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

Abstract

ABSTRACT1. 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: 2 m, 3 m and 4 m) on the sink rates of baited hooks from 0-6 m 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 o2 m underwater 18 s after deployment, including some on the heaviest swivels, and some were o10 m deep after 120 s.3. Within the dead bait group, all three swivel weights on 3 m and 4 m 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 $3.5 m 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 Z120 g swivels r2 m in comparative assessments of the effectiveness of line weighting regimes in reducing seabird mortality.

Influence of electric fishing lights on sink rates of baited hooks in Brazilian pelagic longline fisheries: implications for seabird bycatch

“…Our results indicate that 3.5 m leaders with 75 g leaded swivels are likely to achieve satisfactory sink rates under Brazilian pelagic longline fishing conditions, while the 5.5 m leaders did not, thus supporting international recommendations (ICCAT, 2001;ACAP, 2014) and agreeing with other studies demonstrating that shorter leaders increase sink rates (ROBERTSON et al, 2010;GIANUCA et al, 2011;ROBERTSON et al, 2013).…”

“…squid, mackerel, sardine, skipjack flash) and its state (e.g. dead or alive, frozen or thawed); the mainline tension during deployment; and line weight regimes (the use or not of leaded swivels, as well as their mass and distance from the hooks) (ANDERSON; MCARDLE, 2002;PETERSON et al, 2008, ROBERTSON et al, 2010ROBERTSON et al, 2013).…”

“…Experiments have indicated that ≥ 60 g placed no more than 3 m from the hooks is likely to achieve satisfactory sink rates under most operational conditions (MELVIN et al, 2009b;ROBERTSON et al, 2010;GIANUCA et al, 2011). Accordingly, among the best practices to reduce seabird mortality in pelagic longline fisheries recommended by the Agreement on the Conservation of Albatrosses and Petrels (ACAP, 2014) and the International Commission for the Conservation of Atlantic Tunas (ICCAT, 2011), is to use specific line-weighting regimes that ensure appropriate sink rates.…”

“…Baited hooks of both 5.5 m leaders with and without EFL did not, on average, reach the 10 m depth "low risk" benchmark within 80 m astern, thus continuing to be relatively available for medium size diving petrels beyond the potential coverage of a scaring line (HUIN, 1994;RONCONI et al, 2010;ROLLINSON et al, 2014). However, it should be mentioned that 10 m depth is an arbitrary benchmark necessary for the establishment of sink rate goals in mitigation measurement research (PETERSEN et al, 2008;MELVIN et al, 2009b;ROBERTSON et al, 2010), even when medium-sized petrels can dive deeper than that (HUIN, 1994;RONCONI et al, 2010;ROLLINSON et al, 2014). Said that, the average depth of 9.3 m after 27 sec.…”

Best practices to mitigate seabird bycatch in longline, trawl and gillnet fisheries—efficiency and practical applicability

“…The concept of integrated weight was designed for multifilament demersal longlines, and alternatives to adding external weights for monofilament and pelagic longlines (e.g. weighted swivels, 'safe leads') should be a research priority (see Robertson et al 2010a, Sullivan et al 2010. The line weighting regime, appropriate sink rate and combination of other mitigation measures need to be determined for a specific fishery.…”