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

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Among all eight patches, the species found in the highest abundance overall were juvenile rockfish and pygmy rockfish, and those in the lowest abundance were lingcod and sablefish (Table 1). The total area of all habitats assessed was 124 [km.sup.2], which is approximately 17% of the total area of the sonar survey, and the total number of estimated fish and standard error for that area was 156,598,000 [ or -] 16,854,000. The coefficient of variation was relatively low for greenstriped rockfish, rosethorn rockfish, sharpchin rockfish, shortspine thornyhead, and Dover sole (between 3.7% and 6%) and slightly higher for pygmy rockfish, yellowtail rockfish, lingcod, sablefish, and rex sole (between 7.8% and 11.9%).

Discussion

A primary finding of our study was that distinct bottom types found on Heceta Bank are distinguishable through the use of sonar data and that these interpreted habitats correlate with direct submersible observations of bottom type. The determination of habitat information from sonar data is significant in that it provides a broad-scale view of the seafloor habitat, previously unavailable, and allows a habitat-based groundfish assessment. Although seafloor imaging and GIS techniques have previously been used in the study of marine habitats (e.g. Meaden, 1999; McRea et al., 1999; Sherin, 1999;Yoklavich et al., 2000), ours is one of the first published studies where GIS technique was used to combine a detailed analysis of fish and habitat transect data with broad-area high-resolution sonar seafloor imagery and where total fish abundances were calculated for large areas of the seafloor (see also O'Connell et al.9).

Habitat type could not be determined by bathymetry or backscatter data alone, but the information provided by both data sets, in addition to groundtruthing by direct submersible observation, gives a clearer picture of the overall habitat environment. The use of the backscatter data combined with the bathymetric data has the advantage of providing an indication of substrate type, which is clearly important in fish-habitat associations. In general, however, backscatter provides a better indication of habitat for fish association purposes. Bathymetric data can provide geological structure on the resolution of five to ten meters, whereas backscatter data give an indication of structural variation on a smaller scale, which is of ecological importance in influencing the distribution of groundfish. For example, the physical properties of a substrate influence the types of invertebrates that colonize the seafloor, and local relief can provide microhabitats for some fish.

The extent to which a groundfish habitat can be effectively mapped by remote sensing is determined by the resolution of the system used. In general, sonars are optimized for specific operational depth ranges. A system designed for very shallow water can have sufficient resolution to provide contours of features or objects that deeper water (lower frequency and longer range) systems will only "see" as backscatter changes. As more sites are studied by combining visual seafloor transects, high-resolution sonar, and GIS techniques, it is likely the geologic indices most relevant to groundfish habitats will become apparent. These methods should lead to a more coherent approach to habitat-based stock assessments.