The mid-slope demersal fish community off southeastern Australia

Abstract

The mid-slope (800–1200 m) demersal fish community off southeastern Australia was sampled at 376 random, depth-stratified trawl stations. The mean density of demersal species was 4.82 g m −2. Thirty-seven families and 111 species of demersal fish were represented in the catch. The density of mid-slope fishes off southeastern Australia was comparable to that observed in the Northern Hemisphere. However, landings and acoustic and egg surveys of orange roughy ( Hoplostethus atlanticus) indicate that densities of that species alone are an order of magnitude higher than the total fish density indicated by trawl surveys. Water-column productivity over the mid-slope region appears insufficient to support the higher range of density estimates, implying a significant flux of energy into the region either from offshore or downslope. The dominant mid-slope demersal fishes appear to comprise an identifiable community within a biogeographic province that extends at least from the Great Australian Bight to the Chatham Rise (New Zealand), a distance of ∼5000 km. Distinct assemblages of demersal fish were found at upper (500 m) and mid-slope (800–1200 m) depths off southeast Australia. The mid-slope community could be sub-divided into assemblages by depth (shallow, intermediate and deep) and area (east and west Tasmania), which were statistically robust although with considerable overlap of species composition. There was no overlap in species composition of the southeast Australian mid-slope demersal fish community with fish communities at similar latitudes and depths in the North Pacific, but there were affinities with those in the North Atlantic. These biogeographic patterns, which appear consistent with oceanic circulation at intermediate depths, provide strong evidence that negates the recent hypothesis that deepwater fish communities cannot be defined over broad areas and are only random assemblages ( Haedrich and Merrett, 1990, Progress in Oceanography, 24, 239–250).