Line weighting & bait sink rate

line weighting
© ACAP & Birdlife International

This description is drawn from the Line Weighting Factsheet (May 2019) published by Birdlife International and ACAP for Pelagic Longline Fisheries.

Seabirds
In brief, longlines are weighted to get baited hooks rapidly out of the range of feeding seabirds. Line weighting is a primary mitigation measure and a key component in all successful reductions in seabird bycatch in pelagic longline fisheries. Line weighting should be used in conjunction with streamer lines and night-setting.
 
What is line weighting?
Seabirds are vulnerable to being caught during the short period between when the hooks leave the vessel and when they sink below their diving ranges. Line weighting helps sink hooks beyond the dive depths of surface- and shallow-foraging seabirds and thus helps reduce the likelihood of birds accessing baited hooks. Because most seabird dives occur in the upper reaches of the water column (down to 10 m), effective line weighting should sink hooks rapidly beyond this depth.
 
The sink rate of a hook primarily depends on:
  • The mass of the weight attached to it
  • The distance between the weight and the hook
Heavier weights closer to the hook are the most effective at sinking baited hooks quickly and therefore reducing seabird bycatch; lighter weights further from the hook can result in the hook remaining close to the surface for a period before sinking beyond the danger zone for seabirds. To counteract this effect, weights placed further from the hook need to be heavier. ACAP recommends that the following minimum line-weighting standards represent best practice:
 
Line weighting

 

A number of research projects have shown that adding weights to branch lines does not affect the catch rates of the fish that are being targeted and reduces the loss of bait to birds.

 

Problems and troubleshooting

Crew safety: ‘fly-backs’ (weights flying back towards the vessel after bite-offs or line breaks) are a concern when line weighting is used. Sliding leads that slide down the branch line during bite-offs greatly reduce the incidence of fly-backs. In the USA, fishers address fly-backs by altering the angle at which lines are retrieved so that crew members are not directly in the path of the weight should the line break. Personal safety equipment, such as helmets and face screens, and ensuring safe hauling practices, can help to minimise risks.

Propeller wash: to ensure that hooks sink quickly, they should be cast beyond the propeller wash, but still under the protection of bird-scaring lines.

Combination with other measures

Line weighting is considered to be one of the most important mitigation measures, but to maximise its effectiveness, it should be combined with bird-scaring lines and night setting. When used in combination, bird-scaring lines protect the area behind the vessel in which the baited hooks are still accessible to seabirds (up to 10-m depth), while the line weighting shrinks the extent of the area that the bird-scaring lines need to protect.

Effect on Other Bycatch Species

None.

Compliance
Vessels <35 m total length: Line weights crimped into branch lines are technically very difficult to remove at sea. Inspection before departure from port of all gear bins on vessels is considered an acceptable form of implementation monitoring.
 
Vessels >35 m total length: Technically possible to remove and/or re-configure gear at sea. Implementation monitoring by monitoring line sets using appropriate methods (e.g. observer inspection of line setting operations; electronic (e.g. video) surveillance; at-sea compliance checks).
Further Research
Continued refinement of line weighting configurations (mass, number and position of weights and materials) with regard to effectively reducing seabird bycatch and safety concerns through controlled research and application in fisheries is recommended.
References
  1. ACAP. 2014. Report of Seabird Bycatch Working Group. Agreement on the Conservation of Albatrosses and Petrels, Eighth Meeting of the Advisory Committee. Punta del Este, Uruguay, 15-19 September 2014, AC8 Doc 12.
  2. Birdlife International, ACAP (2019) Pelagic Line-weighting Factsheet. English, Bahasa Indonesian, Japanese, Portuguese, Simplified & Traditional Chinese.
  3. Gianuca, D., Peppes, F.V., Cesar, J.H., Sant’Ana, R., and Neves, T. 2013. Do leaded swivels close to hooks affect the catch rate of target species in pelagic longline? A preliminary study of southern Brazilian fleet. Agreement on the Conservation of Albatrosses and Petrels, Fifth Meeting of the Seabird Bycatch Working Group. La Rochelle, France, 1-3 May 2013, SBWG5 Doc 33.
  4. Jimenez, S., Domingo, A., Abreu, M., Forselledo, R. and Pons, M. 2013. Effect of reduced distance between the hook and weight in pelagic longline branchlines on seabird attack and bycatch rates and on the catch of target species. Agreement on the Conservation of Albatrosses and Petrels, Fifth Meeting of the Seabird Bycatch Working Group. La Rochelle, France, 1-3 May 2013, SBWG5 Doc 49.
  5. Melvin, E., Guy, T. and Sato, N. (2011) Preliminary report of 2010 weighted branch line trials in the Tuna Joint Venture Fishery in the South African EEZ. 4th Meeting of the Seabird Bycatch Working Group. Agreement on the Conservation of Albatrosses and Petrels, SBWG-4 Doc 07.
  6. Robertson, R., Candy, S., Wienecke, B. and Lawton, K. (2010) Experimental determinations of factors affecting the sink rates of baited hooks to minimise seabird mortality in pelagic longline fisheries. Aquatic Conservation: Marine and Freshwater Ecosystems. 20: 419-427.
  7. Robertson, G., Candy, S.G. and Hall, S. 2013. New branch line weighting regimes to reduce the risk of seabird mortality in pelagic longline fisheries without affecting fish catch. Aquatic Conservation: Marine and Freshwater Ecosystems. doi: 10.1002/aqc.2346
  8. Sullivan, B.J., Kibel, P., Robertson, G., Kibel, B., Goren, M., Candy, S.G. and Wienecke, B. 2012. Safe Leads for safe heads: safer line weights for pelagic longline fisheries. Fisheries Research (134-136): 125-132.