Preliminary Catch, Discards and Selectivity Results of Trawl Survey on Deepwater Slops of the Rockall Trough

Abstract

A deepwater trawl survey programme has been operated since 1993 by the Fisheries Research Centre (FRC) in the deep waters of the Rockall Trough and Porcupine Bank. The present survey took place over a period of 10 days in October and November 1997 on the eastern and southern slopes of the Rockall Trough from 54°N to 59°N. Fishing was carried out in five separate areas, in four depth strata: 500-700m, 700-900m, 900-1100m and 1100-1300m. The primary objective of the survey was to obtain samples of chondrichthyan and teleost fish for the FRC deepwater research programme, for contaminant analysis offish by the FRC chemistry section and for food technology analysis at the Teagasc National Food Centre. The survey was carried out on a commercial trawler using commercial deepwater demersal otter trawl gear. In total 15 species of chondrichthyan, 41 species of teleost fish and 5 species of cephalopods were taken. Among the most abundant species in the catch were roundnose grenadier, Portuguese dogfish, leafscale gulper shark, and Baird's smoothhead. Over the entire survey discarding was estimated as 50.5% of the total catch. Discard rates expressed as kg discarded per tonne roundnose grenadier landed and as a percentage of the total catch when compared with those of previous years showed no appreciable change. The main species discarded were rabbitfish, birdbeak dogfish, Baird's smoothhead, roundnose grenadier and Lepidion eques. Catch per unit effort rates expressed as kg caught per hour fished compared with rates for previous trawl surveys showed marked declines. Length frequency distributions for the main chondrichthyan species showed absence of smaller individuals from the samples and sexual dimorphism with respect to length. Attachment of fine-mesh cod-end liner suggested that the commercial gear selects all length frequencies present and that mesh size may not be an effective management measure in this fishery. This leaflet documents the survey and presents some preliminary results. The data from this survey are currently under analysis at the FRC and results will be published in the scientific literature.