National Academies Press: OpenBook

Effects of Trawling and Dredging on Seafloor Habitat (2002)

Chapter: 7 Findings and Recommendations

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Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

Findings and Recommendations

The most challenging aspect of evaluating the effects of trawling and dredging on seafloor habitats is translating observed effects from experimental studies to the scale of actual fishing effort in the various fisheries around the United States. Studies that examine changes in seafloor structure and biological communities after disturbance by various types of mobile fishing gear have yielded consistent patterns of acute effects that can be categorized by gear type, habitat characteristics, composition of the benthic community, and frequency of disturbance. To convert those results into an assessment of ecosystem-level effects on seafloor habitats requires analysis of the frequency of bottom trawling and dredging and fine-scale mapping of this effort relative to the geography of seafloor habitats in the fishing grounds. Further research will be necessary to fully understand the effects of chronic disturbance by mobile bottom gear and to more accurately assess the effects of habitat disturbance on the productivity of commercial and recreational fisheries.

The acute, gear-specific effects of trawling and dredging on various types of habitat are well documented (Chapter 3). Many studies indicate that stable communities of low mobility, long-lived species are more vulnerable to acute and chronic physical disturbance than are communities of short-lived species in changeable environments. Habitat complexity is reduced by towed bottom gear that removes or damages biological and physical structures. The extent of the initial effects and the rate of recovery depend on the stability of the habitat. The more stable biogenic, 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 these habitats tend to be populated by opportunistic species that recolonize more rapidly, recovery also is faster.

Data on the geographic distribution and frequency of trawling and dredging are limited in spatial resolution, and there is considerable regional variation in reporting methods and in records for recent years (Chapter 4). However, data collected in the early 1990s indicate that the most intensive effort (Table 4.1) was in the fishing grounds of the Gulf of Mexico and New England regions. Bottom trawling in the mid-Atlantic, Pacific, and North Pacific regions was relatively light, with less than 1 tow per year in many reporting areas. Even in heavily trawled regions, effort was not evenly distributed; thus, some areas were trawled several times per year while others were trawled infrequently if at all. Throughout the 1990s and into 2001 there were significant reductions in the intensity and spatial extent of bottom trawling (Figure B.38). These decreases resulted from reductions in fishing effort, area closures, and gear restrictions instituted by managers in response to problems with declining fish stocks, bycatch, or interactions with endangered species.

The largest information gap is in the spatial distribution of different habitat types in trawled or dredged areas. For most areas only coarse maps are available on habitat distribution. This mismatch in the spatial scales of experimental results, habitat maps, and trawl effort reporting data makes it difficult to accurately assess effects of trawling and dredging on marine ecosystems. Nonetheless, there is enough information

Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

currently available to support efforts to improve the management of the effects of fishing gear on seafloor habitats. Specific recommendations for making the best use of current information and suggestions for research are provided below.


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. Recommendations for research appear at the end of this section.

Interpretation and Use of Existing Data


Fishery managers should evaluate the effects of trawling based on the known responses of specific habitat types and species to disturbance by different fishing gears and intensity of fishing effort, even when region-specific studies are unavailable. The direct responses of benthic communities to trawling and dredging (Chapter 3) are consistent with ecological models of how biological communities and ecosystems respond to acute and chronic physical disturbance. Although area-specific studies on the effect of trawling and dredging gear will allow more targeted management approaches, adequate information is available to address fishing effects on seafloor habitat. Predictions developed from common trends observed in comparable habitats will provide reasonable estimates of fishing effects to serve as the basis for management. Estimates should be revised as more site-specific information becomes available. An adaptive management strategy could be used both to reduce the effects in the short-term and to provide additional information for improving management in the long-term.


The National Marine Fisheries Service and its partner agencies should integrate existing data on seabed characteristics, fishing effort, and catch statistics to provide geographic databases for major fishing grounds. The potential consequences of fishing can be most efficiently assessed by the simultaneous and consistent presentation of all available data on the characteristics of the seabed and fishing effort. Although some data exist on habitat characteristics and on the location and intensity of fishing (Chapter 4), the available data have been prepared by different agencies, in different formats, at variable levels of resolution, and are collected in separate archives. Integration of these databases into a single, geographic information system will assist managers in evaluating 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 ecological effects of fishing, including, but not limited to, effects on habitat. Other management tools (gear restrictions or modifications and closed areas) may also require effort reduction to achieve maximum benefit. However, effort reduction alone might not be sufficient to reduce effects in highly structured habitats where there is low potential for recovery.

  • Modifications of gear design or restrictions in gear type. Disturbance depends on the extent of contact of the gear with the seafloor; gear designs that minimize bottom contact can reduce habitat disturbance. In addition, shifts to a different gear type or operational mode 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 effectively protect biogenic habitats (e.g., corals, bryozoans, hydroids, sponges, seagrass beds) that are damaged by even minimal fishing.

The appropriate combination of management approaches will depend on the characteristics of the ecosystem and the fishery—habitat type, resident seafloor species, frequency and distribution of fishing, gear type and usage, and the socioeconomics of the fishery. Each characteristic should be evaluated during development of a mitigation strategy.


The regional fishery management councils should use comparative risk assessment to identify and evaluate risks to seafloor habitats and to rank

Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

management actions within the context of current statutes and regulations. Risk assessment provides a scientifically informed approach to clarifying environmental policy issues by elucidating the environmental consequences of particular policy choices (Chapter 5). Comparative risk assessment can be used when there is incomplete scientific information because it relies on a combination of available data, scientific inference, and public values. Mobile bottom gear is only one of many factors contributing to the degradation of benthic habitats; the comparative approach provides a method for simultaneous consideration of a wide range of risks, including pollution, drilling, and natural disturbance. Comparative risk assessment enables stakeholder involvement in the decision-making process and improves the sharing of information among different groups to aid in the development of solutions that have broad societal support.

Policy Issues Raised by Existing Legislation


Guidelines for designating essential fish habitat (EFH) and habitat areas of particular concern (HAPC) should be established based on standardized, ecological criteria. The EFH concept recognizes that management of exploited fish populations requires addressing effects on other parts of the ecosystem upon which fish depend. Its effective use rests upon a clear understanding of the population biology and the spatial distribution of each managed species. The current designation of EFH does not require the use of consistent criteria with respect to the assignment of habitat to each life stage of species covered by fishery management plans. Instead, the regional councils develop the criteria, often based on data availability. Typically, EFH designations are too extensive to form a practical basis for managing fisheries (Chapter 1). Although this approach could assist in mitigating some habitat threats, it provides little guidance for evaluating the effects of trawling and dredging. For example, in some management plans habitat is identified based on the frequency with which fish are found in a particular area. Although this method is based on sound ecological principles, it is important to refine the use of frequency distributions to identify the habitats that support the main fraction of the population rather than to simply document where the fish are found.

The term HAPC should be clearly and narrowly defined with establishment of specific guidelines for regulating fishing activities. The effectiveness of the designations should be reviewed periodically. HAPC forms a subset of EFH based on the ecological value of the area, its susceptibility to perturbation, and whether it is rare or currently stressed (National Marine Fisheries Service, 1997). However, current policy does not require additional protection for HAPC. Because of the demonstrated importance of HAPC in the life cycles of exploited fish populations, HAPC sites should receive priority in fishery management plans.


A national habitat classification system should be developed to support EFH and HAPC designations. A classification system with common habitat designations will improve efforts to protect, inventory, and construct regional or national maps of habitats of importance or concern. Standardizing classifications would facilitate tracking changes over time and provide the basis for developing functional links between the underlying mechanisms that structure the ecosystem and the biological systems that support fisheries production. A habitat classification system would assist in ranking the relative importance of different habitats for fisheries and for biodiversity, estimating the vulnerability of the habitat to disturbance, and facilitating the application of research conducted in one region to the management of habitats in other regions.


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

Gear Impacts and Modification

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. Active engagement of resource users in the research will help ensure that mitigation strategies are practical, enforceable, and acceptable to the fishing community. This could be accomplished through cooperative research programs

Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

involving fishermen and scientists. Topics for future research include the following:

  • identification of the forces produced by fishing gear on the seafloor and the threshold forces that injure and dislodge a range of benthic organisms;

  • use of various empirical approaches, such as sidescan sonar, to assess the spatial extent and overlap of trawl and dredge effects in conjunction with higher resolution effort reporting data;

  • development of fishing gear to reduce damage to habitat and to meet other conservation goals such as bycatch reduction and maintenance of biological communities; and

  • investigation of why some areas appear to continue to produce fish despite chronic disturbance by fishing gear.

Habitat Evaluation

Habitat disturbance has been studied mainly at small spatial scales with short-term observations of acute disturbance. Development of a landscape-scale perspective of the effects of trawling and dredging on the seabed will require a long-term commitment to higher resolution mapping of the continental shelf and slopes. Because most studies have focused on animal communities, more studies are needed on ecosystem processes (e.g., productivity, nutrient regeneration). Topics for future research include the following:

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

  • the dose-response relationship as a function of gear, recovery time, and habitat type to evaluate effects of repeated disturbance by fishing gear;

  • recovery dynamics, with consideration given to estimating large-scale effects at current fishing intensities;

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

  • recovery rates in stable and structurally complex habitats;

  • relative magnitude of different sources of bottom habitat disturbance;

  • long-term trend data for benthic production versus fisheries production; and

  • the effects of habitat fragmentation on total production.


Constructive interactions among stakeholders and policymakers can be facilitated through user group funding of research and through collaborative research that involves scientists and fishermen. Increased participation also will support cooperative development of alternative gears and practices. Comparative risk assessment can be used to identify priorities for acquiring quantitative data to improve risk analysis. Monitoring and evaluation of the consequences of existing management measures (e.g., gear restrictions, area closures, effort reductions) should be used to support the development of new management plans, especially in understudied regions. 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. Topics for future research include the following:

  • develop testable hypotheses of how biological communities in different habitat types respond to fishing;

  • establish baselines for characteristic habitats and regions to document the effects of various fishery practices;

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

  • validate the use of frequency-dependent distribution approaches for designating EFH and HAPC through analysis of community structure and life history parameters; and

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

Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
Page 65
Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
Page 66
Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
Page 67
Suggested Citation:"7 Findings and Recommendations." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
Page 68
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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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