THE VERTICAL-DISTRIBUTION OF EGGS AND LARVAE OF WALLEYE POLLOCK, THERAGRA-CHALCOGRAMMA, IN SHELIKOF STRAIT, GULF-OF-ALASKA

The vertical distribution of walleye pollock eggs and larvae in Shelikof Strait, Gulf of Alaska, was investigated by using data from 36 Multiple Opening-Closing Net and Environmental Sensing System (MOCNESS) tows taken in April and May, 1986-88. Most eggs were found below 150 m to near bottom (approximately 300 m) but progressively shallower later in the season. Eggs in middle stages of development were found at shallower depths than were younger or older eggs. The vertical distribution of eggs was positively related to observed differences in seawater temperature but showed no relationship to density. Larvae hatch at incubation depth and quickly rise to the upper 50 m of the water column where they remain during larval development. Larger larvae (approximately 7-10 mm standard length [SL]) undergo limited diel vertical migration within the upper 50 m. They are deepest during the day, shallowest at dusk, slightly deeper at night, and even deeper at dawn. Their mean depths of occurrence were between 21 and 37 m at all times. At these depths, prey (copepod nauplii) generally were at densities sufficient for larval pollock growth as determined in laboratory studies. Pronounced thermoclines and pycnoclines were present in the part of the water column inhabited by the larvae in late May. Larvae appear to remain below the upper mixed layer during periods of increased turbulence, but at depths during daytime where light was sufficient for feeding and where prey densities were adequate.