Patterns of depredation in the Hawai‘i deep‐set longline fishery informed by fishery and false killer whale behavior

Abstract

False killer whales (Pseudorca crassidens) depredate bait and catch in the Hawai'i-based deepset longline fishery, and as a result, this species is hooked or entangled more than any other cetacean in this fishery. We analyzed data collected by fisheries observers and from satellite-linked transmitters deployed on false killer whales to identify patterns of odontocete depredation that could help fishermen avoid overlap with whales. Odontocete depredation was observed on˜6% of deep-set hauls across the fleet from 2004 to 2018. Model outcomes from binomial GAMMs suggested coarse patterns, for example, higher rates of depredation in winter, at lower latitudes, and with higher fishing effort. However, explanatory power was low, and no covariates were identified that could be used in a predictive context. The best indicator of depredation was the occurrence of depredation on a previous set of the same vessel. We identified spatiotemporal scales of this repeat depredation to provide guidance to fishermen on how far to move or how long to wait to reduce the probability of repeated interactions. The risk of depredation decreased with both space and time from a previous occurrence, with the greatest benefits achieved by moving˜400 km or wait-ing˜9 d, which reduced the occurrence of depredation from 18% to 9% (a 50% reduction). Fishermen moved a median 46 km and waited 4.7 h following an observed depredation interaction, which our analysis suggests is unlikely to lead to large reductions in risk. Satellite-tagged pelagic false killer whales moved up to 75 km in 4 h and 335 km in 24 h, suggesting that they can likely keep pace with longline vessels for at least four hours and likely longer. We recommend fishermen avoid areas of known depredation or bycatch by moving as far and as quickly as practical, especially within a day or two of the depredation or bycatch event. We also encourage captains to communicate depredation and bycatch occurrence to enable other vessels to similarly avoid high-risk areas.

Exploring odontocete depredation rates in a pelagic longline fishery

“…However, there was large variability in odontocete depredation rates between teleost species, while all species of captured elasmobranchs had negligible odontocete depredation. This is consistent with previous studies of the observer program database for the Hawaii tuna longline fishery [ 44 , 45 ]. Findings suggest that odontocetes selectively depredate a subset of the teleost species captured within sets.…”

“…More robust estimates of species-specific odontocete depredation rates could be derived through research using observer data from longline fisheries that standardize fishing effort to account for potentially informative predictors of odontocete depredation. As conduced previously to explore a response of odontocete depredation of combined species of catch, models with a response of species-specific depredation rate could include predictors such as unique vessel, consecutive sets within a trip, season, geospatial location, hooks per set, mainline length, soak duration, bathymetry and sea surface temperature [ 26 , 32 , 37 , 45 ].…”

“…Knowledge of species-specific odontocete depredation rates might contribute to the development of promising, new mitigation methods, such as geospatial, depth and temporal separation of predictable catch rate hotspots of species with both low commercial value and high odontocete depredation rates and hotspots of catch rates of principal market species, and passive acoustic decoys, deterrents and echolocation disruptors. This study analyzed observer data from the Hawaii tuna longline fishery to estimate teleost and elasmobranch species-specific mean posterior marine mammal depredation rates using a Bayesian binomial likelihood estimator with a Bayes-Laplace prior, expanding upon previous assessments [ 44 , 45 ]. The study also assessed the correlation between species-specific depredation rates and length.…”

Focusing on the human dimensions to reduce protected species bycatch

“…Previous research suggested that vessels may need to move at least 100 km (~ 62 miles) and as much as 250 km (~ 155 miles) to avoid repeat depredation events (Fader et al, 2021b;Forney et al, 2011), but more recent research by Fader et al (2021a) found that fishing vessels traveled a median distance of 46 km over 4.7 h following a depredation event during observed fishing trips. Fader et al (2021a) noted that vessels may need to move much further due to vessel clustering while fishing along with the elevated depredation risk following a depredation event, with the greatest conservation gains realized if a vessel moved 400 km and traveled 9 days.…”

Depredation of Demersal Reef Fishes Released with Descender Devices Is Uncommon off North Carolina, USA

“…2018; Fader et al. 2021). In the context of descending, some anglers believe that jaw‐attached devices may restrict the ability of released fish to escape predators, leading to higher release mortality than for undescended fish (Drymon et al.…”