Oceanic sharks are under increasing pressure from pelagic fisheries, particularly tuna longlines. Silky sharks (Carcharhinus falciformis) and blue sharks (Prionace glauca) dominate global pelagic shark catches. Silky sharks are a vulnerable to overfishing due to their slow growth, late maturation, and low fecundity, and are listed as Vulnerable by the IUCN and on Appendix II of CITES and the CMS. Despite bans on finning—and in some cases retention—implemented by some Regional Fisheries Management Organizations, these measures fall short of scientific advice, and evidence of compliance and effectiveness is lacking. Although blue sharks are more fecund than silky sharks and a good candidate for sustainable fishing, retention (and probably targeting) has increased substantially, stock assessments are limited, and overall blue sharks are afforded fewer protections.
Catch of non-target species is typically managed with fleet-level controls on fishing, such as time-area closures, technology requirements, fleet-wide bycatch quotas, or retention bans. These approaches overlook the potential role of individual operators in driving fleets’ bycatch rates. We posited that avoiding bycatch is a skill, which varies across individual operators. To test this hypothesis, we analyzed vessel-level variability in protected species bycatch using observer datasets from Australia, including the pelagic longline sector. We found the “vessel effect” was a dominant driver of bycatch variability across a range of bycatch types, including hammerheads in prawn trawls (bycatch) and shortfin makos in tuna longlines (byproduct). However, the data were completely anonymised, precluding exploration of what aspects of the individual vessels drive the observed effect on bycatch rates.
Here, we further explore variable operator performance using observer and logbook data from the foreign tuna longline fleet operating in the Republic of the Marshall Islands’ Exclusive Economic Zone. The data were collected as part of an ongoing trial of electronic monitoring systems implemented by the Nature Conservancy. We test whether 1) there is an operator effect for shark catch that is independent of the physical characteristics of the vessel and 2) if the registered company drives the performance of individual vessels.