Autoliners and seabird by-catch: do line setters increase the sink rate of integrated weight longlines?

Line setters are used with integrated weight (50 g/m lead core, IW) by some autoline vessels in the Kerguelen and Crozet Islands Patagonian toothfish fisheries to deter seabirds, ostensibly by expediting gear sink rates. A trial was conducted in the Ross Sea in 2007 to determine the effectiveness of line setters in increasing sink rates of IW longlines. Time-depth recorders were deployed along magazines of line set with and without a line setter using a paired treatment design. Sink rates of longlines set with and without a line setter were identical, including in the first few metres of the water column where seabird interactions are likely to be most intense. The results reveal that line setters confer no sink rate advantage to IW longlines. Longlines deployed with a line setter enter the water several metres closer to the stern of vessels, thereby increasing slightly (<0.5 m) the depth of longlines with distance behind vessels. It is doubtful that this small increase in depth will result in substantial reductions in interactions between longlines and seabirds in the Kerguelen and Crozet fisheries.