Abstract:
A 50 m² pelagic trawl equipped with a codend opening-closing device was used to sample micronektonic fishes between 500 m and 1000 m. Horizontal tows were made at 500, 650, 800 and 1000 m. Twelve of 15 common species captured showed significant diel differences in abundance at one or more depths indicative of vertical migration. Distinct vertical separation of day and night peaks in abundance of Bathylagus milleri shows that it migrates from 650 m during the day to 500 m at night. Catches of Cyclothone acclinidens and C. atraria were significantly higher during night than day at 800 m, suggesting upward migration from deeper depths at night. Several species showed changes in migratory behavior with size. Only small (<240 mm) Tactostoma macropus migrated vertically, whereas only large (>110 mm) Bathylagus pacificus appeared to migrate. Many species exhibited trends in size with depth, with size either increasing or decreasing with depth. Congeneric species usually had well separated population centers. Bathylagus mulleri and B. pacificus had similar abundance peaks by day,
but B. milleri were shallower at night. Stenobrachius leucopsarus and S. nannochir were clearly separated vertically both day and night. The four common Cyclothone species were divided into an upper and a lower mesopelagic pair. Cyclothone pseudopallida and C. signata were most abundant at 500 m day and night, whereas C. acclinidens and C. atraria were most abundant at 800 m. All four common Cyclothone species appeared to migrate.
