Elsevier

Fisheries Research

Volume 230, October 2020, 105665
Fisheries Research

Is it all about the haul? Pelagic false killer whale interactions with longline fisheries in the central North Pacific

https://doi.org/10.1016/j.fishres.2020.105665Get rights and content

Abstract

Pelagic false killer whales (Pseudorca crassidens) are killed or seriously injured in the Hawai‘i-based deep-set longline fishery more than any other cetacean, with bycatch regularly exceeding allowable levels. Telemetry data from five satellite-tagged whales (from three groups) and longline logbook entries (4182 sets) from the Hawai‘i-based longline fisheries are used to assess the range of the population and potential interactions with longline gear. A switching state-space model with a 4 -h time step was used to assess the behavior of the tagged whales. Two of the groups remained within the U.S. EEZ surrounding the Northwestern Hawaiian Islands, while one group spent 87.5 % of its time in international waters to the east of the Hawaiian archipelago. Tagged whales came within 100 km of only 26 sets over the 184 days of tag data, with only two of the three groups coming within 50 km of a set. Only twice were whales (from only one group) known to approach closely enough to interact with gear, during two series of three deep-sets, with only one of the six sets recording no catch (indicating probable catch depredation). Movement towards the sets was most dramatic during the haul phase, in one case the tagged whales moved almost 100 km towards the gear in 7 h. During one set in each of the two interactions, whale behavior changed to ‘area restricted search’ (indicative of foraging) during periods that overlapped with hauling of the gear. Overall, our results show that pelagic false killer whales spend a relatively small proportion of their time interacting with U.S. longline gear, and suggest that hauling gear may be an important cue initiating interactions.

Introduction

Fisheries bycatch, the taking and discarding of non-target species, is one of the greatest threats faced by many cetacean populations (Read, 2008), including false killer whales (Pseudorca crassidens) in Hawaiian waters (Carretta et al., 2017). The false killer whale is the most common species of cetacean bycaught in the Hawai‘i-based deep-set longline fishery that targets bigeye tuna (Thunnus obesus), with a smaller number of animals taken by the shallow-set longline fishery targeting swordfish, Xiphias gladius (Bradford and Forney, 2014, 2017; Forney, 2010). Bycatch of false killer whales in Hawai‘i-based longline fisheries has been recognized as a potential management and conservation issue since observer data first became available in the mid-1990s (Forney, 2004). Estimates of pelagic false killer whale bycatch based on data collected by observers of the deep-set longline fishery have exceeded allowable levels for most of the last 15 or more years (Carretta et al., 2017).

Vessels fishing in the Hawai‘i-based deep-set longline fishery deploy monofilament main lines that can be 30–100 km in length, with lines to floats at regular intervals of ∼0.4 km, intended to maintain a fishing depth of 40 m–400 m (Boggs and Ito, 1993; Gilman et al., 2007). Between floats there are 12–40 branch lines clipped to the main line, with a single baited hook hanging approximately 10−12 m below the main line (Bayless et al., 2017; Boggs and Ito, 1993; Gilman et al., 2007). Longlines are usually set in the morning, which takes 4−5 hours, and soaked for several hours before the haul in the afternoon though evening. Depending on the catch, hauling can take 10+ hours (Bayless et al., 2017). False killer whales are known to approach longline gear to remove both bait and hooked fish from the lines (depredation), often leaving the heads of catch attached to the hook (Bayless et al., 2017; Thode et al., 2016). Interactions with shallow-set longlines also occur, but at much lower rates, and shallow-sets represent a small proportion of the total fishing effort in the two fisheries.

Based on interviews, most Hawai‘i-based captains of longline vessels believe they are just as likely to suffer depredation losses during setting or soaking phases as during hauling (TEC Inc., 2009). Longline depredation by cetaceans occurring during hauling (Gilman et al., 2006) might be a result of animals responding to the acoustic signature of the gear hauling equipment and/or the vessel travelling slowly to retrieve the line. Acoustic monitoring of the Hawai‘i-based deep-set longline fishery revealed that most acoustic detections of false killer whales were during the haul phase of operation and that the animals tended to move along the line away from the boat during the haul, moving in the same direction as the boat (Bayless et al., 2017). This monitoring also found that only 16 % of the sets with acoustic detections of false killer whales had any sign of depredation as recorded by an onboard observer (Bayless et al., 2017).

In addition to the 145 vessels active in the Hawaiʻi-based longline fisheries in 2017 (Pacific Islands Fisheries Science Center, 2018), fleets from other nations also operate in the central North Pacific outside the U.S. Exclusive Economic Zone (EEZ), and presumably experience both depredation from and bycatch of false killer whales. These include Japan (Uosaki et al., 2016), Taiwan (Fisheries Agency et al., 2017), Republic of Korea (Kim et al., 2017), China (Dai et al., 2017), and Vanuatu (Vanuatu Fisheries Department, 2015). The non-U.S. high seas longline fishery in the central Pacific is managed by the Western and Central Pacific Fisheries Commission (WCPFC). Member nations have at least a 5% observer coverage on their longline vessels and file annual reports with WCPFC, while the Hawaiʻi-based fisheries have 20 % observer coverage for the deep-set fleet and 100 % coverage for the shallow-set fleet.

There are three stocks of false killer whales around the Hawaiian archipelago, an endangered insular population around the main Hawaiian Islands, another insular population around the Northwestern Hawaiian Islands, and a pelagic population that ranges far offshore (Baird, 2016; Bradford et al., 2015). Most of the false killer whale bycatch in the Hawaiʻi-based longline fisheries comes from the pelagic stock, as the fishery is excluded from nearshore areas that represent the core part of the range of the insular stocks. Pelagic false killer whales around Hawai‘i are a transboundary stock, with a range that extends outside the U.S. EEZ, and most of the observed takes by the Hawaiʻi-based longline fisheries occur outside the EEZ (Bradford and Forney, 2014, 2017). The most recent abundance estimate available for pelagic false killer whales within the U.S. EEZ around the Hawaiian Islands, based on the Hawaiian Islands Cetacean and Ecosystem Assessment Survey (HICEAS) shipboard line-transect survey in 2010, is 1540 animals (CV = 0.66) (Bradford et al., 2014, 2015). There are no estimates for abundance or trends in abundance for the entire stock that includes animals outside the U.S. EEZ. To address the unsustainable bycatch of pelagic false killer whales, the U.S. National Marine Fisheries Service (NMFS) set up a Take Reduction Team, which produced a draft Take Reduction Plan (TRP) in 2010. In 2013, the final TRP was put in place in an attempt to reduce bycatch of both pelagic and insular false killer whales in the Hawai‘i-based deep-set longline fishery. The primary tool used to attempt to reduce bycatch was a gear change, effectively involving weak circle hooks and strong terminal gear (hooks with a maximum wire diameter of 4.5 mm and branchlines with a minimum diameter of 2.0 mm). The gear changes were combined with handling guidelines for the captain and crew, to put tension on the gear to allow hooked whales to straighten the hooks, as well as research recommendations to help better understand how false killer whales detect and interact with gear. A review of data available through 2017 suggests that the gear changes implemented under the TRP are not significantly reducing rates of mortality and serious injury (NMFS Pacific Islands Regional Office, unpublished), thus there is a need for additional information to inform future modifications to the TRP.

Cascadia Research Collective (CRC) and the NMFS Pacific Islands Fisheries Science Center (PIFSC) have ongoing studies of false killer whales in Hawaiian waters (Baird et al., 2008, 2013, 2015; Bradford et al., 2014, 2015, Bradford et al., 2018). As part of these studies, satellite tags have been deployed on individuals in all three populations. Observation and tagging opportunities for pelagic false killer whales are considerably rarer than for insular individuals, but in 2013 satellite tags were deployed by CRC and PIFSC on five pelagic false killer whales in three different groups (Bradford et al., 2015). Two of the tags exceeded 100 days in duration, offering the first opportunity to examine pelagic false killer whale spatial use and interactions with longline vessels.

This paper compares satellite tag telemetry data to logbook data from the Hawaiʻi-based longline fisheries to gain a better understanding of how pelagic false killer whales interact with fisheries. These analyses looked for potential interactions between tagged false killer whales and longline sets to determine how often the whales interact with nearby vessels, the timing of the interaction compared to vessel activity, and whether the presence of the whales affected the catch. A greater understanding of the behavior of pelagic false killer whales around longline sets will help inform the efforts of the Take Reduction Team working to reduce false killer whale bycatch and injury rates in the Hawaiʻi-based longline fisheries.

Section snippets

Methods

Pelagic false killer whales were encountered in 2013 by PIFSC during a ship-based cetacean survey of the Northwestern Hawaiian Islands and by CRC during a small-boat survey for odontocetes off Hawai‘i Island in the main Hawaiian Islands. For each group encountered, group size was estimated, individuals were photographed, and skin biopsy samples were obtained. Whales were tagged using location-only satellite tags (SPOT-5, Wildlife Computers) using the Low-Impact Minimally-Percutaneous

Results

Satellite tags were deployed on five pelagic false killer whales in three separate groups, one (PcTagP01) in a group with an estimated 23 individuals on 15 May 2013 in the Northwestern Hawaiian Islands (PIFSC), one (PcTagP02) in a group with an estimated 15 individuals on 26 May 2013 in the Northwestern Hawaiian Islands (PIFSC), and three (PcTag039, PcTag040, PcTag041) in a group with an estimated 16 individuals on 22 October 2013 off Hawai‘i Island (CRC; Table 1). A total of 28 individuals

Discussion

This study provides the first assessment of the behavior and movements of pelagic false killer whales across large portions of their range and in relation to longline vessels and fishing gear. We provide corroboration with earlier studies that found that false killer whales respond to the hauling of gear, while providing context to the frequency and distance of the interactions. Our analyses have implications for understanding the interactions between pelagic false killer whales and longline

Data availability statement

Longline fishery logbook data is restricted due to confidentiality requirements, and is available only through non-disclosure agreement with NMFS. CRC false killer whale location data is available from RWB upon reasonable request. PIFSC false killer whale location data is available from EMO upon reasonable request.

CRediT authorship contribution statement

David Anderson: Conceptualization, Methodology, Software, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Robin W. Baird: Conceptualization, Resources, Investigation, Writing - original draft, Writing - review & editing, Supervision. Amanda L. Bradford: Investigation, Data curation, Writing - review & editing. Erin M. Oleson: Resources, Data curation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding for CRC’s field work was provided by the Office of Naval Research [grant number N00014-13-1-0648] and a grant from the PIFSC [grant number NA13OAR4540212], with funding for one of the satellite tags provided by a grant from Dolphin Quest. Funding for analyses were provided by a grant from the Bycatch Reduction Engineering Research Program [grant numbers NA14NMF4720319, NA15NMF4720381]. We thank Daniel Webster and Allan Ligon for tag deployments, Sabre Mahaffy and Annie Gorgone for

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