Commercial deep water trawling at sub-zero temperatures – observations from the Faroe-Shetland channel

Abstract

Commercial deep water trawling occurs north and west of Scotland on the continental slopes of the Faroe-Shetland channel (FSC) and the Rockall trough. From existing hydrographic knowledge, reported depths being fished in the FSC suggested that trawling was taking place in waters of zero or sub-zero temperatures. Fishing in deep water at these temperatures may be unique in terms of global commercial fisheries. Instrumentation of a Scottish deep water commercial trawler's net with a temperature and depth recorder showed that indeed some fishing time (<8%) was spent in sub-zero water. However, best catches of both Greenland halibut ( Reinhardtius hippoglossoides) and Roughhead grenadier ( Macrourus berglax) and most fishing time were associated with boundary waters in the range 0–4°C. Variable conditions on a seasonal or shorter time scale were demonstrated as were changes in ambient water temperature of almost 6°C during individual hauls. Sub-zero temperatures were recorded as shallow as 600 m whilst at the same depth modal haul temperature showed a range >6°C. The variable and much colder conditions of the deep water in the FSC were contrasted with the stable and warmer conditions of the Rockall trough. Further research is required if management of this growing fishery is to be scientifically based.