Physical And Biological Factors Affecting The Diel Vertical Migration Of Walleye Pollock

Abstract

The mechanisms underlying diel vertical migration (DVM) in marine fishes are unclear, although it is generally thought that this behavior is influenced by light, hydrography, food availability and predator avoidance. In the North Pacific Ocean, walleye pollock (Theragra chalcogramma) undergo DVM as juveniles, ascending to the surface at night and returning to the bottom at dawn. Adults are generally considered demersal. The objective of this research was to examine the effect of light, temperature and prey availability on the DVM of adult pollock. The work was undertaken to further our understanding of pollock biology, and the mechanisms underlying DVM in marine fishes in general. The study was conducted in the northern Gulf of Alaska in April, August and November 2003. Trawls < 80 m in April, and < 50 m in August, suggested that at least some portion of the pollock population was ascending to within 20 m of the surface in spring and summer. In November, acoustic data and targeted hauls > 100 m indicated that adults were not ascending to the surface at night, and that DVM behavior was occurring at depth. Euphausiids were the primary component of the diet in April and August. Decapods, primarily the shrimp Pandalus borealis, were the main component of the diet in November. Pollock passed through the thermocline during their ascent to the surface at night in August, and there was no relationship between the mean depth of pollock and the isolume (line of equal light intensity) necessary for visual foraging. In contrast, there was a significant relationship between the biomass of adult pollock above the 200 m isobath and the isolume necessary for visual foraging in November. Pollock did not pass through the thermocline at this time. It was concluded that in August adults ignore the isolume and thermocline, simply tracking the movements of euphausiid prey to feed upon them near the surface at night. In November, when euphausiids are no longer in patches, pollock shoals migrate up and down with the isolume necessary for visual foraging to feed on decapods.