Effects of Fishing Gear on the Sea Floor of New England
edited by:
Eleanor M. Dorsey/Conservation Law Foundation
Judith Pederson/Massachusetts Institute of Technology Sea Grant College Program
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This book began with a conference that we chaired on May 30, 1997 at the Warren Conference Center in Ashland, Massachusetts. The purpose of the conference, convened by the Conservation Law Foundation and the Massachusetts Institute of Technology Sea Grant College Program, was to present available information about effects of fishing gear on New England's sea floor and on fishing productivity, benthic habitats, and biodiversity. We were motivated by a concern that widespread and intensive use of fishing gear that contacts the New England sea floor may be damaging fish habitats and threatening biodiversity. A paucity of information about the effects of fishing gear, however, dictated that the first step to addressing the issue must be to gather the available information. Hence the conference and this volume. 

Important information about the topic resides both with scientists who study the sea floor and with fishermen who derive their livelihood from it. Most of the contributors to this volume practice one of these two professions, as do most members of the steering committee that we convened to help plan the conference (seefollowingpage). Fishermen were selected to include users of diverse bottom tending gear types, including both mobile and fixed gear. The steering committee helped select topics and speakers for the conference and recommended that fishermen be asked to talk only about their own gear types, in order to avoid the conflicts and antagonism that sometimes surface between users of different gear types. 

This book contains all of the papers delivered at the conference, but we have not limited the book to a simple reporting of what was presented that day. We have requested from the speakers clarifications of their presentations, we have invited them to expand and update where appropriate, and we have sought additional illustrations for the topics presented. We have added two papers that were not presented at the conference-an initial overview of the geology of New England's sea floor and a final "Short Take" by an invited speaker who could not attend the conference. To conclude this volume, we provide a summary of the discussion session with which the conference ended. A full discussion of management implications and options is, however, beyond the scope of both the conference and this book. 

In our editing, we have preserved differences in voice and style among the diverse speakers, and in order to avoid the cumbersome addition of scientific names to papers of nonscientists, we provide an index to scientific names as an appendix. 

The views expressed in this volume do not necessarily reflect the views of the funders listed in the Acknowledgments. 

Eleanor M. Dorsey 
Judith Pederson, Ph.D. 

Habitat alteration by the fishing activities themselves is perhaps the least understood of the important environmental effects of fishing.

Fishing gear makes contact with thousands of square miles of New England's sea floor each year while targeting fish and shellfish that live on or near the bottom. Fishing effort increased dramatically in the region after 1976, when the U.S. declared exclusive rights to fishery resources within 200 miles of shore. Effort in some bottom fisheries has declined in recent years as a result of regulations to address overfishing, but still remains high. No one questions the impact of heavy fishing on the species that are targeted, like cod, haddock, and scallops: too much fishing has caused serious declines in their populations, with resulting disruption of New England's fishing industry. 

What about the sea floor itself? How has the ocean bottom been affected by repeated passes of trawls and scallop dredges? These gear types disturb the sea floor and remove many untargeted species that grow there. Is fish productivity reduced by bottom habitat damage from fishing? Is New England's marine biodiversity threatened by fishing activities? 

This book presents currently available information about these questions in a collection of papers by scientists and fishermen. The first five papers set the stage by describing the sea floor of New Englandits topography and substrates, the organisms that inhabit soft and hard bottoms, natural disturbance to the sea floor, and bottom habitat requirements f groundfish. The next paper describes and illustrates the major types of bottom tending fishing gear used in the region-bottom trawls, scallop dredges, lobster traps, sink gillnets, and bottom long lines. 

The following three papers are by scientists studying fishing gear impacts, who summarize the results of research in New England, eastern Canada, and Europe. Fishermen who use different gear types offer their understanding of how their gear works and interacts with the sea floor, in the next section of papers. Options are presented for minimizing gear impacts to the sea floor, and the provisions on essential fish habitat (EFH) in the 1996 MagnusonStevens Fishery Conservation and Management Act are reviewed. A section of short observations by fishermen and scientists addresses a variety of other topics relating to the effects of fishing gear. The book ends with a summary of the discussion session that concluded the 1997 conference at which most of the papers were first presented. 

The information presented in this volume is directly relevant to one of the important new requirements of the Magnuson-Stevens Act. Fishery management plans are now required by law to "minimize to the extent practicable adverse effects on EFH caused by fishing." 

This introduction references specific papers in this volume by the last name of the author(s). 


Bottom tending gear is either towed (bottom trawls and scallop dredges) or fixed (lobster traps, sink gillnets, and bottom longlines) during fishing operations. Fixed gear rests on the bottom while fishing and may be pulled over it for short distances during retrieval or storms. Most of the concern about fishing gear impacts and most scientific studies have been directed at mobile bottom gear, because the area of sea floor contacted is so much larger than with fixed gear. This volume considers both types of bottom gear. 

Two sources of data document the widespread  use of mobile bottom gear in New England. One source is the plots of tows of known location (Figure 1), which represent a small fraction of the total tows, those made with a fisheries observer on board. The other source is data on number of days fished, a measure of fishing effort, collected by the National Marine Fisheries Service  (NMFS) for each gear type and attributed to approximate location. These fishing effort data have been combined with assumptions about vessel speed and gear dimensions to derive estimates of total area swept by mobile fishing gear. Each year in recent years, bottom trawls and scallop dredges swept an area of sea floor in the Gulf of Maine equivalent to the size of the gulf (65,000 km'), according to these estimates. On Georges Bank, these gears covered an area estimated at more than three times the size of the bank (41,000 k M2) (see summary by Collie). 

This means that, on average, every square inch of the sea floor was contacted by mobile fishing once a year in the Gulf of Maine and three to four times per year on Georges Bank. The towing does not occur evenly over the sea floor, but is concentrated in areas that produce better catches for fishermen and contain no impediments to towing. Thus some places are swept by fishing gear more often than average and other places are swept less often or not at all. 

Improvements in fishing gear and electronics during the last few decades have allowed bottom towing to occur in places that were previously too rough or too deep, so the extent of sea floor protected from mobile gear by natural features has been shrinking. Precise positioning instruments and electronics that improve visualization of the sea floor now allow fishermen to tow closer to bottom irregularities that may damage trawls and dredges. Scallop dredges are heavier and stronger than in the past and can fish on hard bottom that was off limits before the late 1960s. Bottom trawls can now be towed over much rougher terrain because of the innovations of rollers, rock hoppers, and, just recently, "street sweeper" gear. 

These and other trawl configurations are illustrated in this volume by Carr and Milliken. Lighter and stronger lines, combined with more powerful hydraulics, now permit towing at depths that fishing gear could not reach previously. 

Technological advances have also improved our ability to understand the geology and ecology of the sea floor and the impacts to it from fishing gear. Underwater video cameras, side scan sonar, computer driven mechanical grabs, manned submersibles, and remotely operated vehicles now allow observations and experiments that were previously impossible. Many papers in this volume are based on these new technologies. Because of its versatility and low cost, underwater video is particularly widely used, both by scientists and by fishermen, to view fishing gear and the sea floor. 


Research on the effects of fishing gear has been conducted on a wide variety of sea floors, differing in depth, substrate type, benthic fauna, fishing gear, and degree of human and natural disturbance. Some studies have compared fished areas with unfished areas of similar depth and substrate, while other studies have looked at the same area before and after fishing disturbance. The most rigorous studies are planned experiments with fishing gear that make both kinds of comparisons. Such experiments have been performed in Canada and Europe, but not yet in New England. Research on fishing gear impacts in New England has been impeded by the paucity of unfished areas to use for comparison with fished areas, as well as by the high cost of conducting research on the sea floor. 

One of the few general conclusions that can be drawn from the various studies to date is that mobile fishing gear reduces habitat complexity on the sea floor. This effect has been documented on various substrates in New England, eastern Canada, and Europe, as well in other parts of the world. It results from the tendency of fishing gear to (1) smooth out structures on the bottom and (2) remove bottom fauna that contribute to sea floor complexity. Reduced habitat complexity is expected to reduce shelter for juvenile fish and thus increase their mortality rates due to predation from larger fish. At the same time, removal of benthic fauna may reduce the availability of food from invertebrates for fish of all sizes. The loss of benthic fauna from fishing gear is expected to be greater on stable sea beds with long-lived benthic organisms than in sediments that are frequently disturbed by natural processes, such as the shifting sand dunes on Georges Bank and Nantucket Shoals. 

The three major studies conducted in New England on fishing impacts to bottom communities are reviewed in this volume by Collie. One study found significantly greater habitat complexity on cobble-shell and sand-shell bottoms inside a conservation area closed to mobile gear, compared with outside the area. In another study, scientists reported a substantial reduction in sponges and other invertebrates on boulders on a deep bank where trawling was observed and had apparently moved the boulders. A third study found significantly lower abundance, biomass, and species diversity of invertebrates living on gravel in a heavily fished part of Georges Bank, compared to unfished areas of similar depth. This study also documented a partial recovery in the benthic community 18 months after one site was protected by the groundfish closure that began in late 1994. 

Studies of communities inhabiting hard and soft substrates in the Gulf of Maine offer further insights into the resilience of these communities to the type of disturbance caused by fishing gear. Witman has observed markedly slower recovery of natural and experimental clearings on hard substrates at depths below 30 meters in his long term studies of disturbance and colonization. Soft-bottom communities, which are described by Watling, have not been the subject of comparable long term studies. However, long lifespans and infrequent reproduction in some of the dominant mud-dwelling organisms in deeper water (sea pens and tube-dwelling anemones) suggest that they would take years to recover from one-time disturbance and may never return to areas that are trawled frequently. 

Bottom trawls create clouds of muddy water as they move over the sea floor, and the mud clouds help to herd fish into the net. Sensors deployed on the sea bed in southern New England have detected turbidity that was apparently caused by trawling, and modeling indicates that trawling is the principal cause of sediment resuspension in the deeper waters of the outer continental shelf, whereas storms and currents are the principal cause in shallower waters. Churchill speculates on some possible implications of these findings, one of which is that phytoplankton productivity may be increased by trawling because of the release into the water column of nutrients trapped in sediment. This in turn may lead to increased productivity of invertebrates and fish at the ocean bottom. Another possible effect of trawling-induced sediment resuspension, not discussed in this volume, is reduced survivorship of juvenile bivalves and fish due to clogging of their gills. 

Other research from New England reported in this volume sheds light on fishing gear effects on the sea floor. Moderate harvesting of an estuarine oyster bed with a dredge very much smaller and lighter than the dredge used for sea scallops resulted in fewer large oysters, better recruitment of small oysters, and no changes in the associated benthic invertebrates (see Langan). Lindholm and colleagues stress the importance of shelter provided by undisturbed bottom habitat in the survivorship of newly settled cod. Results from their model suggest that protection of sea floor habitat from fishing disturbance may significantly improve cod recruitment at current low population levels. 

The Canadian government has undertaken a major research program to investigate the impacts of mobile fishing gear on benthic ecosystems in eastern Canada. After developing technology to sample and image the sea floor, researchers have begun experimental studies of different gear types. Gordon and colleagues report the initial results. Trawling on sandy substrates in the same spot on the Grand Banks over three years reduced the biomass of several invertebrates (snow crabs, sand dollars, soft corals, and brittle stars), but not that of hard-shelled mollusks. The complexity of sediment structure was reduced and an influx of scavengers occurred, but the consequences for harvested species is unknown. 

In northern Europe, research on the effects of fishing gear has a longer history than in either New England or Canada. The paper by Rogers and colleagues summarizes many different studies from the eastern North Atlantic. One experiment with beam trawling in the Irish Sea showed a significant reduction in invertebrate numbers and species inhabiting stable sediments of sand, gravel, and shell, but no effect on invertebrates in nearby mobile sediments. Another study found slower recolonization after fishing disturbance for large long-lived clams than for small polychaetes and bivalves. Intensive fishing has removed fragile or long-lived species, like reef-building tube worms and calcareous algae that formerly dominated areas of the European sea floor and provided microhabitats for many other species. 

Many benthic invertebrates are exposed and damaged or killed by the passage of a trawl. These provide food for scavengers, both invertebrates and finfish, and a short term influx of scavengers into experimentally trawled areas has been observed off the coasts of Europe and Canada. One harvested finfish species on the sandy floor of the North Sea (sole) appears to have benefited over the long term from intensive trawling, probably because fishing disturbance has shifted the benthic community to small, opportunistic species appropriate for the sole's small mouth. Although research on trawling impacts in Europe was initiated as long ago as 1955, all studies began after much of the European sea floor had already been intensively trawled for many decades. Rogers calls for targeted sampling of benthic invertebrates and higher spatial resolution of data on fishing effort and fish populations to improve understanding of the effects of mobile bottom gear. 

A number of scientists make the point that effects of fishing disturbance are likely to be more significant in areas that are not subject to high levels of natural disturbance. Organisms living in areas of high natural disturbance are adapted to disturbance by anatomy, behavior, or life history characteristics, but many organisms living in naturally undisturbed areas lack such adaptations and may take years, decades, or even centuries to recover from fishing disturbance. If fishing disturbance occurs more frequently than the recovery time, then susceptible species will be eliminated from the area of fishing and biodiversity will be lowered. This may be occurring with the long-lived deep sea corals for which abundance appears to be reduced (see Butler). 


Fishermen have much to contribute to understanding the effects of fishing gear on the sea floor, but their views are rarely solicited. Fishing gear that contacts the sea floor may bring up diverse forms of marine life and pieces of bottom substrate. Because fishermen have opportunities to observe samples from the bottom every day they are fishing, the frequency with which they can examine bottom samples often far exceeds most sampling regimens that scientists can sustain. Furthermore, experienced fishermen may have decades of observations to bring to bear on this topic. 

Many fishermen keep very detailed records of exactly where and when they fish and what they catch. Sophisticated electronic instruments on many fishing vessels allow for precise depth measurement and positioning, as well as improved visualization of fish schools and bottom textures, although fishermen often cannot identify substrate types. Although the goal of fishing is to provide income from the catch, rather than to test hypotheses, many fishermen seek to understand the very questions about the sea floor that motivate this book. 

Twelve commercial fishermen have contributed papers to this volume. Their experience includes all five major bottom-tending gear types used in New England-bottom trawls, scallop dredges, lobster traps, sink gillnets, and bottom longlines. Fishermen provide descriptions of each gear type and its variations, how it is fished, how the gear and fishing has changed (or not) over time, and how they understand it to interact with their target species and with the sea floor. Some describe specific sea floor habitats that they seek out to catch their fish. As the fishermen in this volume describe the effects that they believe fishing gear to have on the sea floor, they argue a wide diversity of positions. Some report severe adverse impacts on fish habitat; others say there are no impacts; and a few maintain that the effects can be positive. 

The greatest concern about fishing gear impacts is expressed by fishermen who report seeing significant losses of habitat good for fishing. Three distinct types of habitat are reported by New England fishermen to have disappeared: topographic features; benthic invertebrates, some of which previously formed large beds; and a hardened clay formation called pipe clay or clay pipes. Fishermen who report these losses attribute them to mobile gear disturbance. 

Bennett reports that bottom peaks that formerly supported good cod fishing in two different locations have decreased in height or disappeared entirely after mobile bottom gear fished in their vicinity. Williamson mentions the disappearance of topographic features on which gillnetters formerly set their nets. It is not certain that these topographic changes are due to fishing gear rather than to natural disturbance, because storm waves and tidal currents are capable of major changes in bottom topography in shallower water. If fishing gear is indeed the cause, these reports represent reductions in bottom structure on a much larger scale (up to tens of meters) than observed to date in any of the scientific studies of fishing gear impacts. 

A variety of benthic invertebrates that longliners have for decades associated with good fishing (beds of mussels, several species of ascidians) have been significantly reduced in abundance because of mobile gear disturbance. This is according to reports by Bennett and by Leach. As with the topographic changes mentioned above, it is not certain that mobile fishing gear is the cause. The claim is plausible, however, because reductions in similar benthic fauna have been documented in a number of scientific studies. Longliners reported reduced fishing success after these bottom changes occurred. 

Pipe clay (also known as clay pipes) is another bottom habitat type that fishermen associate with good fishing and report to be much reduced in abundance because of mobile fishing gear (see Williamson, Bennett). This unusual bottom formation from Nantucket Shoals and the western Gulf of Maine was unknown to scientists until reported by fishermen, and it merits further study (see Dorsey; Valentine). 

In other places, fishermen describe repeatedly trawled areas that remain productive in spite of intensive disturbance from trawling. Pendleton characterizes a muddy area outside of Casco Bay as being intensively trawled for decades and argues that trawling does not appear to have damaged the bottom or the fishing. Mirarchi reports that catches in heavily trawled areas on and near Stellwagen Bank declined after years of high fishing effort, but have started to rebound recently in the same areas after fishing pressure was reduced. He concludes that either trawling did not damage the area or the area has recovered from its damage over time. 

Other fishermen maintain that disturbance from mobile gear can actually enhance a fishery. Kendall reports improved scallop recruitment on the southwest corner of Georges Bank soon after scalloping was resumed on a neglected scallop bed. Doughty states that stirring the bottom by mobile gear can help keep the bottom from going "sour" (becoming hypoxic or anoxic) and driving fish away. Porter mentions a bed of mahogany clams that he believes was improved by harvesting with a clam dredge. 

All of these observations from fishermen deserve careful consideration. It is possible that all of them are valid and reflect differences in fishing gear impacts depending on substrate type and other variables. Considerable additional research must be conducted and more observations from fishermen collected before the relationship between fish productivity and sea floor habitats is clarified. 

Some of the information from studies in Europe and Canada can be extrapolated to New England to improve understanding of gear impacts in this region, as long as differences in substrate types, gear, and bottom species are considered. Nevertheless, a regional program of additional research on the effects of fishing gear on the sea floor is clearly needed to resolve the many remaining uncertainties about those effects. This program should include a process for integrating fishermen's input into research to take advantage of their experience and observations. The provisions on essential fish habitat in the 1996 Magnuson-Stevens Fishery Conservation and Management Act (see Kurland) will help increase attention to sea floor attributes that support fish populations. 

We believe that information from fishermen and scientists in this volume justifies protecting more areas of the ocean floor from fishing disturbance, both to maintain healthy fisheries and to protect marine biodiversity. We do not find in this material, however, a blanket condemnation of trawling and other mobile gear. As several contributors to this volume noted, bottom trawls can be designed to have reduced impact on the sea floor, and mobile gear probably has negligible or beneficial effects in some situations. 

Mapping New England's bottom substrates according to vulnerability to fishing gear disturbance should be a high priority, a conclusion reached by the conference steering committee and by the people who attended the discussion session at the end of the conference. The other high-priority recommendation to emerge from the conference was for increased research to study the effects of fishing gear on the sea floor and to elucidate the relationship between sea floor habitats and fish productivity. The willingness of a wide diversity of people to engage in serious and productive discussions about the topic of this volume bodes well for New England's ability to address it in a responsible and fair manner. 


Auster, P.J. and R.W. Langton. In press. Indirect effects of fishing. In L. R. Benaka, ed. Fish Habitats: Essential Fish Habitat and Rehabilitation. American Fisheries Society Symposium 22. Bethesda, MD. 

National Research Council. 1994. Improving the Management of U. S. Marine Fisheries. National Academy Press. Washington, D.C. 

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