SIDESCAN SONAR AS A TOOL FOR DETECTION OF DEMERSAL 



FISH HABITATS 



K W Able.i D C. Twichell,2 C B. Grimes,^ and 

 R. S Jones'* 



ABSTRACT 



Sidescan sonar can be an efTective tool for the determination of the habitat distribution of commer- 

 cially important species. This technique has the advantage of rapidly mapping large areas of the 

 seafloor. Sidescan images (sonographs) may also help to identify appropriate fishing gears for differ- 

 ent types of seafloor or areas to be avoided with certain types of gears. During the early stages of 

 exploration, verification of sidescan sonar sonographs is critical to successful identification of impor- 

 tant habitat types. Tilefishes (Lopholatilus and Caulolatilus) are especially good target species be- 

 cause they construct large burrows in the seafloor or live around boulders, both of which are easily 

 detectable on sonographs. In some special circumstances the estimates of tilefish burrow densities 

 from sonographs can be used to estimate standing stock. In many localities the burrow and boulder 

 habitats of tilefish are shared with other commercially important species such as American lobsters, 

 Homarus americanus: cusk, Brosme brosme; and ocean pout, Macrozoarces americanus. 



Acoustic techniques have become important tools 

 in fishery research in the last 20 years. Of these, 

 sonar has proven useful in a number of related 

 efforts for pelagic fisheries (Forbes and Nakken 

 1972) including the detection of fishes in the 

 water column (Harden-Jones and McCartney 

 1962; Anderson and Zahuranec 1977) and estima- 

 tion of fish numbers and biomass (Smith 1970; 

 Hewitt et al. 1976; Suomala and Lozow 1980; 

 Barans and Holliday 1983; Nakken and Venema 

 1983). More recent studies have demonstrated 

 how sidescan sonar, in combination with acousti- 

 cally tagged fish, can be used to evaluate trawling 

 gear (Harden-Jones 1980). Sidescan sonar has 

 been used infrequently to assess critical habitat 

 for demersal fishery resources with the exception 

 of an early attempt to map a herring (Clupea 

 harengus) spawning area (Stubbs and Lawrie 

 1962). Our research has focused on detection of 

 tilefish burrows (Twichell et al. 1985; Grimes et 

 al. 1986; Able et al. 1987), but an outgrowth has 

 been the identification of the habitats of other 

 species. Here we describe the use of sidescan 



^Center for Coastal and Environmental Studies and Depart- 

 ment of Biological Sciences, Rutgers University, New 

 Brunswick, NJ 08903; present address: Marine Field Station, 

 Rutgers University, Tuckerton, NJ 08087. 



2U.S. Geological Survey, Woods Hole, MA 02543. 



^Southeast Fisheries Center Panama City Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, Panama City, FL 

 32407. 



■^University of Texas Marine Science Institute, Port Aransas, 

 TX 78373. 



sonar to map the extent and distribution of differ- 

 ent habitat types and, in the case of tilefish, 

 derive an estimate of standing stock and potential 

 yield. 



TECHNIQUE 



Sidescan sonar is similar to low-angle, oblique, 

 aerial photography except that the images (sono- 

 graphs) are based on differences in the intensity 

 of the reflected acoustic signal rather than the 

 intensity of the reflected light (Belderson et al. 

 1972). The system consists of a towed vehicle (Fig. 

 1) in which is housed two sets of transducers that 

 scan to each side, a conducting tow cable, a winch, 

 and a dual-channel recorder for displaying the 

 signals. The transducers are constructed so that 

 their beams form a very narrow arc (1-2°) in the 

 direction perpendicular to the ship's track, but a 

 broad arc in the direction parallel to the ship's 

 track (Fig. 1). As the ship moves, successive 

 bands of seafloor are insonified, and in this way 

 an acoustic areal map is recorded of the scanned 

 area. 



We used a 100 kHz Klein^ sidescan sonar sys- 

 tem. This system can resolve features as small as 

 0.5 m diameter (see Results) at a scanning range 

 of 100 m to each side of the towed vehicle. The 



^Use of trade names in this report does not constitute endorse- 

 ment by the U.S. Geological Survey or the National Marine 

 Fisheries Service. 



Manuscript accepted June 1987. 



FISHERY BULLETIN: VOL. 85, NO 4. 1987 



725 



