Vertical distribution and migration of oceanic micronekton off Oregon

Abstract

An Isaacs-Kidd midwater trawl system that opens and closes codend nets and monitors depth and water velocity was used to collect micronekton from 12 contiguous depths in the upper 1000 m. Analysis of variance of the catches of common species indicated significant vertical migrations for five of seven fishes and two of seven shrimp and mysid species. Season bad little effect on the pattern of vertical migrations, average depth distributions, or overall variations in abundance. None of the common species was appreciably more numerous in night than in day catches; thus there was no evidence for day-night differences in avoidance of the trawl. Diel vertical migrations, as evidenced by distinct depth separation of day and night modes of abundance, were evident for only 32% of all the species of fishes, 13% of the shrimps and mysids, and 40% of the cephalopods. The range of diel migrations was generally 200 to 400 m. The four most common migrant fishes and the most abundant migrant shrimp numerically dominated upper mesopelagic waters by day and epipelagic waters by night. Lower mesopelagic waters were inhabited by a larger number of generally less abundant species which exhibited little, if any, diel vertical migration. The population of Stenobrachius leucopsarus, the most common fish, was composed of migratory and non-migratory individuals producing nocturnal peaks of abundance at both mid-depths and in near-surface waters. Both groups included a broad range of individual sizes. Individual fish probably do not migrate into surface waters on every night. The depth range of all species during either day or night was large, extending over several hundred meters and several decades of irradiance. As a result, vertical ranges often overlapped broadly. Common migratory species usually occupied depths of 0 to 50 m at night and 300 to 500 m during daytime. Size segregation by depth was not pronounced.