Apex predators such as killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) directly compete with humans for ecosystem services such as fishery resources. One example of this type of competition is whale depredation on longline fisheries in Alaska. Sperm whale and killer whale depredation occurs when whales remove fish from longline gear, damage fish and/or fishing gear. This interaction has negative consequences for the longline fishery and for the whales.
This interdisciplinary dissertation project explores how whale depredation is impacting longline fishing practices and fishery resource use in Alaska. This research will synthesize social research interview and written questionnaire results with quantitative NMFS fishery data to construct a detailed economic analysis of the costs associated with depredation. Results from this study will establish baseline information on the socio-ecological and socio-economic impacts of cetacean depredation and how depredation impacts the long-term sustainability of longline fisheries in Alaska.
Killer whale (Orcinus orca) and sperm whale (Physeter macrocephalus) depredation occurs when whales damage or remove fish caught on longline gear. This project used a mixed methods approach incorporating Generalized Linear and Additive Modeling techniques and social research methods, such as semi-directed interviews and written questionnaires, to evaluate: 1) spatio-temporal depredation trends, 2) depredation effects on groundfish catch rates, and 3) socio-economic implications of depredation avoidance and changing fishing practices due to whale interactions. Catch per unit effort (CPUE) reductions associated with killer whale depredation were estimated for groundfish species using two primary data sets: the National Marine Fisheries Service (NMFS) longline survey and NMFS commercial catch data in the Bering Sea (BS), Aleutian Islands (AI) and the Western Gulf of Alaska (WGOA) management regions. In all three management areas, killer whale depredation was more common on standardized longline survey skates (9.2-34.6%) than commercial sablefish fishery sets (1.0-21.4%). The NMFS survey records depredation at a more refined scale per skate (string of 45 hooks), whereas the commercial fishery records depredation per set (average set = 5900 hooks). More frequent killer whale depredation on the longline survey relative to commercial operations is consistent with other findings reported in this dissertation that commercial fishery operations actively avoid killer whales and generally will not continue to retrieve their gear when killer whales are present. Killer whale depredation had a significant effect on NMFS longline survey catch rates for five of the six groundfish species evaluated: sablefish (Anoplopoma fimbria), Greenland turbot (Reinhardtius hippoglossoides), Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias) and Pacific cod (Gadus macrocephalus). Catch reductions were consistent between the NMFS survey and commercial fishery data despite different modeling approaches (Negative Binomial Generalized Linear Models and Generalized Additive Models, respectively), averaging 35-69% for sablefish, Pacific halibut and Greenland turbot in the commercial fishery and 51-73% for the same species in the survey. Model-based estimates of catch reductions in the NMFS survey and in the commercial fishery were also consistent with fishermen's estimates from the written surveys and depredation data collected on the fishing grounds. Based on the NMFS longline survey data, sablefish CPUE, gear haul time and location were significantly associated with the proportion of skates or sets depredated. Killer whales were more likely to depredate stations with higher average sablefish CPUE, which may be consistent with optimal foraging efficiency and maximizing net rate of energy gain. This finding concurred with data collected from written questionnaires and interviews as part of this research. Survey respondents were asked, in an open-ended question, to list any marine environment characteristics they associated with whale depredation. The majority of respondents (55%) listed "high catch rates," supporting the hypothesis that the whales target fishing grounds with higher CPUEs. Results from both interviews and written surveys, as well as model results from the NMFS survey data, indicate that killer whales targeted areas southwest of the Pribilof Islands and north and south of Unalaska and Umnak Islands. The relatively high prevalence of killer whale-fishery interactions may be related to higher abundances of killer whales in these areas, although abundance data for killer whales are limited in these regions. This project advanced our understanding of toothed whale interactions with Alaskan longline fisheries and associated impacts to the socio-economic sustainability of Alaskan longline fisheries. We refined estimates of catch removals by killer whales and resulting reductions in catch per unit effort (CPUE). We also developed a better understanding of how depredation affects fishing behavior and the economic costs fishermen incur due to whale interactions. This work built upon earlier depredation studies and provided a template for future regional and international research examining apex predator interactions with fisheries.