Walleye pollock (Theragra chalcogramma), an abundant and valuable gadid, occurs throughout the temperate and subarctic North Pacific and adjacent seas and spawns pelagic eggs in numerous defined areas within this range. These areas have different depths and hydrographic conditions. The eggs and then larvae drift with prevailing currents toward presumed juvenile nursery areas. Water temperature and currents often vary with depth, so the vertical position of the eggs can affect their rate of development, and rate and direction of drift. The specific gravity of the eggs determines their vertical position. Results of studies of the vertical distribution and specific gravity of walleye pollock eggs that have been conducted in five spawning habitats (Funka Bay, Japan; Shelikof Strait, Gulf of Alaska; Bering Sea basin; Bering Sea slope; and Eastern Bering Sea shelf are reported. I found that the specific gravity and vertical distribution of eggs varied among these spawning areas. These differences allow the eggs to reside at a depth which will minimize development time and enhance their probability of hatching in a food-rich environment. These variations in specific gravity may reflect genetic differences AMOng spawning populations.

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