clam set, oysters 75-100 ft from dredged area unaffected 

Seafloor appearance, composition, and texture altered; removed invertebrate tubes; trenches 6-8 in. deep, reduced silt-clay fraction; moved buried shell to surface; changes occurred within 75 ft of dredge 

N/A Trenches fi

within 1-2 m

Silt-clay intertidal flats 

Hydraulic escalator dredge 

Harraseeket River, Maine 

Few significant changes: Corophium volutator declined, vegetation uprooted in tracks, clams intact and unbroken, decrease in large clams, decline in polychaetes, increase in Macoma balthica

Solid clay removed from flats creating a trench filled with soft sediments, turbidity briefly increased as a plume but suspended solids returned to low levels, created trenches 30-45 cm deep and mounds of spoils 

Slight changes in chemical parameters, low DO and pH indicate reduced sediment brought up by dredge into water column, was quickly oxidized 

Trenches hwell definemonths, spoheaps lasted months, at months trenhad fine, sosediment 5-below normmajor speciincreased bmonths 

Muddy soil, sandy further toward beach 

Hydraulic clam rake 

Pottery Bridge Flat, St. Andrews, Canada 

Harvested 60% commercial clams, no breakage of marketable clams with short nozzle, some mortality with long nozzle 

Created a track that remained soft for a few days and was an inch or two lower than undisturbed flat 

N/A N/A 
Table 1b, continued. Dredging effects reported from research studies on the soft-shell clam, Mya arenaria. N/A ind

62 

Intertidal flats 

Mechanical escalator dredge 

Clam Harbour, Nova Scotia Canada 

Initial tests dredge harvested 90% of small clams, 50% of commercial sized, some mortality of seed clams by smothering and overcrowding, after dredging little to no damage to shellfish 

Dug deep trenches in shallow water 

Only 7-10% were too damaged to rebury after 3 hours 

N/A 

Fine sand Hydraulic 

clam rake 

Clam Harbour, Nova Scotia Canada 

Damage to <5% of marketable clams and <5% of small clams remaining after harvest, sand and grit in mantle cavity of clams, shell breakage <1% 

Liquefied upper soil strata into soil-water suspension, tracks 24 inches wide, 2.5 in deep, poor operation can excavate wells in sediment 

N/A N/A 

Oyster beds and clams on soft sediment 

Hydraulic clam dredge 

Maryland Intensive dredging: 

mortality of oysters in dredge zone and 25 ft downstream, no mortality of oysters or spat 75 ft beyond dredge, few clams broken, increase in predatory fish and crabs, dredging not a hazard to tidewater resources away from oyster beds 

Intensive dredging elevated turbidity and caused redeposition of suspended sediment 75 ft downstream, trenches up to 18 in deep but more typically 2-8 in 

N/A Trenches 4

after dredgiinches deerecovery vasediments mtake monthcompact, drareas continrepopulate clams 

Subtidal assorted sediments 

Maryland soft shell clam dredge 

Maryland No damage to fish or blue 

crabs, about 1% breakage of collected clams, reduced number of market sized clams, some breakage of fragile animals in collection baskets at end of conveyer, fish attracted to dredge track due to food availability 

Sediment dislodged from the bottom, and falls through the conveyer or brought to the surface 

N/A After 7 yrs 

harvesting, evidence of impaired reproductioreplacemenstocks 
Table 1b, continued. Dredging effects reported from research studies on the soft-shell clam, Mya arenaria. N/A ind

63 

Clam flat, clean sandy soil 

Hydraulic rake 

Clam Harbour, Nova Scotia, Canada 

Damage to <5% of the catch and <5% of the clams left behind 

Upper substratum is converted to a soil-fluid mixture, track width measured 33 in, track was firm after 24 hours, nozzle settings determine track width 

N/A N/A 

Intertidal beaches, sandy flats 

Maryland style hydraulic escalator clam dredge 

Clam Harbour, Nova Scotia, Canada 

Clams returned after dredging settled on the surface, not buried or smothered, breakage to 7-10% of clams, 90% of small clams returned within 75-100 ft of where they entered the dredge, clams reburied quickly 

Heavy “soil” settled first in tracks, tracks 50-75 in. wide with surfaces 4-6 inches below adjacent levels, crumbling and erosion of tracks with extended widths 

N/A N/A 

Medium to fine sands 

Maryland hydraulic escalator clam dredge 

Potomac River, Maryland 

Small clams overcame dredging effects better, sublegal clams not significantly reduced by dredging, recruitment of young clams increased, where number of adults was reduced by dredging 

No major changes to sediment structure or grain size after dredging 

Organic carbon in first inch of substrate redistributed and concentrated after dredging 

Dredged bosoft for 1 yMarch and dredging shno differenclam (>35 mdensities afmonths, Audredging: dfor 8-12 mo

Sand to sandy mud 

Hydraulic dredge 

Chester River, Tributary to Chesapeake Bay, Maryland 

Increased turbidity and light attenuation from dredging decreased light penetration impacting SAV, SAV tolerated reduced light for a day or two of clamming beyond that negative impacts 

Plumes: significantly higher turbidity/light attenuation than background, decreased sediment compaction due to sediment type and water depth, plume dissipation linked to grain size 

N/A Plumes dis

exponentiarapidly at fcoarse partisettle, slowfine sedimeplumes in swater slowedecay 
64 

Table 1c. Dredging effects reported from research studies on harvest fishery of deepwater North and Mid-Atlantic 

indicates no data. Species Habitat 

Sediment 

Harvest Gear 

Study Location 

Biological Effects Physical Effects Chemical 

Effects 

Recover

Arctic surfclam, Mactromeris polynyma 

Medium grained sandy bank 

Two hydraulic clam dredges, 4 m wide, 12 tons 

Banquereau, Scotian Shelf, southeast Atlantic Canada 

Density of large burrows reduced by 90% due to mortality of clams, polychaete tubes reduced, removal of empty shell from benthos 

Sediment smoothing, dredge created 20 cm deep, 4 m wide curvilinear furrows, margins degraded by slumping, sediment transport and bioturbation 

N/A Lasting e

sediment no recovlarge buryrs, dredpersist foincrease polychae2 yrs, at 3 100% incpredredg

Cyrtodaria siliqua, Arctica islandica, M. polynyma, Serripes groenlandicus

Medium grained sandy seabed 

Two hydraulic clam dredges  4 m wide, 12 tons 

Banquereau, Scotian Shelf Eastern Canada 

40% decrease in macrofaunal abundance in furrows, damage to some clams, reduced biomass of target species, colonizing on-going for 2 years 

Cutting depth to 20 cm 

N/A Marked i

polychaeamphipoabundancopportunspecies inby >100%taxonomdistinctnedeclined, recovery species a

Arctic surfclam, M. polynyma 

Sand with some rocks 

New England hydraulic dredge 

Gulf of St. Lawrence, Canada 

Damage to <10% of surf clams, 50% of razor clams and a small number of other mollusks, 2/3 of clams remained on bottom, no long or short term harm to resident benthic species 

Depth of impact 15 to 30 cm, sediment suspended for up to 30 minutes, sediments in tracks less compacted than adjacent areas 

N/A N/A 
Table 1c, continued. Dredging effects reported from research studies on harvest fishery of deepwater North and M

species. N/A indicates no data. 

65 

Ocean quahog, Arctica islandica 

Very fine to medium sand, recently fished and abandoned bed, currently fished and unfished control 

Hydraulic dredge 

Continental shelf off coastal New Jersey 

Abundance and species composition of benthic macroinvertebrates was not altered by dredging 

Dredged areas had small shell fragments and gravel on the sand surface caused by resorting of sand by water jetting 

N/A 

Atlantic surfclam, Spisula solidissima 

Fine, medium, and silty sands 

1.2 m Hydraulic clam dredge 

Offshore of Rockaway Beach, southwest Long Island New York 

Predators more abundant in dredge track, densities back to normal after 24-hrs except moon snails increased, mortality was 30% when dredge efficiency was high 

Initial dredge track conspicuous with smooth track shoulder, angled walls and a flat floor 

N/A Dredge tr

deterioraand after became sdepressio
66 

Table 1d. Dredging effects reported from research studies on assorted clam species in Florida. N/A indicates no dTarget Species Habitat 

Sediment 

Harvest Gear Study 

Location 

Biological Effects Physical Effects Chemical 

Effects 

Recov

Southern Quahog, Mercenaria campechiensis, Southern surfclam, Spisula raveneli, Sunray Venus clam, Macrocallista nimbosa

Variable sediment and seagrass 

Maryland hydraulic escalator clam dredge 

Tampa and Boca Ciega Bays, Cedar Keys, Tarpon Springs, Florida 

No recolonization of seagrass turtlegrass Thalassia testudinum and

Syringodium filiforme, no increase in clam set, no differences in fauna between dredged and control 

Water jets penetrated to 18 inches, uprooted vegetation, tracks visible from 1- 86 days, some areas soft for >500 days, decrease in silt/clay after dredging 

N/A Some 

of algCauleprolifdays pdredgtrenchfilled immedecresilt/clresolva year 

Sunray Venus clam, M. nimbosa 

Sandy substrate 

Commercial hydraulic Nantucket clam dredge 

northwest coast Florida 

Dredging damaged beds of turtle grass, excessive hydraulic pressure forced organisms under cutting blade damaging them 

Dredge filled rapidly with mud disturbing surface layers 

N/A N/A 

Sunray Venus clam, M. nimbosa 

Loose quartz sand 

Nantucket hydraulic dredge 

Bell Shoal, St. Joseph Bay, Florida 

Numbers of fish increased after passage of the dredge, some shell breakage, overall operation of dredge was not harmful to marine environment, bycatch included other commercial clam species 

Substrate was churned up to free clams 

N/A N/A 
67 

Table 1e. Dredging effects reported from research studies on the Manila clam, Ruditapes philippinarum. N/A indica

Habitat Sediment 

Harvest Gear 

Study Location 

Biological Effects Physical Effects Chemical Effects Time to Reco

Firm coarse substrate with rocks 

Hydraulic escalator dredge 

British Columbia 

Decline in harvest size clams, some mortality of legal and sublegal clams 

Trenches 0.5 m deep, 2 m wide at 2-4 months, deep holes, mounds of side cast material 30 cm deep, empty shells 

N/A No significan

recovery 16 mpostharvest 

Soft bottom, clay 

Rusca (iron cage) and rotating drum (iron) teeth rotate and wash clams from drum to conveyer 

Venice Lagoon, Italy 

Disturbance of benthic community, bottom sediments became azoic, decrease in abundance of benthic organisms 

Resuspension of top sediment layer, brought deep anoxic layer near the bottom, harvest gear extends 10 cm deep, changes to sediment compaction 

Depletion of oxidized sediment layer, effects on redox conditions Likely to affect nitrogen and phosphorus cycling 

N/A 

Sand silt clay 

Simulated sediment dredging 

Sacca di Goro, Italy 

Not measured with regard to simulated dredging 

Resuspension of surface sediment, cultivated sediments more reduced than control 

Rapid depletion of oxygen in water overlying sediments 

N/A 

Intertidal sandflats, Lanice conchilegabeds 

Tractortowed sifter Chausey archipelago

Normandy France 

Decreased densities of worm L. conchilega and abundance and diversity of macrofauna 

Sifted top 10 cm of substrate 

N/A N/A 

Mud flat with clay, fine sand and silt 

Suction dredge 

Southeast England 

Reduction in density of individuals, number of species and diversity 

Removed larger sand fractions down to the underlying clay substrate, sediment resuspended by dredge, exposing clay 

N/A No difference 

infaunal commin dredge and areas by 7 mosediment strurestored by sedimentation 
Table 1e, continued. Dredging effects reported from research studies on the Manila clam, Ruditapes philippinarum

data. 

68 

Silty-sand Hydraulic 

dredge 

Venice Lagoon near port of MalamoccoItaly 

Nonselective reduction in species abundance, both those captured and those resuspended in the sediment plume and transported by currents 

Produced deep 20 cm furrows affecting texture of the bottom 

N/A After 60 days

opportunistic assume opporbehavior durirecolonization areas 

 Lagoon Mechanical 

rusca (iron cage) dredge 

Venice Lagoon, Italy 

Enhanced clam growth, negative effects on some benthic invertebrates and detritivorous fish, positive effects on macrophytal grazers, reduced macroalgae 

Resuspended sediments provide a food source for clams, especially juveniles, decreased light transmittance and water transparency 

Removal of bioturbators affects sediment biogeochemistry since harvesting is a strong mixing force 

N/A 

Transition from silt/ silt-clay (15 years ago) to sand or silty-sand 

Hydraulic dredging 

Central Venetian Lagoon, Italy 

Significant changes in total abundance and biomass, no Zosteracolonization and diffusion, scavengers increased 

Furrow 8- 10 cm deep, no immediate changes in sieve fractions, long term effects on sieve fractions from loss/redistribution of fine sediments 

N/A Furrows visib

months, diffebiological comfor 60 days, lchanges in paand sediment 

Muddy sand 

Suction harvesting 

River Exe, Devon, United Kingdom 

Invertebrate abundance and species diversity reduced by >90% 

Increased sediment load in water, 10 cm deep trench 

Suspended particles settled downstream, dispersed to background levels 40 m from dredge 

Rapid recoveinvertebrates recruitment) wmonths of hartrenches refillmonths 

Muddy sand 

Suction dredge 

River Exe, Devon, United Kingdom 

Immediate 80% reduction in infaunal species abundance 

Created 10 cm deep trenches which took 2-3 months to refill 

N/A Sediment stru

invertebrate icommunity reby 12 months 
69 

Table 1f. Dredging effects reported from research studies on clam species from Portugal. N/A indicates no data. Target species 

Habitat Sediment 

Harvest Gear 

Study Location 

Biological Effects Physical Effects Chemical 

Effects 

Recovery 

Spisula. solida, Donax trunculus, Venus striatula, Pharus legumen, Enis siliqua

Softbottom Portuguese dragged, 

toothed iron clam dredge 

Algarve coast, South Portugal 

Decrease in abundance of meiofauna and macrofauna, target and fragile taxa predators increased 

Dredge penetrated up to 50 cm depth, sediment redistribution 

N/A N/A 

S. solida D. trunculus, V. striatula, P. legumen, E. siliqua

Sand Dragged 

iron bivalve dredges 

Algarve coast, south Portugal 

Macrofaunal distribution, diversity, evenness, number of taxa and abundance varied across dredge track 

A sand buffer formed in front of the dredge mouth pushing sediment sideways 

N/A N/A 

S. solida Sandy 

sediment grain sizes 0.5 and 0.355 mm 

Dragged iron bivalve dredges 

Algarve coast, south Portugal 

Increased number of exposed clams, predators increased 6-9 min after dredging: Ophiura texturata, Pomatochistus spp., Diogenes pugilator, Nassarius reticulatus

N/A N/A N/A 

S. solida Simulated 

sand dredge tracks 

Laboratory simulated bivalve dredge 

Algarve coast, south Portugal 

Sublethal effects on clams: decreased RNA/DNA and N/P lipid ratio, decline in condition 

N/A N/A Clam cond

improved spawning 
Table 1f, continued. Dredging effects reported from research studies on clam species from Portugal. N/A indicates 

70 

S. solida, D. trunculus

Coarse sand and gravel 

Towed clam dredge 

Algarve, southern Portugal 

Impacts greater at 18 m depth: macrobenthic organisms showed reduced abundance, number of taxa, and diversity, decrease in meiofauna abundance number of taxa 

Sediment morphology and texture affected, dredge track measured 30 cm wide and 10 cm deep 

N/A Faster bio

recovery aversus 18 meiofauna recovered days, trackgone after but still vi18 m for 1at 6 m grawas similacontrol by at 18 m 1 dredging, increase in size, by 13 dredged sicontrol 

S. solida, D. trunculus, E. siliqua, P. legumen 

Shallow sandy 

Mechanical metal bivalve dredge 

Vilamoura and Armona, Algarve coast, Portugal 

N/A Formation of a 

furrow exposing underlying sand with a spoil ridge on either side of the depression 

Porewater nitrates, ammonium, organic nitrogen, phosphate and silicate decreased postdredging and increased in near bottom water 

Reestablisthe seabed reached wshort time stations 

Chamelea gallina, D. trunculus, S. solida, Tellina tenuis

Sand and sandymud bottom 

Portuguese clam dredge 

Lagos in South Portugal 

Low damage and mortality of macrobenthic animals in dredge path, scavengers attracted in high densities, but dissipated rapidly 

Suspended sediment settled rapidly in sand and mud, tracks deeper and more persistent in sandy mud , tracks eroded via wave action and currents 

N/A Undamage

slightly dashellfish rimmediateescaping tdredge 
71 

Table 1g. Dredging effects reported from research studies on the Striped Venus clam Chamelea gallina. N/A indicaHabitat Sediment 

Harvest Gear Study 

Location 

Biological Effects Physical Effects Chemical 

Effects 

Recovery 

Two sites: sand vs. mud 

Hydraulic dredge using high and low water pressure 

North Adriatic Sea 

High pressure treated clams had lower adenylate energy charge than low pressure, high pressure treated clams burrowed less 

Grain size influenced speed and operation of dredge, impacts were greater on mud bottom 

N/A Juvenile clam

returned to wdredging rebslowly and ato predation 

N/A Dredging with 

high pressure water jets with sieve sorting versus low pressure water jets 

Lido coast, Lagoon of Venice, Adriatic Sea 

At high pressure clam filtration rates decreased , respiration rates increased, lower scope for growth, hematocrit and phagocytic index decreased, reduced acid phosphotase and increased β-glucuronidase activity 

N/A N/A N/A 

Fine sand 2.4 -3 m wide, 

0.6 - 0.8 ton hydraulic dredge 

Adriatic Sea, Italy 

No effects to macrobenthic community (polychates, crustaceans, detritivores and suspensivores), mollusks and bivalve Abra alba were affected, short term pulse impact on scavengers and predators 

N/A N/A N/A 

Fine sand Hydraulic clam 

dredge 

Adriatic Sea, Italy 

Depth strata and fishing intensity affected dredge impact, moderate disturbance to benthic community and significant difference in species number and evenness between fishing intensity at 4-6 m, reduction in evenness at high intensity, increased species number with decreasing intensity 

N/A N/A Recovery of 

community w
Table 1g, continued. Dredging effects reported from research studies on the Striped Venus clam Chamelea gallina

data. 

72 

Jesolo; fine sand, Lido; medium grain sand 

Hydraulic dredge, high and low pressure without sorting, high pressure with sorting 

Jesolo and Lido, Northern Adriatic Sea, Italy 

Water pressure and sorting increased shell damage, the larger the clam the more damage it sustained, some clams survived damage 

N/A N/A N/A 

Jesolo; fine sand, Lido; medium grain sand 

Hydraulic dredge, high and low pressure without sorting, high pressure with sorting 

Jesolo and Lido, Northern Adriatic Sea, Italy 

High water pressure and mechanized sorting decreased clearance rates, scope for growth, and survival in air, season may increase affects 

N/A N/A N/A 

N/A Experimental 

hydraulic dredge with vibrating bottom grid 

Adriatic Sea North of port of Giulianova, Italy 

As compared to standard gear: larger number of damaged shells, better size selectivity and escape of undersized clams and discarded fauna, reduced by-catch, enhanced quality of commercial product 

N/A N/A N/A 
73 

Table 1h. Dredging effects reported from research studies on the razor clam, Ensis sp. N/A indicates no data. 

Habitat Sediment 

Harvest Gear 

Study Location 

Biological Effects Physical Effects Chemica

l Effects 

Recovery 

Clam bed Hydraulic 

dredge 

Gormanston,  Irish Sea 

Reduced dominant and target species, increased infaunal community diversity, increase in scavengers and predators, Lanice conchilega tube worm eliminated, E. siliqua replaced 

by suspension feeder Lutraria lutraria

Dredging to 30 cm deep, increase in larger grain sizes and sorting coefficients, high content of broken shell 

N/A Initial increase 

diversity, folloa downward tr

Clean sandy bottom 

Two dredges, varying tooth lengths 

Lagos, south Portugal 

10-15% by-catch damage to clams reduced by increased tooth length and decreased tow duration, injury inversely proportional to catch efficiency 

Physical impact of dredge of short duration 

N/A Dredge tracks 

within 24 hrs 

Sand Hydraulic 

suction dredging 

Loch Gairloch, Ross-shire, Scotland 

Reduction in target species, increased scavengers, reduced number of macrofaunal species and individuals after 1 day 

Physical disturbance, trenches and holes, area affected 20-30% 

N/A Trenches and h

resolved within days, macrofaurecovered by 4

Maerl Hydraulic 

blade dredge 

Clyde Sea, Scotland 

Kelp coated with mud, both seaweed and sessile biota buried with silt, fragile organisms damaged, increase in predators post-dredging 

Changed from sandy gravel to gravelly sand, suspended sediment reduced visibility, dredge track formed, snowplough effect, altered sediment to 9 cm 

N/A Sediment settle

within 1hour, tpartially erodewithin 1monthand width reduwave action 

Sand Hydraulic 

blade dredge 

Clyde Sea, Scotland 

Survival of 60-70% of dislodged fauna mostly urchin Echinocardium cordatum, 20-100% of target clams damaged 

Tracks of fluidized sand beds 

N/A Reburial of 80

of clams within min, a few stilunable to rebur3 hrs 
Table 1h, continued. Dredging effects reported from research studies on the razor clam, Ensis sp. N/A indicates no 

74 

Coarse sand and shell 

Hydraulic dredge 

Clyde Sea, Scotland 

Some damage of larger versus clams 

Dredged area 45 cm swath, 80 cm disturbed, surface width of dredge track was 1.01 m 

N/A N/A 

sand Hydraulic 

dredge 

Clyde Sea, Scotland 

N/A Suspended fine sediment 

into water column, resettled in dredge track, sediment reduced from moderately to poorly sorted, dredge left 13.9 cm tracks of fluidized sand, eliminated stratification, sediments vertically homogenous to 20 cm 

N/A After 100 days

action and bioturbation retracks to 2.9 cm depth, tracks nshallow furrowwidth increase100- 115 cm 

Fine sand and open broken shell 

Suction dredging 

Orphir Bay and Bay of Ireland, Orkney Islands, United Kingdom 

Lower density and smaller mean length of clams from dredged area, breaks in shell margin, sand grains embedded in deep clefts in the shell matrix 

N/A N/A Some clams sh

slow initiation escape digging increased vulnerability to predator attack 

Sand Water jet 

dredging 

Outer HebridesWestern Isles, United Kingdom 

Initial removal of infauna, damage to 10-28%, scavengers attracted to tracks, immediate reduction in number of species, individuals, and biomass, no change in diversity, reduced polychaetes, increase in amphipods, damage to large bivalve by-catch 

Immediate physical effects apparent, visible dredge tracks up to 2 m wide, fluidized sediment, reduction in % silt immediately after dredging, elevated turbidity 

N/A Dredge tracks 

after 5 days, anno longer visib11 weeks, sediremained fluid% silt returned normal at 5 day
75 

Table 1i. Dredging effects reported from research studies on Pacific Northwest clam species. N/A indicates no datTarget Species Habitat 

Sediment 

Harvest Gear 

Study Location 

Biological Effects Physical Effects Chemical 

Effects 

Re

Saxidomus gigantea, Leukoma staminea, R. philippinarum

Firm coarse substrate with rocks 

Hydraulic escalator harvester 

British Columbia 

Decline in harvest size clams, mortality of legal and sublegal clams 

Trenches 0.5 m deep, 2 m wide at 2-4 months, deep holes, mounds of side cast material 30 cm deep, empty shells 

N/A No 

sigclaremopo

S. gigantea, P. stamina, Tresus capax, Tresus nuttallii

Sand, gravel and shell substrate 

Hydraulic escalator harvester 

Agate Passage, Puget Sound, Washington 

Reduced abundance of attached kelp, little effect on number of benthic species, reduced number of individuals and weight of organisms 

Changes in substrate distribution, shell left on substrate surface, no effects on percentage of fine substrate 

Chemical measurements in harvested areas were reduced or unchanged versus control likely due to reduced biomass 

Mrecowi

S. gigantea, P. staminea

Sand, gravel and shell, some eelgrass beds (which dredge avoided) 

Hanks hydraulic escalator harvester 

Puget Sound, Washington 

Smothering of some adult clams 

Visible tracks and furrows, decrease in sediment compactness, transient sandbar, loosening, emulsification and loss of vertical stratification 

N/A Be

re1.5 no vicla
76 

Table 2. Dredging effects reported from research studies on the Eastern oyster Crassostrea virginica. N/A indicate

Habitat Sediment 

Harvest Gear 

Study Location 

Biological impacts Physical Impacts Chemical 

impacts 

Recovery 

Intertidal Mechanical 

oyster harvester 

South Carolina 

No detectable damage to oyster shell matrix, 5% of harvested oysters were damaged 

N/A N/A N/A 

Oyster bottom 

Hydraulic escalator dredge 

Patuxent River, Maryland 

Minor effect of heavy particles on oysters within 15 ft of dredging, infauna not significantly reduced in dredge and impact areas, no affect on juvenile clams, 100% mortality and burial of oysters in dredged area, disruption of epibenthic community 

Substrate surface color was lighter than on undisturbed bottom, troughs and ridges in dredged area, suspension of sediment high compared to background, no significant change in particulate fraction in impact area 

No toxic substances detected after dredging 

Reestablishment of infauna in dredged area rapid, 3 days after dredge no alteration of bottom, no accumulation of displaced substrate, no burial of oysters or cultch 

High and low intertidal oyster reefs 

Mechanical oyster harvester 

Beaufort County, South Carolina 

Oyster biomass declined in high and low intertidal, species density correlated with oyster biomass, reduced faunal density in high intertidal with target species number and frequency unaffected by harvest, diversity and evenness in harvested high intertidal > than control 

Some areas appeared undisturbed, others had tracks and deep depressions 

N/A Oyster biomass 

in high intertidal remained low, oyster spat were attracted after harvest 
77 

Table 3. Dredging effects reported from research studies on the blue mussel, Mytilus edulis. N/A indicates no data

Intertidal sublittoral 

Dutch and Baird dredges 

Menai Straits, North Wales 

Shell damage to 13% of mussels from rotary sorting and 1.6% damaged by dredging, up to 5% of small mussels destroyed by dredge and sorting induced shell fractures 

N/A N/A Mussels with light 

damage were still ahours after sorting 

Two sites, sand and mud 

2-3 m width single commercial dredge and otter trawls 

Lower Narragansett Bay and Rhode Island Sound, Rhode Island 

N/A Scars from dredging 

and trawling are short lived in sand and shoal waters and longer lasting in deep water and mud 

N/A Bottom scars in sha

sand substrate reso1 to 4 days, 60 daydeep mud substrate 

Mussel beds with bare mud flats 

Dutch mussel dredge, 1.8 m wide, 250 kg 

Danish Sound, Limfjorden, Denmark 

Reduced density of polychaetes, reduced species number, increase in shrimp Crangon crangon

Dredging formed 2-5 cm deep furrows, no change to sediment texture or organic content 

N/A Reduced species nu

in dredge area lastedays, increase in spnumber outside drearea for 7 days 

Mud, silt and clay 

Commercial mussel dredge 

Maquoit Bay, Maine 

Dragging disturbed 10% of eelgrass in study area, removing plant materials above and below ground 

Dragging did not affect physical characteristics of the sediment 

N/A After 1yr eelgrass s

density, height and biomass reduced, rbiomass persisted fyrs , pattern and ratrecovery proportiondrag intensity 
78 

Table 4. Dredging effects reported from research studies on the cockle Cerastoderma edule. N/A indicates no dataHabitat Sediment 

Harvest Gear Study 

Location 

Biological Impacts Physical Impacts Chemical 

Impacts 

Recovery 

Two sites, mud and sand, intertidal 

Tractor towed cockle harvester 

Burry Inlet, Wales 

Loss of common invertebrates, decreased species richness at both sites, dominance declined in sand area, community in clean sand recovered more quickly than mud 

Physical disruption to the complex layered structure of the sediment 

Anoxic layer was brought to the surface and dispersed 

Modest recovery occurred in sand, mud Pygospio eleand Hydrobia ulv(gastropods) remdepleted for 100 dNephtys homberg(polychaete), Scoarmiger and Bathyporeia pilos(amphipod) for 50 

Silty sediments, coarser toward center of bay 

Hydraulic suction dredge and tractor dredge 

Auchencairn Bay, Dumfriesshire Scotland 

High mortality of nontarget benthic fauna, considerable survival in suction dredged areas, reduced abundance of individuals and species 

Dredge tracks did not persist 

N/A Faunal structure i

suction dredged precovered by 56 d

Intertidal flats 

Suction dredge Dutch 

Wadden Sea 

Densities of M. balthica

lower in dredged areas, reduction in density of non-target species 

Sediment removed and disturbed by dredging, less suitable for settlement by Macoma balthica and Mytilus edulis

N/A Densities of nonta

fauna lower in dreareas up to 1 yr la

Sandy intertidal flats 

Suction dredge Griend, Dutch 

Wadden Sea 

Significant negative effect of dredging on settlement of cockles, declines in bivalve stocks linked to reduced settlement 

Dredging increased sediment grain size while silt was lost 

N/A Initial sediment 

characteristics returned,, long las(8 yr decline) negeffects on bivalve recruitment 
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