Underwater setting techniques are means of deploying baited hooks below the surface of the sea, out of the sight and reach of foraging seabirds. None are currently recommended as a mitigation method, though they are noted as under development (ACAP 2019a).
Examples of underwater setting techniques include:
- the underwater bait setter, also known as a bait setting capsule or underwater bait launcher; and
- underwater setting tubes or chutes.
While versions of the setting tubes or chutes have been trialled in pelagic longline fisheries, they are currently used only in demersal longline fisheries (see Birdlife International Factsheet 6 Demersal Longline: Underwater setting chute) and are not discussed here. Likewise, underwater setting funnels or lining tubes apply to demersal longline fisheries.
An underwater bait setter for pelagic longline fisheries has recently become commercially available. It is a stern-mounted, hydraulically-operated device. Baited hooks are carried underwater in a steel capsule connected to hydraulic winches by Spectra® rope. Baits are flushed from the capsule with water pressure from a spring-loaded door in the bottom section of the capsule, allowing release of baited hooks beneath the dive depths of albatrosses and petrels.
- Baited hooks released 4 m underwater reduced seabird mortality by 87% compared to baits set at the sea surface.
- Baits set 10 m underwater reduced seabird mortality by 100%.
- Baits set underwater were associated with marked reductions in ship following and attacks.
Background on the company developing the underwater bait setter can be found here.
ACAP (2019b), in their ‘Advice on Improving Safety when Hauling Branchlines during Pelagic Longline Fishing Operations’ found that underwear setting devices warrant further research. This is because these technologies may reduce or eliminate the need for branchline weighting, thus avoiding dangerous 'flybacks' or 'tear outs', as setting occurs by stealth at a depth beyond the depth ordinarily reached by diving seabirds.
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