Could tori lines replace blue-dyed bait to reduce seabird bycatch risk in the Hawaii deep-set longline fishery?

The comparative efficacy of tori lines and blue-dyed fish bait as seabird bycatch mitigation measures was assessed for the Hawaii deep-set tuna longline fishery. Tori lines have been shown to be an effective seabird bycatch mitigation measure for many pelagic and demersal longline fisheries, but the effectiveness of blue-dyed bait remains equivocal (Gilman et al 2021b). The Western Pacific Regional Fishery Management Council is considering whether to replace blue-dyed fish bait with tori lines in the prescribed suite of bycatch mitigation measures available to the Hawaii-based deep-set tuna longline fishery. The efficacy of three seabird bycatch mitigation measures (tori line, blue-dyed fish bait, strategic offal discharge) for this fishery has been previously evaluated but the blue-dyed bait effect was equivocal because it was a confounded treatment with offal discharge due to regulatory restrictions (Gilman et al 2021b). A partial factorial experiment that was exempt from the blue-dyed fish bait and offal discharge regulatory restrictions was commissioned to evaluate the comparative efficacy of tori lines and blue-dyed bait to reduce seabird interactions in this deep-set longline fishery - where interactions are attempts to contact a baited hook or actual contacts with a baited hook. The study was conducted under a NOAA Fisheries Experimental Fishing Permit issued to the Hawaii Longline Association on January 27, 2021. The purpose was to support evidence-informed seabird bycatch mitigation policy in this fishery. The field experiment comprised 87 sets deployed during 7 trips from 3 Hawaii-based commercial longline vessels. The partial factorial design involved random assignment on each trip to one of the two treatments (tori line, blue-dyed fish bait) for the initial set and then alternating the two treatments for each subsequent set in the trip. All the sets were deployed with lines shooters, similar branchline weights and no offal discharge during setting operations. Nearly 99% of all the seabird interactions during this experiment were for black-footed (Phoebastria nigripes) and Laysan albatrosses (Phoebastria immutabilis). We used a Bayesian modelling workflow to model statistically the albatross interactions (attempts to contact a baited hook, actual contacts with a baited hook) recorded using onboard stern-view videobased electronic monitoring. This modelling approach supports robust statistical inference about the comparative efficacy of the two mitigation measures evaluated (tori lines, blue-dyed bait). The number of albatrosses captured on the hook (drownings) was also recorded during the gear haulback — only 13 albatrosses were hook-caught in this experiment. We found that tori lines worked as an effective seabird bycatch mitigation measure in this trial fishery, which is consistent with previous findings for this fishery (Gilman et al 2021b). More importantly, we found that tori lines were a far more effective seabird bycatch mitigation measure than blue-dyed fish bait. In contrast to Gilman et al (2021b), we were able to unequivocally determine the comparative efficacy of tori line and blue-dyed fish bait as seabird bycatch mitigation measures in this fishery. Specifically, we found that:
* albatrosses were 1.5 times (95% HDI1: 1-2.2) less likely to attempt to contact a baited hook when tori lines were deployed compared to sets deployed with blue-dyed fish bait
*albatrosses were ca 4 times (95% HDI: 1.5-9.2) less likely to contact a baited hook when tori lines were deployed compared to sets deployed with blue-dyed fish bait
* albatrosses were ca 14 times (95% HDI: 7.8-18.6) less likely to be captured on a baited hook when tori lines were deployed compared to sets deployed with blue-dyed fish bait.

It has been shown elsewhere and for this fishery that tori lines tend to increase the median distance from the vessel stern that seabird attacks occur on sinking longline gear (Gilman et al 2021a). The point being that the further astern that a gear/bait interaction can be deterred the better for reducing capture risk, since the further astern, the more likely the baited hook has sunk to a depth inaccessible to a surface foraging albatross.

We found albatross interactions with longline gear deployed with tori lines occurred significantly further astern than for those sets deployed with blue-dyed bait. This finding further reinforces the benefits of using tori lines rather than blue-dyed bait as an effective seabird bycatch mitigation measure.