Integration of submersible transect data and high-resolution multibeam sonar imagery for a habitat-based groundfish assessment of Heceta Bank, Oregon

Fishery Bulletin, Oct, 2002 by Nicole M. Nasby-Lucas, Bob W. Embley, Mark A. Hixon, Susan G. Merle, Brian N. Tissot, Dawn J. Wright

Abstract--In the face of dramatic declines in groundfish populations and a lack of sufficient stock assessment information, a need has arisen for new methods of assessing groundfish populations. We describe the integration of seafloor transect data gathered by a manned submersible with high-resolution sonar imagery to produce a habitat-based stock assessment system for groundfish. The data sets used in this study were collected from Heceta Bank, Oregon, and were derived from 42 submersible dives (1988-90) and a multibeam sonar survey (1998). The submersible habitat survey investigated seafloor topography and groundfish abundance along 30-minute transects over six predetermined stations and found a statistical relationship between habitat variability and groundfish distribution and abundance. These transects were analyzed in a geographic information system (GIS) by using dynamic segmentation to display changes in habitat along the transects. We used the submersible data to extrapolate fish abundance within uniform habitat patches over broad areas of the bank by means of a habitat classification based on the sonar imagery. After applying a navigation correction to the submersible-based habitat segments, a good correlation with major boundaries on the backscatter and topographic boundaries on the imagery were apparent. Extrapolation of the extent of uniform habitats was made in the vicinity of the dive stations and a preliminary stock assessment of several species of demersal fish was calculated. Such a habitat-based approach will allow researchers to characterize marine communities over large areas of the seafloor.

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Dramatic declines in several groundfish populations have occurred along the U.S. West Coast during the last decade (Ralston, 1998; PFMC (1); Sampson, (2) Bloeser (3)). One problem exacerbating these declines is that current stock assessments are not sufficiently precise or accurate to effect empirically based management. This is especially true for commercially important species of rockfish (Scorpaenidae, Sebastes), which comprise major groundfish fisheries along the Pacific Coast. Although evidence has accumulated for substantial declines in the abundance of several species of rockfish, the overall picture is unclear because 42 of 54 rockfish species (78%) have never been assessed (Ralston, 1998; NMFS, 1999; Bloesera). Of the 12 species that have been assessed by the National Marine Fisheries Service, five were listed as "overfished" and one species was listed as "approaching overfished condition" (NMFS, 1999).

A possible alternative to single-species stock assessments of demersal fishes is a habitat-based community assessment, which serves to estimate groundfish population sizes by recognizing that species are not randomly distributed among varying habitats. It is known that the diversity, quality, and extent of bottom habitats are important in determining the distribution, abundance, and diversity of rockfishes (Carlson and Straty, 1981; Pearcy et al., 1989; Carr, 1991; Stein et al., 1992; O'Connell and Carlile, 1993). It has been previously demonstrated, within local study areas, that species richness and composition correlate with seafloor texture (Hallacher and Roberts, 1985; Richards, 1986; Love et al., 1991; Stein et al., 1992; Krieger, 1993; Yoklavich et al., 2000; Hixon et al. (4); Hixon and Tissot (5)). Correlations over larger regions have been difficult to determine because of the limitations in the resolution and areal coverage of bathymetric charts, which are crucial in providing broad-scale habitat data. However, this difficulty no longer exists with the advent of differential GPS and high-resolution sonar systems (Hughes Clarke et al., 1996). When adequately groundtruthed, these new systems provide bathymetric and backscatter data with sufficient resolution to formulate habitat classifications over broad areas of the continental shelf and slope (Able et al., 1987; Yoklavich et al., 1995; Greene et al. (6); Fox et al. (7)).

We developed this habitat-based groundfish assessment strategy by integrating a comprehensive submersible survey with new high-resolution sonar imagery of the seafloor. Sonar images produce habitat data by using acoustic signals to differentiate areas of hard substrata from surrounding soft sediments based on differences in the intensity of reflected sound. This technology has the distinct advantage of examining large regions of seafloor sediment and geological topographic features without relying on expensive direct underwater bottom sampling and observation techniques. Sonar data can be used in an assessment of seafloor habitat and fish density over large areas by extrapolating from direct observations of fish and seafloor morphology transects using manned submersibles or remotely operated vehicles.

The study site for this assessment was Heceta Bank, a 50-kin long outcrop on the outer shelf of central Oregon (Fig. 1). Heceta Bank is the largest rocky reef of the Pacific Northwest and is characterized by high variability in bottom types and textures. The bank provides a diversity of habitat types for many species of groundfish and invertebrates. These characteristics have made Heceta Bank one of the largest and most important of the heavily fished rocky banks in the Pacific Northwest. Along with its commercial importance, the bank has been the subject of substantial scientific research, which has made it an ideal site for developing these methods.