Document Type



Virginia Institute of Marine Science

Publication Date



Special Scientific Report No. 96


Otter trawl cruises were conducted off the middle Atlantic coast of the U. S. from 1971 to 1975. More than 50,000 specimens of demersal fishes were captured during this program at depths of 75 m to 3000 m. Species assemblages were distributed along a coenocline with bathymetric areas of rapid faunal change (anantoclines) and of more gradual faunal change (aganoclines). Between 75 and 3000 m anantoclines were found at 150 to 200 m, 400 to 600 m, 900 to 1000 m, 1350 to 1500 m, and 1900 to 2100 m. Species diversity (Hl) increased between the continental shelf and slope, remained constant to about 1800 m, then declined rapidly. This pattern was primarily related to species richness,not eveness. Numerical abundance increased bet~veen the shelf and slope, then declined exponentially. Biomass increased between the continental shelf and slope and then remained fairly constant out to a depth of about 1800 m beyond which there was an exponential decline (but not as steep as that for numerical abundance). The average size of fishes was about the same on the shelf and upperslope out to a depth of about 1800 m beyond which size increased by threefold. This size increase is contrary to that found for the meiofauna and macrofauna. The difference in strategies lies in the mobility of fishes which allows them to maintain viable sexually reproducing populations at relatively small population size. Biomass of fishes was of the same order of magnitude as that reported by other workers for the infauna at similar depths. This apparent contradiction of classic trophic pyramid structure is resolved because the mobility of fishes allows them to utilize pelagic as well as benthic food sources. The declines in diversity and biomass and the increase in average size of fishes, all occurred at around 1800 m near the slope-rise border beyond which regular food sources of continental origin are absent.



Fishes -- Geographical distribution; Fish populations -- Mid-Atlantic Bight; Fish populations -- Virginia -- Atlantic Coast; Vertebrate populations