Effects of Trawling and Dredging on Seafloor Habitat (2002)

Chapter: Executive Summary

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Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.

Executive Summary

Fishing has a variety of effects on marine habitats and ecosystems, depending on the spatial extent of fishing, the level of fishing effort, and the type of gear. Expansion of U.S. domestic fisheries after passage of the Magnuson-Stevens Fishery Conservation and Management Act of 1976 fueled advances in gear and navigation technology that greatly increased the geographic extent of these effects. However, declining fish stocks have reduced fishing activities in some areas over the past decade. After passage of the Sustainable Fisheries Act in 1996, which required that fishery management plans address the effects of fishing on habitat, attention focused on how fishing affects the seafloor. The primary fisheries involved in the controversy are trawl and dredge fisheries, which tow gear over seafloor habitats and communities. A complete consideration of the effects of fishing on ecosystems would require evaluation not only of trawl and dredge gear, but also of stationary gear (traps, pots, longlines, gillnets) and other kinds of towed gear (pelagic trawls) on target and nontarget species.

As a first step in evaluating the ecosystem effects of fishing, the National Marine Fisheries Service (NMFS) asked the Ocean Studies Board of the National Academy of Sciences to study the effects of bottom trawling and dredging on seafloor habitats. Specifically, NMFS asked the committee to undertake the following tasks: 1) summarize and evaluate existing knowledge on the effects of bottom trawling on the structure of seafloor habitats and on the abundance, productivity, and diversity of bottom-dwelling species in relation to gear type and trawling method, frequency of trawling, bottom type, species, and other important characteristics; 2)summarize and evaluate knowledge about changes in seafloor habitats associated with trawling and with the cessation of trawling; 3) summarize and evaluate research on the indirect effects of bottom trawling on non-seafloor species; 4) recommend how existing information could be used more effectively in managing trawl fisheries; and 5) recommend research to improve understanding of the effects of bottom trawling on seafloor habitats.

During the study, the committee held public meetings in several regions with participation by fishery scientists and managers, the fishing industry, and environmental groups. Discussions often centered on concerns that habitat protection initiatives would become avenues for the reallocation of resources among stakeholders, including various sectors of the fishing industry, recreational fishing groups, and conservation organizations. Resolution of these allocation considerations to meet ecological and socioeconomic goals often has been contentious.

The policy context for addressing the effects of fishing on habitat is found in the essential fish habitat (EFH) provisions specified by the 1996 Sustainable Fisheries Act amending the Magnuson-Stevens Fishery Conservation and Management Act. The amended act requires regional fishery management councils to describe and identify EFH for each fish stock managed under a fishery management plan, to minimize to the extent practicable adverse effects on such habitat caused by fishing, and to identify other actions to encourage habitat conservation and enhancement. Instead of amending individual fishery management plans, most regional councils developed a single,

Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.

overarching EFH amendment. The Secretary of Commerce approved most of the revised plans, but some environmental groups have mounted legal challenges regarding the adequacy of some EFH amendments. A major complaint was that the regional councils did not sufficiently address the effects of fishing gear on benthic habitats.

Gaps in existing knowledge of the distribution of benthic life stages of fishes and other species and of the physical and biological characteristics of the seafloor made it difficult for the regional councils to develop criteria for designating EFH. Similarly, the councils struggled with the requirement to assess the effects of bottom trawling and dredging because of insufficient data on the spatial scale and extent of bottom fishing. The councils also lacked guidelines for generalizing the results of research on specific gears and habitats. These problems relate to the committee’s task to recommend ways of using existing information in the management of the habitat effects of trawl and dredge fisheries.

A complete assessment of the ecosystem effects of trawling and dredging requires three types of information:

  1. gear-specific effects on different habitat types (obtained experimentally);

  2. frequency and geographic distribution of bottom tows (trawl and dredge fishing effort data); and

  3. physical and biological characteristics of seafloor habitats in the fishing grounds (seafloor mapping).

This report summarizes current data in these three areas and describes how the low spatial resolution and availability of the fishing effort and habitat mapping data restrict a full evaluation of the ecosystem effects of trawling and dredging.

Under the first category of information, many experimental studies have documented the acute, gear-specific effects of trawling and dredging on various types of habitat. The results confirm predictions based on the ecological principle that stable communities of low mobility, long-lived species will be more vulnerable to acute and chronic physical disturbance than will short-lived species in changeable environments. Trawling and dredging can reduce habitat complexity by removing or damaging the biological and physical structures of the seafloor. The extent of the initial effects and the rate of recovery depend on the habitat stability. The more stable biogenic (i.e., of biological origin), gravel, and mud habitats experience the greatest changes and have the slowest recovery rates. In contrast, less consolidated coarse sediments in areas of high natural disturbance show fewer initial effects. Because those habitats tend to be populated by opportunistic species that recolonize more rapidly, recovery is faster as well. Significant alterations to habitat can cause changes in the associated biological communities, potentially altering the composition and productivity of fish communities that depend on seafloor habitats for food and refuge.

The second category of information, the geographic distribution and frequency of trawling and dredging, suffers from limitations in the spatial resolution of the data and in regional variation in reporting methods. For example, trawling effort data are averaged over reporting areas that range 25–2420 km2, depending on the region. Although the data are imperfect, a few generalizations emerged from the analysis presented in this report. Based on estimates of the spatial extent and intensity of trawl and dredge fishing effort, bottom trawling takes place over large areas of the continental shelf and slope. The level of effort varies greatly among regions. The highest intensity of effort, based on rough estimates of the number of times a reporting area is swept (Table 4.1), occurs in the fishing grounds of the Gulf of Mexico and New England regions. In contrast, bottom trawling in the mid-Atlantic, Pacific, and North Pacific regions is relatively light, with less than one tow per year in many reporting areas. Even in heavily trawled regions, effort is not evenly distributed. As a consequence, some areas may be trawled several times per year while other areas may be trawled infrequently if at all. Throughout the 1990s and into 2001 there were significant reductions in the intensity and spatial extent of bottom trawling. Those reductions reflect effort reductions, area closures, and gear restrictions instituted by managers in response to problems with declining fish stocks, bycatch, or interactions with endangered species.

The spatial distribution of different habitat types in trawled (or dredged) areas is the third category of information that must be integrated with the other two to assess the effects of trawling and dredging on ecosystems. Experimental studies on specific gear types in a few well-defined habitats provide small-scale estimates of ecological disturbance, but for most areas only coarse maps are available on habitat distribution.

The mismatch in the spatial scales of experimental results, habitat maps, and trawl effort reporting data

Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.

makes it difficult to assess the ecosystem-level effects of trawling and dredging. Although fisheries managers collect data continuously, limitations in resources and time require them to assess effects in the absence of complete information. In this context, comparative risk assessment provides a promising approach to evaluate the effects of bottom trawling and dredging. This method brings together the various stakeholders to identify risks to seafloor habitats and to rank management actions within the context of current statutes. Because risk assessment requires full use of all available information on seafloor habitats, fishing methods, and effort distribution there is an immediate need to integrate the available data in a readily available format.


Although there are still habitats, gears, and geographic regions that have not been adequately studied and characterized, there is an extensive literature on the effects of fishing on the seafloor. It is both possible and necessary to use this existing information to more effectively manage the effects of fishing on habitat. The following recommendations fall into three categories: 1) interpretation and use of existing data; 2) integration of management options; and 3) policy issues raised by existing legislation. These recommendations are intended to build upon the strengths of existing approaches to management rather than completely transform them.

Interpretation and Use of Existing Data


Fishery managers should evaluate the effects of trawling based on known responses of specific habitat types and species to disturbance by different fishing gears and levels of fishing effort, even when region-specific studies are not available. The lack of area-specific studies on the effect of trawling and dredging gear is insufficient justification to postpone management of fishing effects on seafloor habitat. The direct responses of benthic communities to trawling and dredging are consistent with ecological predictions based on disturbance theory. Predictions from common trends observed in other areas provide useful first-order approximations of fishing effects for use in habitat management. As more site-specific information becomes available on the fine scale distribution of fishing effort and habitat distribution, those estimates should be revised.


NMFS and its partner agencies should integrate existing data on seabed characteristics, fishing effort, and catch to provide geographic databases for major fishing grounds. Management decisions about how fishing affects habitat can be improved by the simultaneous and consistent presentation of all available data on the characteristics of the seabed and fishing effort. There are data that describe seabed types and habitats and the location and intensity of fishing for much of the U.S. continental shelf. Available data sets collected by different agencies currently exist in different formats, at variable levels of resolution, in separate archives. Their integration into a single geographic information system will help managers evaluate regional needs for habitat conservation.

Integration of Management Options


Management of the effects of trawling and dredging should be tailored to the specific requirements of the habitat and the fishery through a balanced combination of the following management tools.

  • Fishing effort reductions. Effort reduction is the cornerstone of managing the effects of fishing, including, but not limited to, effects on habitat. Both of the other management tools (gear restrictions or modifications and closed areas) also can require effort reduction to achieve maximum benefit. The success of fishing effort reduction measures will depend on the resilience and recovery potential of the habitat.

  • Modifications of gear design or gear type. Gear restrictions or modifications that minimize bottom contact can reduce habitat disturbance. Shifts to different gear types or operational modes can be considered, but the social, economic, and ecological consequences of gear reallocation should be recognized and addressed.

  • Establishment of areas closed to fishing. Closed areas are necessary to protect a range of vulnerable, representative habitats. Closures are particularly useful for protecting biogenic habitats (corals, bryozoans, hydroids, sponges, seagrass beds) that are disturbed by even minimal fishing

Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.

effort. Because area closures could displace effort to open fishing grounds, effort reductions could be necessary in some cases to reduce habitat effects.

The optimal combination of these management approaches will depend on the characteristics of the ecosystem and the fishery—habitat type, resident seafloor species, frequency and distribution of fishing effort, gear type and usage, and the socioeconomics of the fishery. Each characteristic should be considered during development of management plans for mitigating the impacts of fishing on the seafloor.


The regional fishery management councils should use comparative risk assessment to identify and evaluate risks to seafloor habitat and to prioritize management actions within the context of current statutes and regulations. Risk assessment, in general, is a scientifically informed way of clarifying public debates over environmental policy by making explicit the environmental consequences of particular policy choices. Comparative risk assessment provides the following advantages for the task of benthic habitat protection:

  • It can be used even in the absence of scientific certainty because it relies on a combination of available data, scientific inference, and public values.

  • It provides simultaneous analysis of a wide range of risks to benthic habitats. Mobile bottom gear is only one of many factors that contribute to the degradation of benthic habitats. Other factors might include pollution, drilling and natural disturbance.

  • It enables stakeholder involvement in the decisionmaking process.

Policy Issues Raised by Existing Legislation


Guidelines for designating EFH and habitat areas of particular concern (HAPC) should be established based on standardized ecological criteria. The underlying aim of the EFH concept is valuable, and it appropriately emphasizes the need to place management of exploited fishes within the context of managing the total ecosystem. The current designation of EFH, however, does not require the use of consistent criteria for the assignment of habitat to each life stage of the species covered by fishery management plans. Instead, the regional councils develop the criteria, often based on data availability. Current EFH designations typically are too extensive to form a practical basis for managing fisheries. Although this approach could help mitigate some threats to habitat, it provides little guidance for evaluating the consequences of trawling and dredging. EFH designations should be based on a clear understanding of the population biology and spatial distribution of each species.

An HAPC constitutes a subset of EFH based on the ecological value of the area, its susceptibility to perturbation, and whether it is rare or stressed (National Marine Fisheries Service, 1997). HAPCs require the strongest safeguards to ensure habitat protection because their value in the life cycle of exploited fish populations has been documented. Nevertheless, no such protection is afforded in the current policy structure. HAPC should be clearly and narrowly defined, specific guidelines should be set for determining permissible activities, and a schedule for reviewing the effectiveness of the designation should be developed.


A national habitat classification system should be developed to support EFH and HAPC designations. Efforts to inventory and construct regional or national habitat maps require a classification system with common designations. Such a system would facilitate tracking of changes over time and would provide a basis for determining functional links between seafloor ecosystems and fisheries production. A classification system would assist in ranking different habitats according to the resilience of their biological communities and associated fisheries; estimating habitat vulnerability; and managing habitat impacts based on the generalized results of research conducted in other geographic areas.


Many gaps were identified in current understanding of the consequences of fishing on the seafloor. The following recommendations are intended to direct research toward filling these gaps. They have been organized into three primary areas of research—gear effects and modification, habitat evaluation, and management—with some overlap between categories.

Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.

Gear Effects and Modification

Fishermen’s knowledge and experience should be used to study gear impacts and to develop new gear technology. Their active engagement in research will help ensure that mitigation strategies are practical, enforceable, and acceptable to the fishing community. Further research on gear effects will be required to develop a predictive capability to link gear type and effort to bottom disturbance, fish production, and recovery times in particular habitats. New research is needed in the following areas:

  • identification of the forces that injure and dislodge a range of benthic organisms;

  • development of fishing gear that is less damaging to habitat and that helps meet other conservation goals, such as bycatch reduction and maintenance of biological communities; and

  • determination of the relationship between fish production and bottom disturbance, especially for areas that continue to support fish despite chronic disturbance by fishing gear.

Habitat Evaluation

Most previous research has addressed habitat disturbance on a small spatial scales. The focus has been on short-term, acute disturbance, and on animal communities rather than ecosystem processes (productivity, nutrient regeneration). Closed areas should be used as control sites to study the chronic effects of seabed disturbance by trawl or dredge gear. Future research should examine:

  • cumulative effects of trawling on sites that have been trawled repeatedly;

  • repeated disturbances by fishing gears to determine the dose-response relationship as a function of gear, recovery time, and habitat type;

  • recovery dynamics, with estimates of large-scale effects at current fishing intensities;

  • acute and chronic effects of trawling in deeper water (>100 m); and

  • recovery rates in stable and structurally complex habitats for which the return time will be measured in years to decades.

Evaluation of the indirect effects of bottom trawling and dredging will require experimentation, modeling, and comparison of different habitat types to analyze trends in benthic production and community structure relative to trends in fisheries production. This evaluation should include:

  • effects of habitat fragmentation on biological communities and the productivity of exploited fish stocks;

  • rates and magnitude of sediment resuspension, nutrient regeneration, and responses of the plankton community in relation to gear-induced disturbance; and

  • long-term trend data on benthic production versus fisheries production.


Productive interactions among stakeholders and policymakers should be enhanced through increased participation in research on the effects of fishing on the seafloor and development of alternative gears and practices. Interactions can be facilitated through user group funding of research and by collaborative research projects that involve scientists and fishermen.

Development of better quantitative data for risk analysis will require research on the habitats and population dynamics of nontarget species, specifically:

  • adequate baselines for particular habitats and regions, to document the effects of various fishery practices;

  • testable hypotheses about how communities in different habitat will respond to fishing;

  • quantitative models to predict fishing effects in areas that have not been studied; and

  • mortality estimates for nontarget species.

NMFS should establish protocols for studying existing trawl and dredge area closures to evaluate ecological, social, and economic effects of habitat management strategies. This will aid assessment of management alternatives in other locations. Aggregation and analysis of existing information on habitats, fishing effort, and efficacy of various management measures will help the regional fishery management councils meet their mandate to protect EFH. Research that will facilitate management decisions include:

  • analysis of community structure and life history parameters to validate the use of frequency

Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.

dependent distribution approaches for designating EFH and HAPC; and

  • collection and analysis of data on the social and economic characteristics of trawl, dredge, and nonmobile gear fisheries to assess the tradeoffs among various management alternatives.


Integration of available data on the effects of trawling and dredging, fishing effort, and the distribution of seafloor habitats can provide a starting point for practical initial evaluations that will inform management decisions. Management measures should be assessed regularly to provide better information about how various restrictions affect fish stocks and habitats and to determine the socioeconomic effects on the fishing industry and fishing communities.

However, existing data are not sufficient to optimize the spatial and temporal distribution of trawling and dredging to protect habitat and sustain fishery yields. Resolution of the different, and at times conflicting, ecological and socioeconomic goals will require not only a better understanding of the relevant ecosystems and fisheries, but also more effective interaction among stakeholders.

Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.
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Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.
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Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.
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Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.
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Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.
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Suggested Citation: "Executive Summary." National Research Council. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press, 2002.
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Concerns over the potential ecological effects of fishing have increased with the expansion of fisheries throughout the marine waters of the United States. Effects of Trawling and Dredging on Seafloor Habitat describes how assessment of fishing impacts depends on gear type, number and location of bottom tows, and the physical and biological characteristics of seafloor habitats. Many experimental studies have documented acute, gear--specific effects of trawling and dredging on various types of habitat. These studies indicate that low mobility, long--lived species are more vulnerable to towed fishing gear than short--lived species in areas where the seabed is often disturbed by natural phenomena. Trawling and dredging may also change the composition and productivity of fish communities dependent on seafloor habitats for food and refuge. The scale of these impacts depends on the level of fishing effort. This volume presents color maps of fishing effort for all regions with significant bottom trawl or dredge fisheries -- the first time that such data has been assembled and analyzed for the entire nation.

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