Mesopelagic fishes in Gulf Stream cold-core rings

Calculations of abundance of midwater fishes in the families Myctophidae, Gonostomatidae, Photichthyidae, and Sternoptychidae for the 1000 m water column were made in cold-core rings and in the nearby Sargasso Sea and Slope Water. The data were considered with respect to the depth to 15 C, which isotherm lies shallow in the Slope Water, deep in the Sargasso Sea. Myctophid-gonostomatid biomass (excluding Cyclothone spp.) varied inversely as the depth to 15 with that for the Slope Water in the ratio of about 4.5:1 to that for the northern Sargasso Sea. For Cyclothone spp. the ratio was about 2.5:1. Benthosema glaciale, a subpolar-temperate myctophid, becomes more and more restricted to the deep part of aging rings and finally disappears from them. Warm-water fishes quickly become abundant in the upper few hundred meters of aging rings. The myctophids Lampanyctus crocodilus, L. pusillus, and Hygophum benoiti appear to be rings exploiters; they were more abundant there than elsewhere. Detrainment of fish from Ring "Frank" was observed at depths near 600 m. Three-month old Ring "Al" already had entrained fish at depths greater than 250 m. The northern edge of the Gulf Stream, an important faunal boundary between temperate and subtropical parts of the western Atlantic, is a differential barrier; warm-water animals cross it to the north more easily than do cold-water animals to the south.