Effect of line shooter and mainline tension on the sink rates of pelagic longlines and implications for seabird interactions

Robertson G, Candy SG, Wienecke B (2010) Effect of line shooter and mainline tension on the sink rates of pelagic longlines and implications for seabird interactions. WCPFC, Nuku’alofa, Tonga

1. The likelihood seabirds will be hooked and drowned in longline fisheries increases when baited hooks sink slowly. Fishermen target different fishing depths by setting mainline through a line shooter which controls the tension (or slackness) in the line. An experiment was conducted in Australia's pelagic longline fishery to test the hypothesis of no difference in sink rates of baited hooks attached to mainline set under varying degrees of tension.
2. Mainline was set in three configurations typically used in the fishery: a) surface set tight with no slackness astern; b) surface set loose with two seconds of slack astern and; c) deep set loose with seven seconds of slack astern. Sink rates of baited hooks were measured using time depth recorders.
3. Tension on the mainline had a powerful effect on sink rates. Baited hooks on branch lines attached to tight mainlines reached 2 m depth nearly twice as fast as those on the two loose mainline tensions, averaging 5.8 s (0.35 m/s) compared to 9.9 s (0.20 m/s) and 11.0 s (0.18 m/s) for surface set loose and deep set loose tensions, respectively.
4. The likely reason for the difference is propeller turbulence. Tight mainline entered the water aft of the area affected by turbulence whereas the two loose mainlines and the clip ends of branch lines were set directly into it about 1 m astern of the vessel. The turbulence presumably slowed the sink rates of baited hooks at the other end of the branch lines.
5. The results suggest that mainline deployed with a line shooter (as in deep setting) into propeller turbulence at the vessel stern slows the sink rates of baited hooks, potentially increasing their availability to seabirds. Unless mainline can be set to avoid propeller turbulence the use of line shooters for deep setting should not be accepted as an effective deterrent to seabirds.
6. It is recommended that the WCPFC revise CMM 2007-04 by deleting deep setting line shooter from the list of accepted seabird bycatch mitigation measures in Table 1 of that measure.
Note: also found among the meeting documents of the Third Meeting of Seabird Bycatch Working Group (ACAP). SBWG-3 Doc 08