Department of Conservation | Te Papa Atawhai
https://www.doc.govt.nz/
https://www.doc.govt.nz/
Introduction
Find out about the seabird-safe practice of line weighting.Effect on fish catch
Overall there is no evidence that the presence of weights near the hook affects target catch.
Costs
- Lead swivels cost about US$0.25.
- The hook with a swivel attached to the eye is US$1.30 – crimps are not needed with this hook type, so this is a cost saving.
- Sliding weights cost between US$0.60 to 0.80 each – when the sliding weight is made of a luminous material, light sticks are not needed so this is a cost saving.
Effectiveness
Line weighting can reduce seabird captures by 40% to 80%. Diving seabirds can still get caught though. The science measuring effectiveness of line weighting is good.
How line weighting works
Line weighting means adding a weight near the hook to make it sink quickly beneath the dive depths of albatrosses and petrels.
Minimum standards to be effective
The closer the weight is to the hook, the lighter the weight can be to get the same sinking rate.
The four weighting options in the image have the same sink rates.
Weights should be metal. The use of fishing lights or other fishing gear is not recommended unless proven to sink as quickly as metal weights.
Weighted swivels
Image: Nigel Brothers | ©
Weighted swivels are commonly used. These are crimped into the branch line.
Image: Nigel Brothers | ©
Some fishers use a hook with the weight attached directly to the eye. Shown on the left. And there is a new heavy hook being developed. Shown on the right. Another option is a sliding weight that is threaded onto the branch line.
Sliding weights stored on board. Image: Becky Ingham, Fishtec | ©
Safer for crew if a shark bites off the hook
The sliding weight is designed to be safer for crew if a shark bites the hook off and the weight flies back towards the boat.
The sliding weight falls off the branch line if a shark bites the hook off.
Case study
In 2014 and 2015, seven Republic of Korea tuna longline vessels operating off South Africa and Western Australia took part in a trial of sliding leads. The fishing masters were satisfied that the sliding weights made it safer for crew and didn’t affect fishing.
The vessels have continued to use sliding weights since the trial. This project involved Sajo Industries, Dongwon Fisheries and Dongwon Industries, the Korean National Institute of Fishery Science, FitzPatrick Institute of African Ornithology and BirdLife South Africa.