Mesopelagic fishes and thermal fronts in the western Sargasso Sea

Analysis of 25 midwater trawl collections, disposed along the meridian 70°20′W from off Hispaniola to the Gulf Stream, showed a change in the mesopelagic fish fauna at about 27°N. The point of faunal change corresponded to a change in the temperature structure of the upper part of the water column, i.e., at a (the?) so-called “thermal front”, perhaps identical to the so-called “North Atlantic subtropical convergence”. Of 44 species occurring in four or more collections, 13 species were collected only to the north of the front and 1 species only to the south. By most criteria, north-of-the-front collections were larger than southern ones. This is in accord with the north-south difference in primary production noted by other workers which, in turn, seems atributable to the north-south difference in temperature structure. To the north the upper part of the water column is well stratified in summer only, while to the south it is well stratified at all seasons. Taken altogether, what is now known suggests that the thermal front logically divides the upper Sargasso Sea into northern and southern portions that differ in many ways.