Semi-pelagic trawling in the U.S. West Coast groundfish bottom trawl fishery: Effects on catch efficiency and seafloor interactions

Abstract

Reducing the impacts of bottom trawling on seafloor habitats is a management priority in the U.S. West Coast groundfish bottom trawl fishery as well as other trawl fisheries internationally. Modifications to conventional bottom trawls, such as semi-pelagic trawl technology, are commonly used in demersal fisheries to reduce trawl-seafloor interactions by elevating the doors and portions of the sweeps off the seafloor. This study evaluated changes in catch efficiency and trawl geometry between a conventional bottom trawl outfitted with bottom-tending doors and the same trawl modified with midwater doors to fish semi-pelagically. We observed the seafloor interactions using Dual-frequency IDentification SONar (DIDSON) and quantified the reduction in trawl-seafloor interactions by periodically placing an altimeter on the semi-pelagic trawl door to measure height off bottom. Across the tows where the altimeter was used, results showed that the midwater doors fished off bottom >96% of all tow durations at a minimum height of 0.6 m. The midwater doors also spread 43 m wider on average than the conventional doors, which was significant (p < 0.001). Catch comparison results showed no significant difference in catch efficiency between the two gear types for any target groundfish species, however, the mean catch per unit effort for sablefish (Anoplopoma fimbria) did substantially increase when switching from the conventional to semi-pelagic trawl. Mean door spread did not significantly affect the catch efficiency of any species. DIDSON and altimeter data showed the midwater doors and raised sweeps provide clearance for low profile and infaunal benthic organisms to pass beneath without contact. This study demonstrates semi-pelagic trawl gear can effectively harvest demersal groundfishes in this fishery while substantially reducing trawl interactions with the seafloor. While our study has direct management implications for the U.S. West Coast groundfish bottom trawl fishery, our findings are also likely to apply to other demersal trawl fisheries internationally where reducing the impacts of bottom trawling on seafloor habitats is a management priority.