Diel behavior of sardine and anchovy schools in the California Current System

Fish schools containing Pacific sardine Sardinops sagax and northern anchovy Engraulis mordax were observed in 2 regions within the California Current System in 2 years using a combination of moored and shipboard acoustics and net sampling. Schools of sardines and anchovies off the Oregon coast followed typical diel patterns consistent with antipredator behavior, dispersing at nighttime, rapidly reforming into discrete schools at sunrise, and maintaining schooling behavior throughout daylight hours. Discrete schools containing primarily sardines in Monterey Bay, California, were observed during both daytime and nighttime in addition to layers and loose aggregations at nighttime, with a peak in the formation of schools occurring several hours before sunrise. Transitions between daytime and nighttime behaviors occurred more gradually in Monterey Bay than off the Oregon coast. The 2 regions experienced different prey environments, with acoustic indices for zooplankton abundance in Monterey Bay much higher than off Oregon. Due to the shallower water column, prey availability was fairly consistent throughout day and night in Monterey Bay. However, prey availability was highly variable at the Oregon site, where diurnally migrating zooplankton were only available to fish in the surface region at night. The combined effects of prey availability and the water column depth may influence the efficiency of school formation, leading to the differences in diel patterns of schooling that were observed among the 2 regions. These environmental influences on schooling behavior likely have significant consequences for predators that rely on sardine and anchovy schools as prey as well as the commercial fisheries in both regions.