This description relies upon: Birdlife International. 2014. Bycatch Mitigation Fact-Sheet 8 (September 2014), Pelagic Longline: Line weighting.
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 mortality during the short period between when hooks leave the vessel and when they sink beyond their diving ranges. Preventing contact between seabirds and baited hooks at this time is crucial. In many pelagic longline fisheries, weights are added to branch lines (snoods) to deliver hooks to target fishing depths as efficiently as possible.
The best practice weighting regimes recommended below are intended to take baited hooks beyond the diving range of seabirds while under the protection of a well designed and properly deployed bird scaring line (streamer or tori line), without compromising fish catch rates.
Reducing seabird mortality in pelagic longline fisheries with line weighting regimes is more complicated than in demersal longline fisheries because of 'secondary' interactions with baited hooks. Secondary interactions occur when seabirds with good diving capabilities, such as shearwaters and petrels, bring sinking bait back to the surface where they can be ingested by larger and more dominant species, such as great albatrosses. Secondary interactions rarely, if ever, occur in demersal longline fisheries because branch lines are extremely short (< 0.6 m) and the mainline is heavy. In contrast, pelagic branch lines can be 15-40 m in length and light-weight. Secondary interactions are implicated in a significant proportion of seabird bycatch in pelagic longline fisheries.
Important aspects of line weighting
Two aspects of pelagic branch line construction are critically important to achieving fast sink rates - the length of the leader (distance between lead weight and baited hook) and the weight of the added weight. Leader length is the main determinant of the initial phase of the sink profile, while the weight of the attached lead is the main determinant of the final phase of the sink profile.
The initial phase starts when the baited hook lands in the water and ends when the leader becomes taut. In this phase the lead weight sinks much faster than the baited hook. The final phase of the sink rate occurs when the slack in the leader is taken up and the baited hook comes under maximum load (pull-down) of the lead weight. The initial sink rate occurs in the first few metres of the water column (depends on leader length) and is increased by adding weight close to the hook (more quickly reduces the slack in the leader). The final sink rate occurs at deeper depths and is increased by increasing the attached weight.
To reduce the availability of baits in all depths of the water column it is important to increase both the initial and final phases of sink profiles. This is achieved by using heavier weights closer to hooks (Robertson et al., 2010; Robertson et al., 2013). Recent experiments indicate that a 60 g lead weight, placed either at the hook or within 1 m of the hook, and a 120 g lead weight < 2 m from the hook, is likely to achieve these sink rates under most operational conditions (Robertson et al., 2013). Other experiments conducted on Japanese vessels, trialled branch lines weighted with 65-70 g lead weight within 3-3.5 m from the hook, using a double-weight configuration; two leads placed at either end of a 1-1.5 m section of wire trace inserted into the branch line 2 m from the hook. In combination with dual streamer lines, this system reduced bycatch by 86% compared with un-weighted lines, with mean target catch rates remaining equal (Melvin et al., 2011).
Sink rate experiments
Sink rate experiments have been undertaken in southern hemisphere countries (Gianuca et al., 2013; Jimenez et al., 2013; Robertson et al., 2013). These studies have demonstrated that branch line weighting configurations with more mass close to the hook sinks the hooks most rapidly, reduces seabird attacks on baits and consequently is most likely to reduce mortalities. Further, these studies, which looked at a range of weighting regimes, including regimes with weight at the hook, have shown no negative effect on target catch rates. The following prescriptions should thus be regarded as a minimum for line weighting regimes:
Swivel weights and leader lengths: Lead weights typically vary between 40 and 80 g. Leader lengths also vary, typically between 3-4 m. High seas fisheries either use minimal, if any, weight on branch lines to improve sink rates. In fisheries with high seabird interaction rates, line weighting options include: > 45 g lead weight at less than or equal to 1 m from the hook (to minimise gear loss from shark bite-offs); > 60 g lead weight at less than or equal to 3.5 m from the hook; or > 98 g lead weight less than or equal to 4 m from the hook. These line weighting regimes result in greatly improved sink rates in both phases of the sink profile without affecting the catch rates of target and non-target fish.
Propeller turbulence: The fastest sink rates are achieved by avoiding setting gear in the area behind vessels most affected by propeller turbulence. Bird scaring lines should be deployed in line with the edge of the vessel wake or outside of this area. Baited hooks should be deployed so as to land beneath the bird scaring lines, either into the vessel wake zone or outside of this area (depends on the position of the streamer line).
Bait thaw status: In fisheries where lead weights are added to branch lines, as long as bait (fish, squid) are thawed to an extent that permits hooks to be inserted without undue force, bait thaw status has no effect on sink rates. In fisheries where leaded swivels are not added to branch lines, the use of unthawed bait slows sink rates. However, the difference is minor and less important than other factors that affect gear sink rates.
Best practice recommendation
Line weighting is a primary measure for reducing seabird bycatch and there is increasing understanding about its effectiveness in combination with other measures. If used with an effective bird scaring line and night-setting, weighting regimes that sink hooks at greater than or equal to 0.3 m/s to 2 m depth and greater than or equal to 0.5 m/s to 5 m depth, should take hooks beyond the reach of most surface-seizing birds. Extensive work is currently underway to determine the most effective line weighting regime, however current minimum standards for branch line weighting configurations are:
> 45 g lead weight at less than or equal to 1 m from the hook (to limit gear loss from shark bite-offs); or
> 60 g lead weight at less than or equal to 3.5 m from the hook; or
> 98 g lead weight at less than or equal to 4 m from the hook.
Leader length: Positioning the weight further than 4 m from the hook is not recommended, as this increases the length of time for the baited hook to sink below the diving depth of seabirds.
Crew safety: To improve crew safety issues associated with the use of a point source of weight (e.g. leaded swivels), use of Sliding Leads is strongly encouraged. These leads slide down the branch line during bite-offs or when the line breaks under tension, thereby greatly reducing the incidence of dangerous fly-backs towards the vessel (Sullivan et al., 2012).
In the USA, fishermen commonly address fly-backs by altering the angle at which the weighted line is retrieved so that crew are not directly in the path of the weight should the hook line break under high tension. Options include welding an open metal loop to the top rail next to the vessel’s fish gate, or welding a short metal post perpendicular to the top rail. An even simpler option is to use the smooth post of the fish gate itself, though a more detailed specification is yet to be published (ACAP, 2014).
Vessel effects: Large industrial and small artisanal vessels may require different weighting regimes to attain the same reduction in seabird bycatch.
Operational effects: In order to achieve the fastest practicable sink rates, hooks must be cast beyond the propeller wash, and yet remain under the protection of the bird scaring line/s.
Target species catch rates
Recent research reveals that adding weight to pelagic branch lines does not affect the catch rates of target and non-target fish (Gianuca et al., 2013; Jimenez et al., 2013; Melvin et al., 2011, Robertson et al., 2013).
Combinations of measures
Line weighting is arguably the most important mitigation measure, but to ensure effectiveness it is recommended that it be used in combination with other measures, including:
- Streamer lines (Fact-sheet 7a and 7b)
- Night-setting (Fact-sheet 5).
Effect on Other Bycatch Species
Ease of Deployment and Safety
Fishermen are concerned about the safety implications of using weighted lines. When the line is stretched during hauling and suddenly breaks (a 'bite-off', usually due to shark bycatch), the lead weights attached to branch lines can be launched back towards fishermen on deck, and in a few cases serious injury and even death have resulted. In some fisheries, protective helmets are worn to reduce the risk of injury.
To improve crew safety issues associated with the use of a point source of weight (e.g. leaded swivels), use of Sliding Leads is strongly encouraged. These leads slide down the branch line during bite-offs or when the line breaks under tension, thereby greatly reducing the incidence of dangerous fly-backs towards the vessel (Sullivan et al., 2012).
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).
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.
- 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.
- Birdlife International. 2014. Bycatch Mitigation Fact-Sheet 8 (November 2014), Pelagic Longline: Line weighting.
- 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.
- 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.
- 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.
- 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.
- 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
- 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.