Effects of monofilament nylon versus braided multifilament nylon gangions on catch rates of Greenland shark (Somniosus microcephalus) in bottom set longlines

Scott M Grant; Jenna G Munden; Kevin J Hedges

doi:10.7717/peerj.10407

Effects of monofilament nylon versus braided multifilament nylon gangions on catch rates of Greenland shark (Somniosus microcephalus) in bottom set longlines

PeerJ. 2020 Dec 3:8:e10407. doi: 10.7717/peerj.10407. eCollection 2020.

Authors

Scott M Grant¹, Jenna G Munden², Kevin J Hedges³

Affiliations

¹ Centre for Sustainable Aquatic Resources, Memorial University of Newfoundland, St. John's, NL, Canada.
² Herring Science Council, Halifax, NS, Canada.
³ Arctic Aquatic Research Division, Central & Arctic Region, Fisheries and Oceans Canada, Winnipeg, MB, Canada.

Abstract

The Greenland shark (Somniosus microcephalus) is the main bycatch species in established and exploratory inshore longline fisheries for Greenland halibut (Reinhardtius hippoglossoides) on the east coast of Baffin Island, Canada. Bycatch and entanglement in longline gear has at times been substantial and post-release survival is questionable when Greenland sharks are released with trailing fishing gear. This study investigated the effect of the type of fishing line used in the gangion and gangion breaking strength on catch rates of Greenland shark and Greenland halibut in bottom set longlines. Circle (size 14/0, 0° offset) hooks were used throughout the study. Behavior of captured sharks, mode of capture (i.e., jaw hook and/or entanglement), level of entanglement in longline gear, time required to disentangle sharks and biological information (sex, body length and health status) were recorded. Catch rates of Greenland shark were independent of monofilament nylon gangion breaking strength and monofilament gangions captured significantly fewer Greenland sharks than the traditional braided multifilament nylon gangion. Catch rates and body size of Greenland halibut did not differ significantly between gangion treatments. Although most (84%) of the Greenland sharks were hooked by the jaw, a high percentage (76%) were entangled in the mainline. The mean length of mainline entangled around the body and/or caudal peduncle and caudal fin was 28.7 m. Greenland sharks exhibited cannibalistic behavior with 15% of captured sharks cannibalized. All remaining sharks were alive and survived the disentanglement process which can be attributed to their lethargic behavior and lack of resistance when hauled to the surface. Thus, as a conservation measure fishers should be encouraged to remove trailing fishing gear prior to release. Our results are used to demonstrate benefits to the fishing industry with regard to an overall reduction in the period of time to disentangle sharks and damage to fishing gear by switching from braided multifilament to monofilament gangions in Greenland halibut longline fisheries.

Keywords: Bycatch mitigation; Circle hooks; Conservation; Cumberland sound; Entanglement; Greenland halibut; Greenland shark; Longline; Monofilament gangion.

Grants and funding

This study was supported by the National Research Council of Canada, the Nunavut Offshore Allocation Holders Association- Nunavut Exploratory Fishery Fund, the Government of Nunavut (Department of Environment), the Canadian Northern Economic Development Agency, Ducks Unlimited Canada, Oceans North, and Fisheries and Oceans Canada. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.