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    Interactions between seabirds and deep water hake trawl gear:
    an assessment of impacts in South African waters 2004/05

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    Numéro du document:
    B.P. Watkins, S.L. Petersen and P.G. Ryan (South Africa)
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    This study estimates the mortality of seabirds in the South African deep water hake trawl fishery. Observations of interactions between seabirds and trawl gear were made on 331 trawls during 20 trips on 14 vessels between mid-2004 and end 2005. Long-winged albatrosses were killed most frequently, with Shy Albatrosses comprising 43% and Black-browed Albatrosses 37% of birds killed. Small numbers of White-chinned Petrels (10%), Cape Gannets (7%) and Sooty Shearwaters (3%) also were killed. Mortalities were greater in winter, when more birds attended fishing vessels, and most occurred during dumping of fishery wastes. Using a bootstrapping approach to estimate 95% confidence intervals (CI), the average mortality rates were 0.56 (0.32-0.82) birds killed per hour during dumping in winter, 0.21 (0.07-0.38) during dumping in summer, 0.09 (0.02- 0.19) when not dumping in winter, and 0.00 (–) when not dumping in summer. 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. Deaths resulting from entanglement in fishing nets occurred at an average rate of 3.02 (0.91-5.44) birds per 100 trawls. Estimating the total impact of the fishery requires assumptions about total fishing effort, the proportion of trawls where dumping takes place and the average duration of dumping. A simple model was developed which allowed these parameters to be varied, but the most sensitive parameters were bycatch rates. Based on conservative estimates of effort and dumping time, the total extrapolated mortality is some 18 000 (95% CI 8 000-31 000) birds killed per year, of which 85% are killed on warps and 15% entangled in nets. Of the birds killed, some 39% are Shy Albatrosses, 29% Black-browed Albatrosses, 14% Cape Gannets and 9% White-chinned Petrels, all listed as globally threatened or near-threatened. Active mitigation measures are required to reduce trawl mortality in this fishery.