Nocturnal Feeding of Pacific Hake and Jack Mackerel off the Mouth of the Columbia River, 1998‐2004: Implications for Juvenile Salmon Predation

Abstract

AbstractPredation by piscivorous marine fishes has been hypothesized to be a primary source of marine mortality for Pacific Northwest juvenile salmon. During the springs and summers of 1998‐2004, we collected predator and prey fishes (forage and juvenile salmonids) at the surface at night off the mouth of the Columbia River. Pacific hake Merluccius productus had relatively low percentages of empty stomachs during cool‐ocean years (2000 through 2002) and high percentages during 1998, a warm‐ocean year. Euphausiids and fishes were the most commonly eaten prey for both species. Pacific hake and jack mackerel Trachurus symmetricus appeared to show some diet selectivity, eating some fish, including salmonids, in a higher proportion than found in the environment. Both Pacific hake and jack mackerel ate juvenile salmonids, but at very low amounts. After considering population sizes in the study area, these two predators do not appear to be responsible for the death of large numbers of Columbia River juvenile salmon smolts. However, we may have underestimated the number of salmonids eaten by hake and mackerel due to the limitations of our study. More work needs to be done to identify and quantify predation of juvenile salmon off the Pacific Northwest.