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Midwater Assessment & Conservation Engineering (MACE)

Gulf of Alaska, East Kodiak Fishery Interaction Experiment

map, see caption
Figure 1.  Chiniak Trough and Barnabus Trough off the east side of Kodiak Island, Alaska, showing acoustic-trawl survey transects for the Fisheries Interaction Team experiment.

Members of the RACE Division’s Midwater Assessment and Conservation Engineering (MACE) Program and the REFM Division’s Fisheries Interaction Team (FIT) collaborated in an acoustic-trawl experiment for the fifth year aboard the NOAA ship Miller Freeman off the east side of Kodiak Island in Chiniak and Barnabas Troughs from 13 August through 5 September.

The principal objective of the study was to describe the spatio-temporal variability in walleye pollock and capelin (Mallotus villosus) abundance and distribution patterns in the two troughs over a period of several weeks before and during the commercial pollock fishery.  This work is part of a larger effort designed to evaluate the effect of commercial fishing activity on the prey availability of walleye pollock and other forage fish species to endangered Steller sea lions (Eumetopias jubatus).

Both troughs were surveyed from near-shore to the shelf-break along parallel transects spaced 3 nmi apart (Fig. 1).  Midwater and bottom trawls were used to confirm the species composition of the acoustic backscattering and collect other biological information. Oceanographic data were collected, including underway sea surface temperature and fluorometry, water column profiles, and drifter deployments.

Five survey passes of each trough were completed. Most of the acoustic backscattering was attributed to three principal groups: age-0 pollock/capelin mix, adult pollock, and age-1 pollock. Adult pollock were generally detected as on-bottom schools (“carpet”). Age-1 pollock or other aggregations composed of both age-0 pollock (5-11 cm standard length (SL)) and capelin were found in midwater. Adult pollock were generally distributed within the northern half of Barnabas Trough and throughout Chiniak Trough. Age-1 pollock (18-25 cm fork length (FL)) were generally more abundant in Barnabas than in Chiniak. They were typically found above the adults in midwater.

Relatively large, dense aggregations of age-0 pollock/capelin were broadly distributed throughout Chiniak Trough and predominantly in the southern portion of Barnabas Trough. Unlike the deeper dwelling adults, the age-0 pollock/capelin mix often extended over relatively shallow bottom depths of less than 100 m. A single, shallow area where adult pollock were found was located near the mouth of Ugak Bay. The presence of relatively large numbers of age-0 pollock during the survey was similar to what was observed in 2004.

Daytime trawl catches often caught more age-0 pollock than capelin, although the selectivity of the midwater trawl to these species is unknown so it is uncertain whether the different catch rates for these two species groups accurately reflects their relative abundance in the water column. Adult pollock were generally large. Adult size compositions based on trawl catches had length modes that ranged between 45-62 cm FL. Size composition of age-1 pollock had prominent length modes near 20-21 cm FL.

A total of 589 marine mammal sightings of groups or individuals were made during the survey.  A northern right whale was sighted in the outer region of Barnabas Trough on 1 September 2006. Humpback whale sightings occurred in both troughs. Sightings of fin whales occurred in the outer portion of Barnabas and Chiniak Troughs, and gray whales were reported in Barnabas Trough in the vicinity of Ugak Bay. Other marine mammal sightings included Dall porpoises, Steller sea lions, and harbor seals. As in earlier surveys, an oceanographic front was observed in the middle of Barnabas Trough.

By Chris Wilson, Anne Hollowed, Libby Logerwell, and Paul Walline

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