Abstract:
Most fishes exhibit differences in activity patterns from day to night, with a switch from
high to low activity at dusk and reverse at dawn. Marine reef fishes show similar behavior
among sites, despite dissimilar species assemblages, suggesting that common selection pressures
are leading to convergent behavior. Few studies to date have examined 24-hour changes in
species composition and activity patterns in temperate deepwater habitats, where day to night
differences in light intensity are low. In 2000, a consortium of research organizations conducted
overlapping surveys during day, night, dawn, and dusk periods using the ROPOS remotely
operated vehicle (ROV) on Heceta Bank, Oregon. Activity in fishes was classified by movement
and contact with the seafloor. Most taxa were broadly distributed by depth, station and habitat
during both day and night. General patterns in activity were similar to other shallow temperate
diel studies with an overall increase in abundance and activity of fishes during the day compared
to night, particularly in shallow areas of large substratum. However, few taxa showed evidence of
diel niche partitioning of habitat and most taxa did not exhibit distinct diurnal or nocturnal
behavior. During dawn, some taxa showed sequential emergence and increased activity over a
2.5 hour time period, although no distinct quiet period was shown. For some fishes with
significant day-night differences in abundance and activity, this work suggests that daytime
groundfish trawl surveys may have an important sampling bias. This research is unique in that
ROV technology is used to examine diel fluctuations in abundance and activity of demersal
fishes at a relatively deep, temperate rocky bank along the outer continental shelf in the vicinity
of one of the most important areas for groundfish fisheries off the Oregon coast.
