On the midwater fish faunas of Gulf Stream rings with respect to habitat differences between slope water and northern Sargasso Sea

Abstract

Forty-three 1000-in oblique Isaacs-Kidd midwater trawl collections were made on six cruises. In most cases, these were accompanied by 1500-m hydrocasts and plankton tows to 800 m. The fish collections comprise 19,400 specimens in 129 species, from Slope Water, Gulf Stream, Northern Sargasso Sea, and four cold-core rings. Temperature, salininty, oxygen, phosphate, and zooplankton biomass information accompany the collections. A model predicts that expatriate populations of Caribbean species can be maintained in the Slope Water and Northern Sargasso Sea in excess of 15 per cent of their Caribbean population density. With Caribbean contamination accounted for, the relative abundances of species in Slope Water and Northern Sargasso Sea are used to assign habitat preferences. Species clusters derived from an ordination using correspondence ana1ysis agree well with the habitat preference groups. The assemblage of species in the rings occupies the middle of a gradient in faunal composition from winter conditions in Slope Water to fall conditions in the Northern Sargasso Sea. The midwater habitat is modeled as two layers, and the ring environment as a hybrid habitat consisting of a surface layer of Northern Sargasso Sea habitat and a deep layer of Slope Water habitat. This model is consistent with patterns of species abundance. It is argued that the dramatic change in the depth of the main thermocline on crossing the Gulf Stream is the major physical factor affecting fish distributions with respect to this boundary.