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Fishery Bulletin 98(3) 



older individuals of valuable rockfish species. There is little 

 information on the distribution, abundance, and habitat 

 characteristics of mature rockfishes associated with deep- 

 water benthic marine habitats off California. This type of 

 habitat is below scuba depths (<30 m), and the rocky het- 

 erogeneous substrata inhabited by many rockfish species 

 prohibit accurate estimates of fish abundance with con- 

 ventional trawl surveys. Diversity, quality, and extent of 

 habitat likely are among the most significant environmen- 

 tal determinants of distribution, abundance, and species 

 richness of adult rockfishes (Larson, 1980; Richards, 1986; 

 Pearcy et al, 1989; Carr, 1991; Stein et al., 1992). Charac- 

 terizing and quantifying elements of suitable habitat, such 

 as substratum type, texture, and relief are therefore criti- 

 cal for evaluating the effectiveness of refugia, both natu- 

 ral and designated protected areas, to maintain regional 

 marine resources. 



Eight submarine canyons cut into the continental shelf off 

 central California, placing deep-water habitats in close prox- 

 imity to shore. We hypothesized that isolated rock outcrops in 

 deep water along the steep walls of these canyons could serve 

 as natural harvest refugia and allow certain rockfish spe- 

 cies locally to attain large sizes and high abundances. Sub- 

 sequent distribution of offspring from these mature fishes 

 could help maintain viable populations and species diversity 

 in adjacent areas of greater fishing activity. 



Our general goal in this study was to characterize rock- 

 fish assemblages and their relationship to specific benthic 

 habitats within submarine canyons by combining geophys- 

 ical and m situ submersible surveys. Because temperate 

 benthic habitats are often defined by geologic attributes, 

 geophysical techniques are critical in determining habitat 

 structure, depth, and lithology. 



Within our general goal, this study had four specific 

 objectives. Our first objective was to characterize the geo- 

 morphology of our study site in Sequel Canyon using 

 bathymetric mapping and side-scan sonar imaging (i.e. 

 sonographs) to classify the substrata and to locate rock 

 outcrops on a spatial scale of 100s of meters to kilometers. 

 Our second objective was to map and quantify the amount 

 of exposed hard substrata at depths suitable to rockfishes 

 within our study site. Third, using a manned submersible, 

 we set out to verify our interpretations of the remotely 

 sensed images of habitat on a smaller scale (i.e. 1 meter 

 to 10s of meters) and determine frequency of occurrence, 

 distribution and type of habitat that support assemblages 

 of adult benthic rockfishes in Sequel Canyon. Our fourth 

 objective was to estimate and compare species composi- 

 tion, abundance, size, and diversity of fishes among habitat 

 types, depth zones, and locations that have been subject to 

 various amounts of fishing activity within the canyon. 



depth of 80 m. This canyon trends southwest for 10 km, at 

 which point it intersects the larger Monterey Canyon at a 

 depth of 915 m (Fig. lA). Soquel Canyon is eroded from the 

 generally flat-lying beds of the Pliocene Purisima Forma- 

 tion, a shallow-water marine deposit comprising interlayers 

 of sandstone, mudstone, and shell hash (coquina). Previous 

 undercutting of the canyon walls has caused extensive lamd- 

 slides and slumping; resultant rock exposures were the tar- 

 gets of our survey of rockfishes and their associated habitats. 

 Our study area covered about 17 km-' of the headward 

 part of the canyon, between 80 and 360 m water depth. 

 This represents about two-thirds (6.7 km) of the length of 

 the canyon's axis; at 6.7 km along the axis, the canyon is 

 3.5 km wide and 650 m deep. 



Geophysical surveys 



Side-scan sonar is a suitable method for distinguishing 

 blocks of hard substrata from surrounding soft sediment 

 by differences in intensity of reflected sound (Able et al., 

 1987; Greene et al., 1995; Yoklavich et al, 1997). Our 

 sonographs of seafloor morphology resemble a black and 

 white photographic negative. Topogi'aphic features such as 

 ledges, vertical walls, and boulders produce dark and light 

 images on the records, depending on the orientation and 

 hardness of the feature. A strong signal or reflector (dark) 

 is received from the side of a relatively hard feature facing 

 the transducer, whereas a weak signal or shadow (light) is 

 received from the side sloping away from the transducer. 



We conducted a side scan sonar survey along 110 km of 

 track lines using a 100 kHz acoustic signal with a swath 

 width of 600 m (300 m per side). The sonographs along 

 each track line were positioned precisely with navigational 

 data from a differential global positioning system (GPS) to 

 form a mosaic of rock type and texture within the canyon. 

 Because of steep relief only one side of the transducer 

 received usable signals and 200'7f coverage was necessary 

 to produce a complete mosaic. 



We used the mosaic to quantify the amount of hard sub- 

 strata at depths suitable to rockfishes. Our interpretations 

 of the sonographs were verified by direct observations 

 made by marine geologists (HGG and DS) during nine 

 dives in the Delta submersible. Type, relief and size and 

 depth range of features were described; these field descrip- 

 tions assisted the marine biologists in planning dives at 

 each site and in assessment of habitat after the cruise. 



We conducted a bathymetric sui-vey of Soquel Canyon 

 using a 3.5-kHz precision depth recorder integrated with 

 Loran and GPS navigational data. The resultant high-res- 

 olution map (20-m intervals) was used to identify areas of 

 high relief and potential slumps. 



Materials and methods 



Study site 



Soquel Canyon (ca. 36°49'N, 121°59'W) is a submarine canyon 

 of inactive sediment that cuts into the continental shelf in 

 Monterey Bay nine miles south of Santa Cruz, at a water 



Fish and habitat surveys 



We used the Delta submersible to assess benthic fish 

 assemblages and associated habitat in Soquel Canyon 

 in August 1992 and October 1993. The Delta is a small 

 ( 4.75-m ) submersible, accommodates one scientific observer 

 and a pilot, has a maximum operating depth of 365 m, 

 and a cruising speed of 1.5 knots. An acoustic track-point 



