Experimental determinations of factors affecting the sink rates of baited hooks to minimize seabird mortality in pelagic longline fisheries

1. An experiment was conducted in Australia's pelagic longline fishery to establish a scientific basis for the introduction of line weighting to reduce seabird mortality. The experiment examined the effects of different bait species (blue mackerel, yellow-tail mackerel and squid), bait life status (dead or alive), weight of leaded swivels (60 g, 100 g and 160 g) and leader length (distance between leaded swivel and hooks: 2m, 3m and 4m) on the sink rates of baited hooks from 0-6m deep. 2. On average, live bait sank much more slowly than dead bait. The sink rates of individual live bait were highly variable: many were <2m underwater 18 s after deployment, including some on the heaviest swivels, and some were <10m deep after 120 s. 3. Within the dead bait group, all three swivel weights on 3m and 4m leaders sank at similar rates. Initial sink rates (e. g. 0-2 m) were 2-3 times slower than final rates (e. g. 4-6 m) for all combinations of swivel weight and leader length. The fastest initial and final sink rates were associated with heavy swivels placed close to hooks. 4. The results show that (a) compared with dead bait, live bait greatly increases the exposure of baited hooks to seabirds; (b) initial sink rates of dead bait are increased by placing leaded swivels close to hooks and final rates by increasing the weight of the swivels; (c) adding weight to long leaders makes little difference to sink rates; and (d) the small (incremental) changes to swivel weights and leader lengths typically preferred by industry will be difficult to detect at sea and unlikely to substantially reduce seabird mortality. 5. We suggest that experiments designed to reduce seabird mortality from that associated with 60 g swivels and similar to 3.5m leaders (the preferred option by industry) should aim to expedite the initial sink rates as well as rates to deeper depths. This objective could be achieved by including branch lines with >= 120 g swivels <= 2m in comparative assessments of the effectiveness of line weighting regimes in reducing seabird mortality. Copyright (C) 2010 John Wiley & Sons, Ltd.