Note: see Swimmer et al 2020 for a meta-analysis of effects on bycatch species for of switching from wire leaders to monofilament.
Monofilament (nylon) line is used widely in the fishing industry. It is commonly used for both the mainline (the longline) and branchlines (which hang off the main longline and are also known as snoods or gangions/ganglions). Branchlines may incorporate a section of line (of variable length) known as a leader, with a lead weight at one end and the baited hook at the other. Wire leaders have implications for sharks and seabirds, as discussed below.
Among the fishing benefits of using monofilament is its low visibility in water. This characteristic improves catch rates of target species but makes discarded monofilament a danger to marine life - including sea turtles, seabirds, marine mammals and sharks - which can ingest or become entangled in these lines (Refer to ALDFG - management of abandoned, lost, discarded fishing gear).
While monofilament is the predominant type of nylon line employed in longline fisheries, multifilament (generally braided polypropylene and polyethylene lines) is still used in some fleets. This has implications for sea turtles, as outlined below.
Bycatch of marine turtles in longline fisheries occurs through hooking or entanglement in longline gear. In artisanal longline fisheries in the eastern Pacific, turtles are most often caught in float lines and branchlines, but can get caught in any part of the line . If gear is lost and sinks, then the turtle will also sink and can drown if not retrieved and released in time. Monofilament line significantly reduces the risk of entanglement, compared with multifilament line [3,10], because monofilament is less flexible. Monofilament, which is relatively thick and strong but light in weight, does not tend to knot or tangle and, if tension is removed, will probably lose any tangles . This is supported by anecdotal evidence from Australian longline fishers, who noted the high possibility of a turtle being easily untangled once cut free, due to the nature of longline gear . Monofilament used in recreational fishing has different properties and will tend to remain tangled on the sea turtle .
Whether there is any difference in hooking of turtles between monofilament and multifilament branchlines has not been clearly established. One study found no difference . Another recommended that further research into branchline characteristics (e.g. material, length, thickness) should be carried out in order to reduce turtle mortality .
Refer also to Soak duration, which affects sea turtle bycatch rates via entanglement and hooking.
Both wire and monofilament leaders are used in longline fishing gear. It is more difficult for sharks to 'bite free' from wire leaders than from monofilament leaders, generally resulting in higher shark catch rates [8,13]. Where there is no regulatory framework and where the market is profitable for shark products, some fishermen will use wire leaders to maximize shark catch [7 in 6].
However, substituting monofilament for wire leaders will not necessarily result in lower shark catch rates. Hook - leader interactions can influence the ability of a shark to sever leaders. For example, if the leader is made of monofilament, a shark may be better able to bite free if it is gut hooked on a J hook (as opposed to, e.g., jaw hooked on a circle hook) . This scenario was noted in the Brazilian longline fishery where 97 percent of the 'bite offs' occurred on nylon leaders, and significant differences in shark catch rates between wire and nylon leaders were observed only for J hooks [1 in 5].
Where wire leaders do result in higher shark catch rates, sharks may be released at haulback by the crew (i.e. in order to recover the hooks and leaders) but have a higher probability of injury or mortality than would otherwise occur with bite-offs or if a nylon leader is cut at the rail .
According to a recent comprehensive review of mitigation measures in longline fisheries, it is important to assess whether banning wire leaders in conjunction with switching from J hooks to circle hooks would result in higher or lower shark hooking and at-haulback mortality rates . The review also noted that additional mortalities may occur after sharks have bitten through the leader, but retain the hook(s) in their jaw, oesophagus or gut. These effects remain largely unstudied and cannot be quantified [6,7,13 in 5] .
With regards to impacts on other bycatch species, replacing wire leaders with monofilament may exacerbate seabird bycatch problems. Fishers may be less likely to attach weights (which increase hook sink rate and get baited hooks rapidly out of the range of feeding seabirds) close to hooks on monofilament branchlines due to safety concerns (i.e., the risk of ‘flyback’ of the weight when a line is severed under tension) . However, this concern is being addressed with Sliding Leads.
- Afonso, A.S., Santiago, R., Hazin, H. and Hazin, F.H.V. 2012. Shark bycatch and mortality and hook bite-offs in pelagic longlines: Interactions between hook types and leader materials. Fisheries Research 131-133: 9-14. doi.org/10.1016/j.fishres.2012.07.001.
- Alessandro, L. and Antonello, S. 2010. An overview of loggerhead sea turtle (Caretta caretta) bycatch and technical mitigation measures in the Mediterranean Sea. Reviews in Fish Biology and Fisheries 20(2): 141-161. DOI 10.1007/s11160-009-9126-1
- Boggs, C. H., and Swimmer, Y. 2007. Developments (2006-2007) in scientific research on the use of modified fishing gear to reduce longline bycatch of sea turtles. WCPFC-SC3-EB-WP-7.
- Clarke, S. 2011. A status snapshot of key shark species in the western and central pacific and potential management options. WCPFC SC7 EB-WP-04.
- Clarke, S., Sato, M., Small, C., Sullivan, B., Inoue, Y. and Ochi, D. 2014. Bycatch in longline fisheries for tuna and tuna-like species: a global review of status and mitigation measures. FAO Fisheries and Aquaculture Technical Paper No. 588. Rome, FAO. 199 pp. http://www.fao.org/3/a-i4017e.pdf
- Cosandey-Godin, A. and Morgan, A. 2011. Fisheries Bycatch of Sharks: Options for Mitigation. Ocean Science Series, The Pew Environment Group.
- Gilman, E., Clarke, S., Brothers, N., Alfaro-Shigueto, J., Mandelman, J., Mangel, J., Petersen, S., Piovano, S., Thomson, N., Dalzell, P., Donoso, M., Goren, M. and Werner, T. 2007. Shark depredation and unwanted bycatch in pelagic longline fisheries: Industry practices and attitudes, and shark avoidance strategies. Western Pacific Regional Fishery Management Council, Honolulu, USA.
- Gilman, E., Clarke, S., Brothers, N., Alfaro-Shigueto, J., Mandelman, J., Mangel, J., Petersen, S., Piovano, S., Thomson, N., Dalzell, P., Donoso, M., Goren, M. and Werner, T. 2008. Shark interactions in pelagic longline fisheries. Marine Policy 32(1): 1-18.
- Hoey, J. J. 2001. Comments on SEC Initial Draft Criteria, In Stock assessments of Loggerhead and Leatherback Sea turtles and an Assessment of the Impact of the Pelagic Longline Fishery on the Loggerhead and Leatherback Sea turtles of the Western North Atlantic. National Marine Fisheries Service Southeast Fisheries Science Centre. NOAA Technical Memo. NMFS-SEFSC-455.
- Mug, M., Hall, M. and Vogel, N. 2008. Bycatch Initiative: Eastern Pacific Programme - A vehicle towards sustainable fisheries. Progress report of fishing experiments with modified gear (2004-2007). WWF and IATTC.
- Robins, C. M., Bradshaw, E. J. and Kreutz, D. C. 2007. Marine Turtle Mitigation in Australia's Pelagic Longline Fisheries. Fisheries Research and Development Corporation Final Report 2003/013, Canberra, Australia.
- Stone, H. H. and Dixon L. K. 2001. A Comparison of Catches of Swordfish, Xiphias gladius, and Other Pelagic Species from Canadian Longline Gear Configured with Alternating Monofilament and Multifilament Nylon Gangions. Fisheries Bulletin 99: 210-216.
- Ward, P., Lawrence, E., Darbyshire, R. and Hindmarsh, S. 2008. Large-scale experiment shows that nylon leaders reduce shark bycatch and benefit pelagic longline fishers. In IOTC-2011-WPDCS08-INFO7.