Impact of Submarine Canyons on Nearshore FisheriesIn August 1995, demersal juvenile and adult Pacific hake (Merluccius productus) were observed in the Scripps/La Jolla canyon system and at nearby non-canyon sites. The submersible Sea Cliff and the Advanced Tethered Vehicle were used at depths between 300 and 900 m in the canyon, and 300 to 500 m at non-canyon sites. Two weeks later the submersible Delta was used for dives ranging from 100 to 300 m at both canyon and non-canyon sites. In July 1996 additional observations (100 -200 m) were made using Delta in Carlsbad, Redondo, and Dume canyons, and at non-canyon stations near Carlsbad and Malibu.
Scripps/La Jolla Canyon. Hake were seen on canyon dives from 100 - 700 m, and on non-canyon dives from 200 - 500 m. Adults were generally observed singly while juveniles were most common in groups of 2-3 out of the canyon and in aggregations ranging from dozens to thousands in the canyon, depending on depth.
Hake were more abundant at canyon than non-canyon stations at all depths; however, numbers of adults were not significantly different between the two habitats at 500 m. Juvenile hake were common in the canyon from depths between 100 and 290, but were especially abundant from about 150 to 230 m, where they were frequently seen lying motionless on the sea-floor in densities as high as 50 m-2. Juvenile hake were rare at all non-canyon stations.
Juvenile hake were collected on several Delta dives with a slurp-gun that emptied into a 300 µm mesh net. The mean size of the captured fish was 79.6 mm. The sampling device did not appear to have a size bias, all juvenile hake that came near the end of the sampler were captured. Stomach and intestine contents from 120 fish were examined. All fish had pink amorphous material in their intestines, along with some recognizable crustacean parts. Eighty four percent of the juvenile hake had food items in their stomachs. Of those, 87% had recently consumed euphausids. The next most common prey item was adult Nebalia spp. (a benthic leptostracan crustacean). Nematodes were found in the body cavity of 6% of the dissected fish. All fish examined were captured between 0800 and 1200 hrs.
In October 1998 large aggregations of juvenile hake were observed in Scripps Canyon. At this time the fish were larger than those observed in August 1995 and were able to avoid capture by the slurp sampler.
Carlsbad, Redondo and Dume Canyons. Large aggregations of juvenile hake were seen in Carlsbad and Redondo canyons. Few or no juvenile hake were seen at control stations off Carlsbad and Malibu and in Dume Canyon. Dume canyon is the northernmost of the canyons investigated and unlike the other canyons which are oriented east-west, it is oriented north-south.
All three Southern California Bight submarine canyons with east-west orientations examined harbored large aggregations of juvenile Pacific hake. The fish may be attracted to submarine canyons by high prey concentrations or physical structure; however they may simply become concentrated in canyons during ontogenetic onshore migrations. If that is the case it could explain why they were absent from the north-south oriented Dume canyon.