Mesopelagic fish zoogeography in the central tropical and subtropical Pacific Ocean: species composition and structure at representative locations in three ecosystems

Mesopelagic fish species are not uniformly distributed throughout the tropical and subtropical Pacific Ocean. There are reasonably clear-cut faunal assemblages with characteristics species. The geographic ranges of the assemblages conform to those patterns shown by zooplankton. As part of a 5 yr (August 1969-March 1974), seven-cruise, ecological study, replicate sampling in central gyral waters included additional quantitative sampling at a point along the equator at 155°W to compare species composition and structure at this location with points in the central gyres. These comparisons show that the distribution of individuals among species displays similar structure in all three areas, but species composition and the relative abundances of shared species differ. There are species restricted to the low-productivity central gyres, and subgroups of these are restricted to specific gyres. The gyral faunas differ markedly from the high-productivity species assemblage found along the equator. With the exception of the eastern tropical Pacific, where low oxygen concentrations probably limit species distributions, the primary productivity regime is the most likely factor defining the distributions of most mesopelagic fishes in the tropical and subtropical Pacific. The mechanisms for the maintenance of these geographic patterns are not clear, but the correlation of the patterns with the productivity regime leads one to hypothesize that the ultimate factor is food.