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Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.


Over the last 25 years there has been increasing interest in the effects of fishing—not only for target species but also for nontarget species. The environmental consequences—how fishing affects habitat— have come into the discussion during the past decade. Numerous studies indicate that habitat complexity improves the survivorship of many fish species. Benthic organisms (plants, corals, and sponges) and sediment forms (mud burrows and gravel) add structure to the seafloor and increase habitat complexity. Seafloor structures serve as nurseries for juvenile fish and provide refuge and food for adults. Even small structures, such as cobbles and clam shells, can form important habitat. Areas of the seafloor that lack these structures do not support the variety of fish populations observed in more complex regions (Collie et al., 1997; Kaiser et al., 1999).

Fishing affects marine habitats and ecosystems in many ways that depend on the type of gear used and on the spatial and temporal extent of fishing (Auster and Langton, 1999). Mobile fishing gear, such as bottom trawls and dredges, can be configured to drag across the seafloor to catch demersal fish and shellfish and some semipelagic fish. But this method of fishing disturbs the structure of the seafloor, affecting the three-dimensional character and availability of fish habitat and changing the composition of biologic communities in the area. The indirect effects of trawling and dredging include disruption of the food web, alteration in the rate of decomposition of organic matter, and recycling of nutrients through resuspension of bottom sediments.

It is difficult to quantify the effects of habitat disruption caused by trawling and dredging because there are so many sources of disturbance: depletion of commercial and recreational fish stocks, pollution, changes in climate, and oceanographic variability. Those factors complicate the evaluation of the general consequences of fishing on marine ecosystems, including the specific effects of habitat degradation. To study the effects of fishing within a background of variability from other sources researchers often compare unfished areas with areas that are experimentally trawled or dredged. The difficulty is in finding analogous untrawled control areas. It also has been difficult to quantify, and therefore to generalize, the relationships among fishing intensity and frequency, fishing methods and gear, seafloor structure, productivity and abundance of economically valuable species, and diversity of other organisms. Finally, some fishermen, fishery managers, and scientists have expressed skepticism about the validity of generalizing data from one region to another as a basis for implementing regulations.

In the 1996 reauthorization of the Magnuson-Stevens Fishery Conservation and Management Act—the Sustainable Fisheries Act (SFA)—Congress increased the regulatory focus on habitat protection through the inclusion of essential fish habitat (EFH) provisions. Regional fishery management councils (FMCs), which manage most of the marine fisheries in the United States, were required by the act to “describe and identify essential fish habitat” for each managed fish stock “to minimize to the extent practicable adverse effects on such habitat caused by fishing.” FMCs must evaluate the effects of all fishing practices on seafloor habitat.

The National Oceanic and Atmospheric Administration asked the Ocean Studies Board of the National

Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

Box 1.1 Statement of Task

This study will be the first in a series that will evaluate available data related to the physical and biological effects of fishing on marine habitats and ecosystems. This first study will 1) summarize and evaluate existing knowledge on the effects of bottom trawling on the structure of seafloor habitats and 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 with trawling and 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 needed to improve understanding of the effects of bottom trawling on seafloor habitats.

Research Council’s Division on Earth and Life Sciences to undertake a series of studies to examine the effects of various fishing practices and to make recommendations for action that could reduce or mitigate effects. This first report addresses the specific effects of bottom trawling on seafloor habitats, as described in the statement of task (Box 1.1). In deliberations at its first meeting on the topic, the committee decided to address the effects of both trawls and dredges, because these are the two major types of bottom-tending mobile gear used in U.S. fisheries.


This report summarizes the literature on the effects of bottom trawling on habitats and discusses management tools that can be applied to mitigate them. Inshore and offshore areas are considered, although there is more emphasis on offshore regions because of the greater amount of information generally available for federally managed waters. The Caribbean and West Pacific regions are not discussed because they have no major trawl or dredge fisheries. The regulatory framework provided by provisions of the SFA is introduced below. Chapter 2 describes different types of mobile bottomtending gear used in trawl and dredge fisheries. Chapter 3 summarizes research findings on the direct and indirect effects of trawling and dredging and it reviews the literature on the postdisturbance recovery of habitat and biota. Chapter 4 describes what is known about the seafloor habitats and about the regional distribution and frequency of trawling and dredging activities. Appendix B presents maps and more detailed descriptions of the distribution and frequency of trawling in U.S. waters. Ecological risk assessment methods are presented in Chapter 5, and management options for reducing the damage caused by trawling and dredging are presented in Chapter 6. In Chapter 7, the committee presents its conclusions and recommends topics for research. Appendix A lists committee and staff members. Appendix C gives a brief explanation of mapping tools.


EFH provisions of the Sustainable Fisheries Act (1996) gave resource managers a new tool to address degradation and loss of fish habitat. The final rule published by the National Marine Fisheries Service (NMFS) (2002) defines EFH as follows:

. . . those waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity. For the purpose of interpreting the definition of essential fish habitat: “Waters” include aquatic areas and their associated physical, chemical, and biological properties that are used by fish and may include aquatic areas historically used by fish where appropriate; “substrate” includes sediment, hard bottom, structures underlying the waters, and associated biological communities; “necessary” means the habitat required to support a sustainable fishery and the managed species’ contribution to a healthy ecosystem; and “spawning, breeding, feeding, or growth to maturity” cover a species full life cycle.

The act requires fishery management plans to describe and identify EFH, minimize to the extent practicable adverse effects on EFH caused by fishing, and identify other actions to encourage habitat conservation and enhancement. EFH must be designated for each life stage of the more than 700 federally managed species. Additionally, NMFS must provide conservation recommendations to all federal or state agencies on actions that adversely affect EFH. Federal agencies must respond within 30 days to recommendations, although the recommendations are nonbinding. Hence, the SFA made habitat conservation a mandate under federal fisheries management.

Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

The regional fishery management councils were charged with identifying EFH for the egg, larva, juvenile, and adult stages of each species they manage, even though in many cases there was little information to assist them. The broad definition of EFH used in the legislation made for a daunting task. Each council took a slightly different approach, developing EFH amendments by species, by multispecies complexes, and by habitat type. The process used by the New England Council illustrates one approach.

The New England Council produced a stand-alone EFH amendment to address the needs of all 18 of its managed species. Amendment development was divided into three distinct phases. In the first and most time-consuming phase, the Council identified and described EFH for managed species. The Council based its descriptions predominantly on NMFS’ “EFH Source Documents,” a compilation of 31 EFH species reports, consisting mostly of NMFS survey data. The adult and juvenile data were compiled from bottom trawl surveys conducted by NMFS (1963–1997) and the larvae and egg data were based on the Marine Resources Monitoring, Assessment, and Predictions ichthyoplankton survey (1977–1987). EFH often was defined by the Council as the area containing 90–100 percent of each life stage for each managed species. Because time and resources were limited, the Council primarily used these data sets to designate EFH for numerous species.

In the second phase of development, the New England Council evaluated fishing related effects on EFH. This involved extensive literature reviews and drew heavily on “The Effects of Fishing on Fish Habitat” (Auster and Langton, 1999). The Council then identified and reviewed measures to reduce the potential for harm from fishing activities. In the third and final phase, the Council identified a range of actions to mitigate damage from nonfishing related activities, although the Council lacks the authority to regulate those activities. Based on its experience, the Council identified many areas of habitat-related research and information that would aid in the evaluation and improvement of its existing EFH designations.

The approach used by the New England Council defined EFH for each life stage of all managed species and thereby satisfied SFA requirements. However, under the criteria used to identify EFH, maps developed for New England included most of the waters off its coast. Even when EFH for only three species and one life stage (juvenile cod, pollock, and haddock) are plotted on a single map, most of the Gulf of Maine region and Georges Bank is classified as EFH (Figure 1.1).

This dilemma is not unique to New England. Many other councils developed EFH maps that covered most if not all of the areas under their jurisdiction. For example, the Gulf of Mexico Fishery Management Council took a cumulative, species-by-species approach to designating EFH. Once an area had been identified as EFH for one species it was not evaluated. The Gulf of Mexico Council concluded that the entire region under its jurisdiction could be classified as EFH. The South Atlantic Council did not use species abundance as a criterion for designating EFH. Instead, it identified habitat types associated with the managed species and designated EFH based on the presence of those habitat types.

NMFS recognized that the definition of EFH was so broad that the mandate to minimize the effects of fishing would be difficult to implement. So it included the concept of habitat areas of particular concern (HAPC) to provide a focus for conservation efforts. There is no requirement that councils designate HAPC for any species, nor does doing so confer additional protection or restrictions (National Marine Fisheries Service, 1997, 2002). HAPC was not a new concept; it had been developed in response to mandates contained in the 1986 amendments to the Magnuson-Stevens Act. NMFS defined HAPC as “EFH that is judged to be particularly important to the long-term productivity of populations of one or more managed species, or to be particularly vulnerable to degradation” (National Marine Fisheries Service, 1997, 2002). Under the EFH regulations, HAPC can be designated based on one or more of the following criteria:

  1. the importance of the ecological function provided by the habitat;

  2. the extent to which the habitat is sensitive to human-induced environmental degradation;

  3. whether and to what extent development activities are, or will be, stressing the habitat type; and

  4. the rarity of the habitat type.

Where there was significant information about the habitat needs of a particular species of fish, some councils defined HAPC. For example, the New England Council designated an area on Georges Bank as HAPC for juvenile Atlantic cod, based on evidence from numerous scientific studies that the gravel–cobble

Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

FIGURE 1.1 Essential Fish Habitat (EFH) designations for juvenile cod, pollock, and haddock in New England waters (data from New England Fishery Management Council).

substrate present in this region provides space for the newly settled juvenile cod to find shelter and avoid predation. All councils except the Pacific Fishery Management Council have used HAPC designations to identify areas of special ecological importance. Descriptions of HAPC designations for each region are available from NMFS (

The concepts of EFH and HAPC provide NMFS and the regional councils with the authority to mitigate habitat degradation from bottom trawling and dredging, among other effects of fishing. Recent controversy and

Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.

litigation regarding EFH amendments submitted by the regional councils and approved by the Secretary of Commerce has made it timely to examine the extent of the problem and the various management options for mitigating impacts.


EFH regulations published by NMFS in the interim and final rules describe a four-tiered approach to organizing information for describing and identifying EFH. The tiers are in order of increasing availability of information about the habitat requirements of managed species, as follows:

  • Level 1: Distribution data are available for some or all of the geographic range of the species. Either systematic presence–absence data or opportunistic observations of the location of various life stages may be used to infer habitat use.

  • Level 2: Habitat-related densities of the species are available. Geographic information on the density or relative abundance of a species at each life stage may be used to assess habitat value compared with the overall species distribution.

  • Level 3: Growth, reproduction, or survival rates within habitats are available. The success of the species in a given habitat—based on growth, reproduction, and survival rates—is used as a proxy for productivity.

  • Level 4: Production rates by habitat are available. Direct assessments of production rates as a function of habitat type, location, quality, and quantity are used to determine the habitat essential for a sustainable fishery and for the species’ contribution to a healthy ecosystem.

In most cases, EFH has been designated at Level 2, using frequency-dependent distributions of fishes as a proxy for habitat. That is, essential habitat lies within the region with the highest density of a species. This method is based on sound ecological principles, but often EFH has been designated using the top 90–100 percent of the distributions of many species, based on

Box 1.2 Managed Species and Habitat Types

Habitat is that part of the environment on which organisms depend directly or indirectly to carry out life processes. For fish, this includes spawning grounds, nursery areas, feeding areas, and migration routes. Habitat includes the physical environment (structure provided by biogenic animals, plants, and sediments; depth of water), the chemical environment (salinity, dissolved oxygen), and the many organisms (plants, invertebrates) that constitute a food web. Density fronts separate water masses or plumes of turbid, low salinity water produced by large rivers. Kelp beds, seagrass meadows, intertidal marshes, mud, sand or cobble flats, and offshore ledges and banks are distinct areas that serve as habitat for fish and other marine organisms. The physical substrate is often the most noticeable aspect of a habitat and is therefore the basis for many habitat classifications (Langton et al., 1995; Auster, 1998).

density or catch per unit effort data as an indicator of abundance. Using more restricted ranges at the top end of the distributions (10–30 percent) would narrow EFH designations to the more preferred habitats, but the optimal range for different species and life stages will require further analysis and definition.

Levels 3 and 4 require significant amounts of information about the relationship between the managed species and the type of habitat (Box 1.2). Fish require a broad diversity of intact habitat functions and processes to survive, grow, and reproduce. Because the physical structure of an area is often the most noticeable aspect of habitat, structure is the basis for most habitat classifications (Allee et al., 2000; Chapter 4) and has been the focus of many studies on the effects of fishing on habitat. However, the biotic component of habitat (food) is equally important to sustaining fish production. Therefore, both the physical and biologic components of seafloor habitat must be included in assessing the effects of trawling and dredging.

Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
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Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
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Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
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Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
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Suggested Citation:"1 Introduction." National Research Council. 2002. Effects of Trawling and Dredging on Seafloor Habitat. Washington, DC: The National Academies Press. doi: 10.17226/10323.
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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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