{"id":6053,"date":"2025-07-07T14:55:25","date_gmt":"2025-07-07T21:55:25","guid":{"rendered":"https:\/\/blogs.oregonstate.edu\/gemmlab\/?p=6053"},"modified":"2025-07-07T15:00:35","modified_gmt":"2025-07-07T22:00:35","slug":"the-final-chapter-concluding-a-phd","status":"publish","type":"post","link":"https:\/\/blogs.oregonstate.edu\/gemmlab\/2025\/07\/07\/the-final-chapter-concluding-a-phd\/","title":{"rendered":"The Final Chapter: Concluding a PhD"},"content":{"rendered":"\n

By Rachel Kaplan, PhD candidate, Oregon State University College of Earth, Ocean, and Atmospheric Sciences and Department of Fisheries, Wildlife, and Conservation Sciences, Geospatial Ecology of Marine Megafauna Lab<\/em><\/em><\/p>\n\n\n\n

At the beginning of a graduate program, it\u2019s common for people to tell you how quickly the time will pass, but hard to imagine that will really be the case. Suddenly, I\u2019ve been working on my PhD for almost five years, and I\u2019ll defend in just over two weeks. As I look back, I am amazed by how much I have learned and grown during this time, and how all the different parts of my graduate school experience have woven together. I began my program in 2020<\/a> with an intense \u201cbootcamp\u201d of oceanographic coursework, and am ending in 2025 with new analytical skills, a few publications, and a ton of new thoughts about whales and the zooplankton krill, the subjects of my research. My PhD work encapsulates all those different elements in an exploration of ecological relationships between baleen whale predators and their krill prey \u2013 which I now see as an expression of oceanographic and atmospheric processes.<\/p>\n\n\n

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Figure 1. One of my favorite sightings during my PhD fieldwork was a group of seven fin whales in Antarctica, on Christmas 2024. Photo: Rachel Kaplan<\/figcaption><\/figure>\n<\/div>\n\n\n

Oceanographic processes drive prey quantity and quality across time and space, shaping the preyscape encountered by predators on their foraging grounds and driving habitat use (Fleming et al.<\/em>, 2016; Ryan et al.<\/em>, 2022). Aspects of prey including distribution, energy density, and biomass therefore represent mechanistic links between ocean and atmospheric conditions (e.g., El Ni\u00f1o Southern Oscillation cycles<\/a>, circulation patterns, and upwelling processes<\/a>) and diverse aspects of marine predator ecology, including spatiotemporal distributions, foraging behaviors, reproductive success, population size, and health. Both predator and prey species are impacted by environmental variability and climate change (e.g., Hauser et al.<\/em>, 2017; Atkinson et al.<\/em>, 2019; Perryman et al.<\/em>, 2021), and events like marine heatwaves and harmful algal blooms can force ecosystem changes on short, seasonal time scales (e.g. McCabe et al.<\/em>, 2016; Fisher et al.<\/em>, 2020). However, many marine species have some degree of plasticity that allows them to still accomplish life history events in the face of ecosystem variability (e.g., Lawrence, 1976; Oestreich, 2022), which may provide the capacity to adapt to climate change processes.<\/p>\n\n\n\n

Observing and describing predator-prey relationships is complex due to the scale-dependent nature<\/a> of these relationships (Levin, 1992). Each chapter of my dissertation considered krill, a globally-important prey type, from the perspective of baleen whales, which are krill predators. Chapter 2 used a comparative analysis to identify the optimal spatial scale at which to observe baleen whale-krill relationships on the Northern California Current (NCC) foraging grounds. We found correlations at a 5 km scale to be strongest, which can provide a useful starting point for further studies in the NCC and other systems. Chapter 3<\/a> used this spatial scale to compare several aspects of krill prey quality and quantity as predictors of humpback whale (Megaptera novaeangliae<\/em>) distributions in the NCC. The best performing metric was a species, season, and spatially informed krill swarm biomass variable \u2013 yet the comparable performance of a simple acoustic abundance metric indicated that it can act as a reliable proxy for biomass. This finding may be advantageous for future research, as measuring the acoustic proxy is less computationally intensive and relies on fewer datastreams. Interestingly, one of this study\u2019s best-performing models was based on only the proportion of Thysanoessa spinifera <\/em>in krill swarms, which is also a highly accessible variable due to effective krill species distribution modeling in the NCC (Derville et al.<\/em>, 2024). Integrating the acoustic abundance proxy and krill species distribution predictions, two relatively simple metrics, could support predictions of humpback whale distributions in the NCC and inform whale-prey research in other ecosystems.<\/p>\n\n\n

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Figure 2. Collecting samples of individual krill gave us the opportunity to learn about their quality as prey for whales in the Northern California Current. Photo: Courtney Flatt<\/figcaption><\/figure>\n<\/div>\n\n\n

Studies relating predator foraging to prey characteristics often rely on metrics such as prey biomass or energy density (Schrimpf et al.<\/em>, 2012; Savoca et al.<\/em>, 2021; Cade et al.<\/em>, 2022), but the tendency of krill to form aggregations introduces dimensionality to krill prey quality. Chapter 4 showed that elements of krill swarm structure (particularly depth, proportion of T. spinifera<\/em>, and metrics describing how krill occupy space within swarms) may be mechanistic drivers of variable blue, fin and humpback whale distribution patterns on the NCC foraging grounds. These findings suggest that krill swarm characteristics may be important links between baleen whales and the foraging environment. Swarm characteristics may be considered a component of krill prey quality for baleen whales, and future research could illuminate direct causal relationships between oceanographic conditions, krill swarming responses, and niche expression in baleen whale predators. <\/p>\n\n\n\n

The relationships between baleen whale distributions and krill quantity and quality explored in the first chapters of my dissertation may also shed light on other aspects of baleen whale ecology. The final chapter considers overwintering trends in global baleen whale populations, and examines the wintertime Western Antarctic Peninsula (WAP) as a case study. Extended humpback whale presence on the WAP feeding grounds may be driven by the profitable feeding areas and elevated energy content of krill during the winter months, and may reflect the high energetic needs of certain demographic subgroups (e.g. lactating females, juveniles). Wintertime humpback whale presence may also reflect adaptation to multifaceted competitive pressure on krill resources that are declining due to climate change (Atkinson et al.<\/em>, 2019), including consumption by growing baleen whale populations (Johnston et al.<\/em>, 2011) and a fishery whose catch limits may be impacting krill predators (Watters et al.<\/em>, 2020; Savoca et al.<\/em>, 2024). This work demonstrates how investigating prey quality during the winter months can contextualize baleen whale overwintering on the foraging grounds. It also provides a meaningful violation of the canonical baleen whale migration paradigm central to marine mammal science, which may lessen the efficacy of whale monitoring programs and management policies. <\/p>\n\n\n

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Figure 3. We were surprised to see humpback whales like this one in Antarctica during the winter months — which raised a number of questions about overwintering of baleen whales on foraging grounds around the world. Photo: Giulia Wood<\/figcaption><\/figure>\n<\/div>\n\n\n

Management efforts<\/a> that aim to mitigate risk to whales often hinge on predictive modeling of whale distributions. Species distribution models (SDMs) can provide managers with spatially and temporally explicit predictions of protected species occurrences (Wikgren et al.<\/em>, 2014; Santora et al.<\/em>, 2020), but species distributions in rapidly changing ecosystems are difficult to predict (Muhling et al.<\/em>, 2020). Findings from this dissertation may inform modeling efforts by suggesting meaningful predictor variables for SDMs, such as krill species on the NCC foraging grounds and swarm energy density at the WAP. This work also speaks to meaningful spatial scales for analyzing predator-prey relationships (i.e., 5 km), and relevant elements of temporal variability (e.g., seasonal cycles of krill energy density).<\/p>\n\n\n\n

Just as marine predator-prey relationships are shaped by ocean processes, they likewise have consequences for those processes. For example, krill and other zooplankton are capable of generating large-scale mixing that can overcome stratification of water masses and alter water column structure (Noss and Lorke, 2014). Baleen whales influence global carbon cycles due to the huge amount of prey they consume (Savoca et al.<\/em>, 2021; Pearson et al.<\/em>, 2023) and transport important nutrients along the \u201cgreat whale conveyer belt\u201d during their vast migrations (Roman et al.<\/em>, 2025). Baleen whales seek krill as an essential prey resource on foraging grounds around the globe, and the impact of this trophic interaction scales up, with implications for ecosystem functioning and management. Continued research into the spatiotemporally dynamic relationships between krill and baleen whales improves our understanding of ocean functioning, and can improve our capacity to live as part of this system.<\/p>\n\n\n

By Rachel Kaplan, PhD candidate, Oregon State University College of Earth, Ocean, and Atmospheric Sciences and Department of Fisheries, Wildlife, and Conservation Sciences, Geospatial Ecology of Marine Megafauna Lab At the beginning of a graduate program, it\u2019s common for people to tell you how quickly the time will pass, but hard to imagine that will … Continue reading The Final Chapter: Concluding a PhD<\/span><\/a><\/p>\n","protected":false},"author":10751,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1310784,1310817,1310535],"tags":[1310820,195384,677522,513,148762,1310687],"class_list":["post-6053","post","type-post","status-publish","format-standard","hentry","category-antarctic-baleen-whales","category-opal","category-oregon-whale-distribution","tag-baleen-whales","tag-field-work","tag-foraging-ecology","tag-marine-mammals","tag-oregon-coast","tag-rachel-kaplan"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/posts\/6053","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/users\/10751"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/comments?post=6053"}],"version-history":[{"count":4,"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/posts\/6053\/revisions"}],"predecessor-version":[{"id":6060,"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/posts\/6053\/revisions\/6060"}],"wp:attachment":[{"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/media?parent=6053"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/categories?post=6053"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.oregonstate.edu\/gemmlab\/wp-json\/wp\/v2\/tags?post=6053"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}