Effects of a Warm-Core Eddy on Fish Distributions in the Tasman Sea of East Australia

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Abstract
Distributions of fishes were determined in and near a warm-core eddy off the east coast of Australia from Nov. 28-Dec. 13, 1978. Samples were collected with a midwater trawl (39) in the upper 500 m of the water column during day and night. Of the 14,602 fish caught, 88% belonged to the family Myctophidae. Few fish (< 4% of total) were caught during the day. At night, the structure of the fish community within the eddy was recognizably distinct from that in surrounding water masses. Most fish distributions correlated well with thermal structure. Common species were categorized as eddy species (5), outside eddy/cold-water species (5), warm-water species (1), cold-water species (8) and widespread species (7) based on nocturnal distributions. Eddy species were largely restricted to warm water within the eddy; warm-water species were also abundant near the surface outside the eddy. Outside eddy/cold-water species were caught almost exclusively outside the eddy or at the eddy edge. Of these species, 2 were concentrated in the cold water outside the eddy. For most cold-water species, the extent of the upward vertical migration at night did not appear to penetrate into the warmer surface layers. Widespread species included 4 of the 5 most abundant species caught (60% of total catch) and occurred inside and outside the eddy and at the eddy edge. Individuals of 4 spp. [Ceratoscopelus warmingii, Scopelopsis multipunctatus, Lampanyctus alatus and Diaphus thermophilus] caught inside and outside the eddy differed in size, and individuals of 2 spp. [C. warmingii, S. multipunctatus] differed in diet. Distributional patterns are discussed in relation to competition, predation and thermal ecology of the species.