Patterns of Odontocete Depredation and Bycatch in Pelagic Longline Fisheries
The leading threat to the conservation of whales and dolphins worldwide is incidental mortality (or bycatch) in fisheries. In longline fisheries, these interactions are commonly driven by an attraction to feed on bait or fish secured on fishing gear, a process known as depredation. Depredation is particularly common and costly in longline fisheries, where bait and catch are typically unprotected prior to gear retrieval. Bycatch of depredating whales can occur when animals become hooked or entangled in the gear while attempting to remove bait or catch. Depredation, and associated bycatch, is an exceedingly difficult problem to solve in open-ocean, or pelagic, longline fisheries, because the behavior often occurs at depth or at a large distance from the fishing vessel. Evidence of depredation may consist only of large stretches of missing bait or fish heads on hooks, because odontocetes have eaten the bodies of captured fish. Observations of bycatch are also rare and depend on on-board observers to carefully report details of the interactions. In my dissertation, I examined two longline fisheries in the United States heavily impacted by depredation and bycatch of cetaceans: the Hawai‘i deep-set longline fishery depredated by false killer whales (Pseudorca crassidens) and the Atlantic pelagic longline fishery depredated by short-finned pilot whales (Globicephala macrorhynchus). These two interactions are data rich (relative to most other pelagic longline fisheries), with detailed observations of cetacean-fishery interactions and long-term, dedicated behavioral research programs focused on the two depredator species. Both fisheries also benefit from resources and institutional will as a consequence of legislative mandates to reduce bycatch by the U.S. Marine Mammal Protection Act. Nevertheless, managers and stakeholders in both fisheries have struggled to fully resolve the problem. I used a variety of different data sources and interdisciplinary methods to better characterize depredation and bycatch interactions in both fisheries, with the ultimate goal to identify potential mitigation solutions that may help reduce the negative impacts of depredation and bycatch on both whales and fishermen. In my first chapter, I reviewed common mitigation approaches to depredation and bycatch by marine mammals, and I conducted a case study analysis on the management process and its effectiveness in addressing depredation in the two focal fisheries. I identified past successes and failures and summarized the current situation for both fisheries and where bycatch management is going next. The next two chapters focused in-depth on the Hawai‘i deep-set pelagic longline fishery and depredation interactions by false killer whales. I first 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. I identified several broad-scale patterns of depredation but found the best indicator of depredation to be the occurrence of depredation on a previous set of the same vessel. I identified spatiotemporal scales of this ‘repeat’ depredation and analyzed satellite-tagged false killer whale data to better understand fine scale whale movement that may be relevant to avoiding depredation. I brought this information together to provide simple recommendations to fishermen that could reduce the occurrence of depredation from 18% to 9% on average (a 50% reduction). Next I used the same fishery-dependent data sources (observer-collected and logbook data) and a novel, multi-stage modeling approach to derive quantitative estimates of catch losses by depredating odontocetes in the Hawai‘i deep-set longline tuna fishery. Although depredation is relatively rare and variable on a per set basis, I estimated the total lost biomass and economic value of the top three catch species to average 100 t and one million USD per year, respectively. I also identified broad-scale spatiotemporal patterns where the relative losses of depredation by odontocetes are expected to be greatest, providing additional recommendations for how fishermen may be able to minimize costs of depredation. My final chapter focused on the Atlantic pelagic longline fishery and depredation and bycatch of the short-finned pilot whale. I conducted a baseline analysis of pilot whale behavior from a rich satellite tag dataset, identifying a previously undetected diel behavioral pattern and adding additional novel insights to a growing body of research on this population of whales. I then described a novel behavioral state that occurred when whales were in close proximity to fishing activity (< 50 km). Although the resolution of these data precluded a detailed characterization of depredation events, my observations that whales change their behavior when in the proximity of vessels may help inform move-on rules that fishermen could employ to reduce the occurrence of depredation and bycatch.