Draft review of potential mitigation measures to reduce fishing-related mortality on silky and oceanic whitetip sharks (Project 101)

Citation
Bigelow K, Carvalho F (2021) Draft review of potential mitigation measures to reduce fishing-related mortality on silky and oceanic whitetip sharks (Project 101). In: WCPFC Scientific Committee 17th Regular Session. WCPFC-SC17-2021/EB-WP-01, Electronic Meeting
Abstract

The paper develops and applies a model for how silky (Carcharhinus falciformis) and oceanic whitetip (C.longimanus) shark might interact with longline gear in the Western and Central Pacific Ocean (WCPO) and potential reductions in mortality with two different management measures: 1) removal of shark lines and 2) transition from branchlines with wire leaders to monofilament leaders. Using Regional Observer Program (ROP) data, the study compared absolute values of total catch and total mortality across scenarios and the relative change in fishing related mortality from the status-quo option given a conversion from wire to monofilament leaders, no shark lines used and both a conversion to monofilament leaders and no shark lines. The analysis also explores reduction rates of both shark species under a variety of management scenarios, including banning both shark lines and wire leaders. The study provides an update to Harley et al. (2015) by using recently available observer information (2010–2018) on longline gear characteristics and spatial distribution of effort (2015–2019). The study used previous assumptions (Harley et al. 2015) on: 1) results of previous studies on catchability and survival and 2) spatial differences in the density of the two species.
The key conclusions of the current analyses are:
• Banning shark lines has the potential to reduce fishing mortality by 2.6% and 5.4% for silky shark and oceanic whitetip shark, respectively. These percentages are lower than predicted estimates from Harley et al. (2015) which may be explained by a decrease in use of shark lines in more recent observer data.
• Banning branchline wire leaders has the potential to reduce fishing mortality by 28.2% and 35.8% for silky shark and oceanic whitetip shark, respectively. These percentages are higher than estimates from Harley et al. (2015) and are perhaps due to improved characterization of gear use in the distant-water longline fisheries.
• Banning both shark lines and wire leaders has the potential to reduce fishing mortality by 30.8% and 40.5% for silky shark and oceanic whitetip shark, respectively.
• Submission of ROP observer data has increased in recent years. Future analyses would benefit from both in-zone and ROP data to estimate catchability effects for shark lines, wire and monofilament leaders and further characterize WCPFC member longline gear characteristics.