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Groundfish Assessment

Juvenile Pacific Ocean Perch Research in the Aleutian Islands

figure 2, see caption
Figure 2.  Juvenile Pacific ocean perch survey area. Study sites are outlined.

Two research cruises aboard the Ocean Explorer were completed by Groundfish Assessment Program scientists during summer 2004 to map and study juvenile Pacific ocean perch (POP) habitat. The research was conducted around Samalga Pass and the Islands of Four Mountains, in the eastern half of the Aleutian Islands archipelago (Fig. 2 above). The goal of this study was to assess the value of AI habitat to juvenile Pacific Ocean perch (POP). The specific objectives of this research were to 1) map five study areas using multibeam and sidescan sonar and groundtruth these acoustic observations; 2) model relationships between juvenile rockfish abundance and habitat characteristics, such as depth, slope, temperature, sponge and coral abundance; and 3) link the habitat to the condition of juvenile rockfishes by determining if energetic content of juvenile rockfish is similar in different habitats.

  Sidescan sonar mosaic
Figure 3.  Sidescan sonar mosaic from the Islands of Four Mountains west study location showing interesting geographic features on the seafloor.

During the initial cruise, beginning and ending in Dutch Harbor, Alaska, (28 May to 9 June 2004) habitat mapping was completed. Current speeds were high around the islands and passes (exceeding 6 knots), which slowed the mapping progress. Each of the five study areas surrounding the Islands of Four Mountains was mapped using towed side-scan sonar (Klein 3000) and multi-beam sonar (Simrad SM2000) systems. These instruments collected bathymetry and reflectivity data using sound characteristics reflected from the seafloor. This acoustic data was geo-referenced using an ultra-short baseline (USBL) tracking system to pinpoint the exact position of the bottom information. Acoustic data collection and tracking were performed by technicians from the U. S. Naval Undersea Warfare Center (NUWC), Keyport Washington. Much of the data processing was completed aboard the Ocean Explorer and mosaics were produced that depicted bottom depth and roughness (i.e., Fig. 3). In total, 25 km2 were mapped using side-scan sonar, and multi-beam data was collected over almost twice that area.

Fish and zooplankton were collected at nine transects in the five study areas using a bottom trawl. Atka mackerel and Pacific cod dominated the catch. Other species collected in large numbers were adult Pacific ocean perch, northern rockfish, Pacific halibut, and walleye pollock. Juvenile POP were collected from two of the five study areas, but were not found at the other sites. Sponge and coral were found at 85% of transects where trawls were made.

Preparing to launch the camera system
Figure 4.  Researchers and vessel crew prepare to launch a towed underwater camera system aboard the Ocean Explorer.

The second cruise (12-23 August 2004) benefited from excellent weather for 6 days that allowed researchers to make substantial progress in meeting project objectives. Researchers concentrated on completing underwater video collections and sediment sampling to verify acoustic habitat maps produced during the first cruise. The underwater video system was towed behind the research vessel from 1-2 m off the seafloor, and technicians from the NUWC used a USBL system to geo-reference the video camera position (Fig. 4). Video was collected at 12 transects in three of the study areas that bisected interesting areas from the multibeam and sidescan sonar mosaics. Scientists viewed the video feed aboard the research vessel in real time which allowed areas with juvenile POP present to be located using the camera system. Preliminary results indicate habitats sampled at each area varied widely, from bare sand fields to rocky ledges, ridges and pinnacles. Sponge and coral were the dominant epibenthic invertebrates observed in the video and trawl collections, although diversity in benthic organisms was quite high at some sites.

Sediment samples were collected from the seafloor using a van Veen grab (20 samples total). The samples were composed of coarse volcanic sands and rocks, some of which were quite large. These samples will be used to interpret reflectivity collected using the sidescan sonar system by determining the grain-size of each sediment sample.

Biological sampling was also carried out during the August cruise. Both bottom trawls and ring nets were used to sample the biota of three of the study areas. Zooplankton samples were collected using a 1-m ring net and were preserved for laboratory analysis. Juvenile POP were collected from five transects in three areas. These samples were taken at locations where juvenile POP were observed in the video, and the samples were frozen for laboratory analyses. Sponge and coral were observed at all sites where juvenile POP were present in August.

During the fall and winter of 2004-05, video, sediment samples, zooplankton, and fish collections will be analyzed in the laboratory. In the coming months we will determine the energetic content (indicating the relative condition) of juvenile POP collected from the different sites. Using biological (plankton abundance) and physical (temperature) data combined, we will predict the potential for growth of juvenile POP at each study site. This three-tiered approach (abundance, condition, and growth potential) will provide a method to determine the relative value of habitats to their inhabitants, as well as providing insight into the processes controlling fish-habitat relationships.

By Chris Rooper



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