Biology

The FOCI program has done much to add to our understanding of the physical environment and life history of walleye pollock in the Gulf of Alaska. Before FOCI was established in 1984, walleye pollock from strong year classes in the mid-1970's produced a large commercial fishery. Hydroacoustic surveys conducted to estimate the size of the adult walleye pollock population showed that large concentrations of prespawning walleye pollock migrated into Shelikof Strait. These observations served as a starting point for our program. In order to study natural changes in the abundance of walleye pollock, our initial hypothesis was that optimum survival and subsequent recruitment result when larvae are transported to nursery grounds in coastal regions along the Alaska Peninsula, rather than into the Gulf of Alaska.

The timing and location of spawning and egg and larval development of walleye pollock have been found to be remarkably consistent. In late winter, adults enter Shelikof Strait by heading north through a deep sea valley. Spawning occurs in early April near Cape Kekurnoi. Each female produces about one-half million free-floating planktonic eggs (1.8 mm diameter) in a series of about ten batches over a few weeks. The eggs reside near the bottom at depths of 150-200 m and remain there for about 2 weeks until they hatch. The dense, localized spawning pattern creates a large patch of eggs detected in plankton surveys. Young larvae are 3-4 mm long at hatching and are relatively undeveloped, without functioning mouths or eyes. As they develop, they quickly rise to the upper 50 m of the water column, where they drift in the prevailing currents during late April and May. Larvae remain in patches and have been found caught up in mesoscale circular currents (eddies). During this time, larvae are about 10 mm and grow about 0.2 mm/day with a mortality rate of about 0.09/day. They eat copepod nauplii and other small zooplankton; the size range of prey increases as the larvae grow. They are visual feeders eating mostly during the day.

By late May, considerable year-to-year variation exists in abundance and location of larvae as they are slowly carried further southwest by the current which splits near the Semidi Islands. Two primary tracks characterize westward larval drift: along the Alaska Peninsula in the slow-moving flow over the shoreward edge of the sea valley and offshore in the rapidly moving Alaska Coastal Current. FOCI scientists hypothesize that the best conditions for survival and growth probably occur in the more productive coastal waters. Larvae appear to stay just below the turbulent, wind-mixed upper layer of the water column. Larger larvae undergo vertical migrations in the water column (15-20 m deeper during the day). By mid-summer larvae transform into juveniles, and by late summer are schooling and concentrated in nearshore areas along the Alaska Peninsula. After this period we know little about their life history until they enter the fishery and become sexually mature at age 3.

Caption for figure to be added: Geographic distributions of walleye pollock eggs and larvae for four time intervals during their first two months of life. Distributions are represented by ellipses computed from ichthyoplankton surveys from 1987 through 1992.