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Hybrid streamer lines with weighted and unweighted branch lines were tested for their effectiveness in reducing seabird interactions with longline gear during the 2010 tuna joint venture fishery in the South African EEZ. Weighted branch lines with hybrid streamer lines dramatically reduced seabird attacks, secondary attacks and mortalities and had a minimal effect on fish catch. Bird mortalities on weighted branch lines were reduced by 86% compared to unweighted branch lines. Primary attack rates were four times lower on weighted branch lines. In addition, no seabird attacks occurred between two streamer lines, suggesting that streamer lines flown in pairs are highly effective at preventing attacks. The average tuna catch was almost the same on both types of branch lines, but weighted branch lines tangled on themselves three times more often than unweighted branch lines.