Megafauna bycatch in drift nets for albacore tuna ( Thunnus alalunga) in the NE Atlantic

Abstract

In this study, results from an observer programme on a drift net fishery in the eastern North Atlantic are presented. Observers recorded all fish, marine mammals, seabirds and turtles caught and discarded and coverage ranged from 47.8% of the fleet ( n = 7 vessels) in 1996, to 2.2% ( n = 21 vessels) in 1998. A minimum of seven fish species were caught and landed during the fishing period; albacore tuna ( Thunnus alalunga) comprised approximately 99% of the total landings (by number) in both years. Eleven fish species were discarded, of which blue shark ( Prionace glauca) was the most frequently recorded representing 68% of all fish discarded by number. At least four species of seabird (Northern Gannet Morus basanus, Northern Fulmar Fulmarus glacialis, Manx shearwater Puffinus puffinus, Atlantic Puffin Fratercula arctica) and two species of turtle, including the leatherback turtle ( Dermochelys coriacea), were also entangled. Eight species of Cetacea were recorded as bycatch during these fishing operations, including common dolphins Delphinus delphis and striped dolphins Stenella coeruleoalba. Length–frequency distributions for both these dolphin species suggest that age segregation occurs. Using landings of albacore tuna as an indicator of effort, the extrapolated decadal scale data from Irish and other driftnet fleets operating in this area suggest that during the period 1990–2000, a minimum (95% confidence intervals) of 778,452 (622,520–934,384) blue sharks were caught, with a substantial proportion discarded. An estimated 24,358 dolphins were killed during these years by these fleets, of which 11,723 (7670–15,776) were common dolphins and 12,635 (10,009–15,261) were striped dolphins. Although this type of fishing was effective at catching the target species, it removed a large biomass of megafauna and likely accelerated the decline of blue sharks in this area.