Zooplanktivorous fish and variable diel vertical migration in the marine planktonic copepod Calanus pacificus

As a field test of the hypothesis that variable diel vertical migration in zooplankton is a response to variations in the abundance of visually orienting predators, we sampled juvenile and adult planktivorous fish inhabiting a temperate fjord (Dabob Bay, Washington) concurrently with the vertical distributions of a population of adult female Calanus pacificus (Copepoda: Calanoida) known to exhibit seasonally variable diel vertical migration. Results of nighttime trawling on seven dates between April 1985 and October 1986 and subsequent stomach content analyses showed that eight species of fish, comprising 45 different size classes, fed on adult female C. pacificus; all species of fish showed marked seasonal and interannual variations in their abundances. The extent to which the population of C. pacificus exhibited diel vertical migration was determined for each cruise date by calculating the proportion of the population crossing 75‐m depth on a diel cycle. A statistically significant regression was obtained between strength of diel vertical migration in C. pacificus and abundance of fish actively preying on the copepod. We propose that the timing and magnitude of changes in the migration behavior of C. pacificus in Dabob Bay are directly linked to the year‐class strengths of the dominant species of planktivorous fish.