Vertical Size–Depth Distribution Properties of Midwater Fish off Bermuda, with Comparative Reviews for Other Open Ocean Areas

Abstract

The nighttime (N) and daytime (D) size–depth distributions of mesopelagic fish at Ocean Acre, a typical deepwater site in the North Sargasso Sea, are described using archival discrete-depth data from the surface to about 1500 m. The N/D abundance distributions n(z) for a representative group of 18 swtmbladdered species indicate that fish are concentrated in the 0–150 m euphotic zone at night and in the 400–1000 m zone during the day in spring, summer, and winter. Seasonal succession in the average life stage size classes of the bladdered group occurs, but all classes are present year-round due to the variety of specific spawning patterns. The noncoincident N/D depth niches of individual bladdered species apparently are organized to reduce competition. Normalized N/D depth distributions [Formula: see text] for the bladdered group are similar throughout the year, and appear to be determined, respectively, by the N distribution of zooplankton biomass and the D underwater light field, other hydrographic variables having little direct influence. Also, it seems likely that the form of [Formula: see text] is relatively insensitive to species composition for such groups. These hypotheses are verified qualitatively by comparing specific and group distribution properties at other ocean sites with those at Ocean Acre.