Interactions between seabirds and deep‐water hake trawl gear: an assessment of impacts in South African waters

Abstract

AbstractMany seabirds are killed or injured by fishing gear, contributing to the high proportion of threatened seabirds. This study estimates the impact of the South African deep‐water hake Merluccius spp. trawl fishery on seabirds. At least 30 birds were killed in 190 h of dedicated observations of trawl warps during 2004 and 2005. Most were killed when their wings were entangled around the trawl warp and they were dragged under by the force of the water passing over the warp. Albatrosses were killed most frequently: shy albatrosses Thalassarche cauta (43% of all birds killed) and black‐browed albatrosses Thalassarche melanophrys (37%), with smaller numbers of white‐chinned petrels Procellaria aequinoctialis (10%), Cape gannets Morus capensis (7%) and sooty shearwaters Puffinus griseus (3%). Mortalities occurred mainly during dumping of fishery wastes, and were more frequent in winter, when more birds attended fishing vessels. Average mortality rates were 0.56 (95% confidence interval 0.32–0.82) birds killed per hour during dumping in winter, 0.21 (0.07–0.38) during dumping in summer and 0.09 (0.02–0.19) when not dumping in winter. No birds were killed in the absence of dumping in summer. Albatrosses suffered a disproportionately high mortality rate, with 15% of birds dragged under drowning, compared with 4% of all other species. Deaths resulting from entanglement in fishing nets mainly affected Cape gannets M. capensis, and occurred at an average rate of 3.0 (0.9–5.4) birds per 100 trawls (n=331 trawls). Serious warp incidents were independent of age among albatrosses, but there was a tendency for immature gannets to have a higher interaction rate than adults. Crude extrapolation suggests that total mortality is c. 18 000 (8000–31 000) birds per year, of which 85% are killed on warps and 15% entangled in nets. These estimates are of the same order of magnitude as estimates of long‐line bycatch in South African waters. Mitigation measures have been implemented to reduce mortality in this fishery.