Time series acoustic observations of macrozooplankton diel migration and associated pelagic fish abundance

Abstract

In April 1997, we deployed an upward-looking 165-kHz water acoustic structure profiler (WASP) at a depth of 240 m near the head of Barkley Canyon on the west coast of Vancouver Island. Acoustic backscatter intensity, recorded every 10 s at 2-m intervals through the water column, revealed that zooplankton aggregated in well-defined layers near the canyon floor during daylight hours. Several hours prior to sunset, the animals began a slow vertical ascent, which ended 7.2 ± 4.2 min after sunset with a rapid ascent to the surface. Upward migration attained maximum ascent rates of 6.3 ± 0.5 cm·s-1 and ended 55.4 ± 2.3 min after sunset (18.6 ± 2.4 min before nautical twilight). Zooplankton remained near the ocean surface for several hours before beginning a slow descent, followed by a rapid-descent 24.6 ± 3.2 min before sunrise. Zooplankton quickly descended below major fish aggregations and ended their rapid descent 22.6 ± 8.7 min after sunrise, reaching the canyon floor 2-4 h after sunrise. Small fish targets were observed near the ocean surface during the night. Large fish targets generally did not arrive until dawn and were more concentrated at middepth.