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Effects of Trawling and Dredging on Seafloor Habitat
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.
IDENTIFYING ESSENTIAL FISH HABITAT
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 1.2Managed 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.