David Rosen

Sea otters (Enhydra lutris) are keystone predators with voracious appetites that impose immense predation pressure on their prey, thereby significantly altering the structure of local ecosystems. To better understand how much food sea otters must consume to meet their energy needs and impacts of predation, I developed bioenergetic models for sea otters. These models infer how the energy expenditures of individual sea otters vary under changing conditions, while accounting for sex, age, and reproductive status. I created three models— females without a pup, females with a pup, and males— using peer-reviewed, published data on sea otter ecology, physiology, and behaviour. The models demonstrated that females caring for a pup have the highest total energy expenditures when compared to adult males (3-10% less on average) and females without pups (33% less). Sensitivity analyses showed that the body mass parameter had the largest effects on model outcomes, due to many input values being mass-specific. The second variable affecting model outcomes was metabolic rate, which helps inform which types of data should be collected to improve model accuracy. The amount of food sea otters require to satisfy their energy expenditures depends on complex relationships between the energy density, digestive value, and foraging costs of obtaining different prey items. I converted the energy expenditure estimates generated by the models into ingested food mass (IFM) and biomass removed to investigate the effects of diet and prey availability on prey consumed by sea otters. I chose five diet scenarios to compare: one general otter diet and four specialist diets. Otters that prefer more energy dense prey species (e.g., abalone) had lower IFM than otters consuming less dense prey (e.g., urchins). The amount of food required by mothers with pups on a low energy diet may present a significant challenge. I further explored how changes in food availability affect otters’ foraging efficiency and found that as foraging efficiency decreases, foraging time and energy expenditure both increase. Therefore, prey selection by sea otters has critical energetic consequences effecting their overall energy balance, and may contribute to limiting range expansion or population growth of sea otters in certain areas.

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Migratory movement is integral to the life histories of many animal species, as they travel in pursuit of essential resources, favourable environmental conditions, and reproductive opportunities. The migrations of marine mammals are among the most impressive, spanning vast expanses of open ocean over months at a time. How they navigate this seemingly featureless landscape remains largely unknown, as are the potential environmental cues that may guide them. Southern elephant seals (Mirounga leonina) provide a compelling model for studying open ocean navigation, as they spend most of their lives at sea and migrate thousands of kilometers to reach small, isolated breeding islands. Using ARGOS satellite locations from 271 southern elephant seals tracked across the Southern Ocean between 2009 and 2019, I quantified the characteristics of their long-distance movements and evaluated them against theoretical predictions that assumed reliance on lunar, solar, and geomagnetic environmental cues. I found that the migratory paths of southern elephant seals were remarkably precise and efficient when returning to breeding islands. Individuals maintained straight-line trajectories that accounted for the curvature of the Earth and corrected for drift caused by ocean currents, despite initiating travel from widely dispersed locations. These findings suggest that southern elephant seals are capable of sophisticated navigation, likely guided by specific environmental cues. My results indicate it is unlikely that lunar and solar cues are used for orientation, but the observed movement patterns closely align with predicted use of geomagnetic cues. Seals consistently followed gradients of geomagnetic inclination, total intensity, and declination, with many paths following a combination of two gradients. Sensitivity analyses suggest that successful navigation would require seals to possess high sensitivity to subtle changes in geomagnetic values, whether following individual cue gradients or using them in tandem. This study is among the first to quantitatively characterize the navigational abilities of southern elephant seals and provides some of the first evidence that pinnipeds may use geomagnetic information for navigation and goal-finding. These findings also highlight the utility of tracking information to investigate animal navigation in species that cannot be studied using traditional experimental approaches, advancing understanding of animal movement and ultimately informing conservation.

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Marine mammals routinely travel vast distances across the oceans, but there is ongoing debate on how they manage these feats. The typical approach to investigating how animals navigate involves first identifying a specific cue or environmental correlate that may influence their decisions, and then looking for behavioural evidence to support this hypothesis. However, selecting a cue to investigate without a priori knowledge of an animal’s movements can be inefficient and lead to confirmation bias. Therefore, I set out to demonstrate how a meta-analysis that identifies movement patterns in previously collected data is a valuable tool to generate useful hypotheses in navigation research. I illustrate this process using telemetry data from the Antarctic fur seal (AFS; Arctocephalus gazella), a model species due to its extensive at-sea movement and the vast amount of existing publicly available location data spanning two decades. The data was pre-processed to split journeys into outbound and inbound transiting periods whilst discarding intermediary foraging behaviour. In total, reliable data was obtained from 86 individuals from Marion Island and 132 individuals from Bird Island for my analyses. I then deployed four different approaches to quantify potential patterns in their movement to ultimately determine how they navigate. Circular statistics identified consistent preference for island-specific dispersal direction during outbound legs from their home island, as well as similarities between outbound and inbound directions. Area analysis identified potential corridor usage during return legs suggesting regular travel routes that can be potentially used to identify environmental cues. Block design statistics showed the seals’ ability to gradually correct their heading during inbound legs to the island. However, I did not find any correlation between lunar fraction and the timing of the start of these return legs. This study with AFS also provides a demonstration of how clearly delineating patterns in movement will permit unbiased testing of environmental correlates to determine how marine mammals navigate. For marine mammals in general, this research illustrates the importance of meta-analyses as an efficient, informative approach to analysis decision-making.

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The global population of California sea lions (Zalophus californianus) has declined in the Gulf of California (Mexico), while numbers have increased along the California coastline (U.S.). It is unclear what is behind the divergent population trends, but differences in diets likely play a role. I used diet data to investigate whether the changes in sea lion population numbers that occurred in sea lion numbers from 1980–2020 could be explained by differences or shifts in diet quality — specifically energy density and diet diversity. I also explored whether diet quality in the Gulf of California was affected by increased sea surface temperatures that occurred in 2014. I considered rookeries in California (Channel Islands) to be a single ecological Zone and divided the Gulf of California breeding islands into nine Zones based on geographic proximities and similarities in population trajectories. Years with matching population and diet data within all these Zones were used to test for relationships between measures of diet quality and population changes. My results showed that diet variability and composition differed between the Channel Islands and the Zones within the Gulf of California. In general, sea lions breeding in the Gulf of California consumed a large variety of mostly benthic species and schooling fish, whereas sea lions at the Channel Islands primarily consumed schooling fish and squid. Contrary to expectations, no significant relationships were found between population changes and measures of diet quality across all Zones and times. However, the average energy density of sea lion diets in certain Zones within the Gulf of California declined as sea surface temperatures increased. While my results did not reveal a direct relationship between population changes and diet quality, they demonstrate the significance of considering the influence of environmental heterogeneity on regional population dynamics. My results also highlight the importance of better understanding the ecosystem dynamics of the Gulf of California at small regional scales. Such findings may be key to fully understanding the interplay between environmental changes, diets, and future population trajectories of California sea lions and other pinniped species in geographic locations throughout Mexico and the U.S.

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Marine mammal diving behaviour is influenced by multiple physiological processes, both at depth and at the surface. To date, the majority of research in diving physiology has focused solely on how quickly marine mammals utilize their O₂ during a dive, as seen in the numerous studies of the aerobic dive limit (ADL) and calculated aerobic dive limit (cADL). In this thesis I investigated other physiological limits, namely how long it takes for marine mammals to recover after a dive, and how these animals transition between aerobic and anaerobic metabolism at depth. Specifically, I 1) determined how post-dive rates of O₂ and CO₂ gas exchange are affected by dive behaviour, and 2) measured how lactate accumulates with increased dive time, and examined how this indicator of metabolic transition affected post-dive recovery times. To measure gas exchange, I used flow-through respirometry to determine the time required for Steller sea lions (Eumetopias jubatus) to reach within 5% of stable rates of O₂ uptake and CO₂ excretion following a dive. These times were interpreted as the O₂ and CO₂ recovery times, respectively. CO₂ recovery time was longer and became more extended with increasing dive time when compared to O₂, requiring an extra 44 sec per minute submerged for CO₂ as opposed to 33 sec per minute submerged for O₂. This indicates that recovery time was limited by CO₂ as opposed to O₂, and this difference became greater with increased dive time. Contrary to traditional models, plasma lactate concentration was present even after short dives, and increased linearly with dive duration. Neither O₂ nor CO₂ recovery rates were affected by levels of blood lactate. This indicates that anaerobic metabolism may be used long before the body’s total O₂ -stores have been consumed. These results support the idea that there is not a distinct threshold between aerobic and anaerobic pathways, but rather a progressive transition, which casts doubt on the usual interpretations of the ADL and cADL. My thesis challenges long-held assertions in diving physiology, and underlines the need to further examine how CO₂ and lactate accumulation may act as limits to diving behaviour.

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Seasonal changes in the daily energy expenditure (DEE) of captive northern fur seals (Callorhinus ursinus) and key components of their energy budget (cost of resting metabolism, thermoregulation, activity and growth) were examined to elucidate potential reasons for the species’ population decline in the wild. The average DEE of 6 females was 527.8 ± 65.7 kJ kg-¹ d-¹ and fluctuated seasonally (~20% greater in the fall than in the winter). Resting metabolism also changed significantly with season, and was higher in the fall (potentially due to molting or as preparation for migratory activity). While resting metabolism was the largest component of the DEE (~80% on average), it did not follow the same seasonal trend as DEE, and therefore was not the source of the seasonal variation in DEE. Cost of activity was the second major component of DEE and may explain the observed seasonal variations. Energetic costs associated with thermoregulation appeared to be negligible. The northern fur seals were thermally neutral in all seasons for all water temperatures tested (2 °C – 18 °C), except during the summer when immersed in 2 °C water. Comparing this broad thermal neutral zone to the average sea surface temperatures encountered by fur seals in the wild during annual migrations indicates that fur seals can likely exploit a large geographic area without added thermal metabolic costs. While the direct energetic costs of growth appeared to be negligible compared to DEE, the higher growth rates in the summer and elevated resting metabolism in the fall suggests that inadequate nutrition could have greater negative effects during these seasons. Two alternative proxies for measuring energy expenditure were tested and calibrated against respirometry for potential application to wild individuals. The doubly labeled water (DLW) method over-estimated DEE by 13.1 ± 16.5% compared to respirometry. In comparison, accelerometry over-estimated DEE, using fine time scale intervals of 60 and 15 min, by an average of 5.4 ± 29.3% and 13.8 ± 39.5%, respectively. Importantly, seasonal effects (and time of day for accelerometry) must be accounted for when estimating energy expenditure from measures of DLW and acceleration in free-swimming northern fur seals.

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The diving ability of marine mammals is limited by body oxygen stores (TBO) and rates of oxygen depletion (diving metabolic rate; DMR), which can be expressed as the calculated aerobic dive limit (cADL). Diving ability must also be influenced by CO₂ production and control of ventilation. I investigated the factors that limit the diving ability of Steller sea lions (Eumetopias jubatus), including the effect of nutritional stress on the cADL. Specifically, I 1) determined the cADL of Steller sea lions by measuring TBO and DMR, 2) determined whether nutritional stress alters the cADL and 3) examined the post-dive elimination of CO₂, and the sensitivity of Steller sea lions to hypercapnia (high inspired CO₂). TBO was estimated from measured blood oxygen stores and body composition―and metabolic rate, breathing frequency and dive behaviour were recorded prior to and during a period of nutritional stress where animals lost ~10% of their mass. Animals breathed ambient, hypercapnic or hypoxic (low O₂) air to experimentally alter pCO₂ levels and decrease rates of CO₂ elimination and O₂ consumption. I found that the TBO (35.9 ml O₂ kg-¹) and cADL (3.0 minutes) in actively diving Steller sea lions were lower than previously reported for other species of sea lions and fur seals. I also found a significant increase in mass-specific DMR and blood volume (resulting in higher TBO) in nutritionally stressed animals that resulted in a longer cADL. Hypercapnia was found to significantly affect ventilation, but had no effect on dive behaviour―and elimination of CO₂ between dives took longer than replenishing O₂ stores. Overall, nutritional stress and hypercapnic conditions did not directly limit the diving ability of the Steller sea lions, but had an indirect effect on foraging efficiency by increasing the time they spent on the surface between dives. Accumulation of CO₂ over several dives in a foraging bout also appeared to reduce foraging efficiency, which likely ultimately limits the time a sea lion spends in apnea and therefore overall foraging duration and net energy intake.

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