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



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement