The ecology of the deep-sea benthic and benthopelagic fish on the slopes of the Rockall Trough, Northeastern Atlantic

A total of over 28,000 benthic and benthopelagic fish belonging to 34 families and comprising at least 85 species were collected from the Hebridean Terrace in the Rockall Trough between soundings of 500 and 2000 m. Commercial type trawls (20.6 m Granton or 140 foot German bottom trawls) fished on paired warps at 33 stations accounted for 89% of all individual caught, the remainder being caught by a 16.4 m prawn trawl fished on a single warp (22 stations) and a 3 m Agassiz trawl (12 stations). The stations sampled, with a few exceptions, fell into discrete bathymetric zones separated by increments of approximately 250 m and different combinations of nets were used at each of these zones. The catch composition of the commercial trawls differed from those of the other nets. The most obvious difference was that squalid sharks, the alepocephalid Alepocephalus bairdii and the black scabbard fish Aphanopus carbo were important in the commercial type trawls but were absent or poorly represented in the other nets. Net size and towing speed were considered to be important factors influencing the catch composition. Net selectivity was most apparent on the upper and mid slopes but less apparent on the lower slopes. Relatively few families contribute to the total biomass at a given bathymetric zone and because the families Squalidae and Alepocephalidae contribute significantly to the biomass on the upper and mid slope it is therefore concluded that small nets must grossly underestimate the biomass at these depths. The greatest biomass occurred at mid slope depths (750–1000 m).