Effects of Experimental Otter Trawling on the Feeding of Demersal Fish on Western Bank, Nova Scotia
Ellen L. Kenchington, Donald C. Gordon, Jr., Cynthia Bourbonnais, Kevin G. MacIsaac, Kent D. Gilkinison, David L. McKeown, and W. Peter Vass
Abstract. The impact of otter trawling on demersal fish feeding was studied over 3 years (1997– 1999) on Western Bank, off Nova Scotia, Canada. This site is within the 4TVW Haddock Closed Area, which has been closed to otter trawling since 1987. The seabed was gravel (mostly pebbles and cobbles), and water depth averaged 70 m. A rich and diverse benthic community of 341 taxa characterized the site. Experimental trawling (at least 12 sets along the same line per year) was conducted using an Engel 145 otter trawl with 1,250-kg otter boards and rockhopper ground gear. Twenty-two species of fish and five invertebrate species were captured. Atlantic cod Gadus morhua, haddock Melanogrammus aeglefinus, and winter flounder Pseudopleuronectes americanus increased significantly in the trawl catches after the first trawl pass, indicating movement into the area. Detailed analysis of the stomach contents of Atlantic cod, haddock, American plaice Hippoglossoides platessoides, yellowtail flounder Limanda ferruginea, and winter flounder showed changes in diet attributed to the trawling. Collectively, these fish preyed upon 177 taxa, with haddock having the most diverse diet and American plaice the most specialized. Two classes of effects attributed to the trawling disturbance are documented: (1) quantitative changes in the abundance of prey species consumed and (2) qualitative changes through opportunistic feeding on novel food items. All five predator species investigated demonstrated statistically significant changes in their diets. Qualitative changes in diet were seen in Atlantic cod, American plaice, and yellowtail flounder. Each of these fish species increased consumption of the horse mussel Modiolus modiolus (also known as northern horsemussel), with American plaice and yellowtail flounder found feeding on the mussels only after trawling commenced. Increases in consumption of the tube-dwelling polychaete Thelepus cincinnatus were observed across all species, with high propensity and individual selectivity within species. Our results demonstrate that repetitive otter trawling produced both quantitative and qualitative changes in the diets of demersal fish, presumably caused by changes in prey availability brought about by trawl disturbance.