Andrew Trites

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Biologging tags that record high-resolution tri-axial accelerometry data are proving to be integral to the study of foraging ecology of large, free-roaming marine mammals, such as whales. They have been applied to a number of baleen whale species that feed pelagically through lunges or ram filtration to quantitatively define behaviours and estimate energetic costs. However, fewbehavioural ecology studies using accelerometry data have been conducted on grey whales, a unique baleen whale that performs benthic suction feeding. Using suction cup tri-axial accelerometer tag deployments on 10 Pacific Coast Feeding Group (PCFG) grey whales along the Oregon and Washington coasts, I defined signals of foraging behaviour at both the broad state (dive) and foraging tactic (roll event) scales. I then estimated the relative energetic cost of these behaviours using energy expenditure proxies derived from the accelerometry data—Overall Dynamic Body Acceleration (ODBA; ms⁻²), stroke rate (Hz), stroke amplitude (radians per s), and duration of dives with different foraging tactics performed (min). Hidden Markov Models (HMMs) defined three biologically distinct states—forage, search, and transit—using turn angle, dive duration, dive tortuosity and presence of roll events. Classification and Regression Tree (CART) models best defined the foraging tactics of headstands, benthic digs, and side swims using median pitch, depth to body length ratio, and absolute value of the median roll. These definitions of grey whale foraging signals using accelerometry data add to the quantitative descriptions of foraging behaviours previously described for baleen whales. Stroke rate identified foraging and headstanding as being the most energetically costly activities at the broad state and foraging tactic scales. These findings contribute to the foundational understanding of grey whale foraging energetics needed to assess the impacts of various conservation concerns on the fitness and interpret patterns of behaviour choice of this unique group of grey whales.

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An array of foragers prey on salmon in rivers and estuaries while salmon smolts out-migrate from their natal streams—and may account, in part, for the poor returns of adult salmon to the Salish Sea. However, the Pacific great blue heron (Ardea herodias fannini) has not been identified as a predator of smolts despite being regularly seen near salmon streams. I investigated the role that herons may be playing in the depredation of salmon by scanning fecal remains under heron nests for Passive Integrated Transponder (PIT) tags that had been implanted in wild and hatchery-reared salmon smolts from 2008-2018. These nests were located in three heron rookeries that were within 35 km of the mouth of the Cowichan, Big Qualicum, and Capilano Rivers. Using a mobile PIT antenna, I recovered 1,199 smolt tags, representing a minimum annual predation rate of 0.3–1.3% of all smolts in the three rivers. Correcting for tags consumed by herons and defecated outside of the rookery raised the estimated proportion of smolts to 0.7–3.2% of the outmigrating fish, but predation rates as high as 6% were documented during a low river-flow year in the Cowichan River. The distribution and timing of tag depositions under the heron nests indicated that most great blue herons prey on salmon smolts and that consumption occurs in late spring during the chick-rearing phase of the breeding season. Energetic analyses suggest that smolt consumption provides a substantial proportion of the heron chick diet during a time of peak energy demand. Predation on smolts occurred primarily in the lower rivers and upper estuaries. Smaller salmon smolts were significantly more susceptible to heron predation in all systems, and predation rates were comparable between wild and hatchery-reared smolts. Recovering so many tags from smolts at heron rookeries was unexpected and indicates that great blue herons are a new predator of wild and hatchery-reared juvenile salmon. Locations of heron rookeries relative to salmon bearing rivers are likely good predictors of heron impacts on local salmon runs, and a potential means to assess coast-wide effects of great blue herons on salmon recovery.

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The hearts of marine mammals frequently sustain marked changes in heart rate and vascular resistance when diving. However, it is not known how marine mammal hearts facilitate these changes. I examined cardiac function and electrical activity of marine mammal hearts to understand how they might differ from terrestrial mammals. I measured electrocardiographic parameters in 8 Steller sea lions, 5 northern fur seals, and 1 walrus—and echocardiographic function in all 8 Steller sea lions. I also compiled electrocardiographic parameters from 17 species of marine mammals (including my measurements) for comparison with 50 species of terrestrial mammals. I found that atrial and ventricular depolarization are slower in marine mammals after accounting for differences in body mass—and that the left ventricle of Steller sea lions contracts less than expected for a mammal of that size. These differences in cardiac timing and function may reflect specialized adaptations for diving. Electrocardiographic measurements of Steller sea lions, northern fur seals, and a walrus also varied between species and among the individuals of each species. For example, sinus arrhythmias occurred in 5 out of 8 individual Steller sea lions, but not in northern fur seals or the walrus. Mean electrical axes were also unique to each individual and varied greatly. Measurements ranged from -124° to 80°—with 3 of the Steller sea lions having extreme right axis deviation (-111° to -124°). Echocardiographic measurements showed that left ventricular form was similar between Steller sea lions and terrestrial mammals, except that Steller sea lions have larger aortic roots and larger left ventricular end-systolic dimensions than terrestrial mammals. Overall, my results show that marine mammals have functionally similar hearts to terrestrial mammals with a number of notable differences that likely support anatomical adaptations to diving.

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There is increasing evidence that predation by harbour seals (Phoca vitulina) on salmon smolts out-migrating from rivers may be a significant source of mortality for coho (Oncorhynchus kisutch) and Chinook (O. tshawytscha) salmon populations in British Columbia. Studies supporting this have focused on documenting what and how much seals eat—and the potential impact this has on salmon populations. However, little attention has been given to understanding where, when and how this predation occurs, and the extent to which it might be opportunistic or specialist feeding behaviour by a few or many individuals. I documented the spatiotemporal foraging behaviour of harbour seals in the Salish Sea by equipping 17 seals with biologgers—and tracking them before and after the release of thousands of coho and Chinook smolts from the Big Qualicum Hatchery. Reconstructing the high-resolution movements of the seals―and quantifying feeding using counts of prey chasing events (PCEs) detected by accelerometry—revealed that the Big Qualicum estuary was a feeding hotspot for 47.0% of the seals, but was relatively small (accounting for 3% of PCEs) compared to the largest feeding area outside the estuary (26% of PCEs). Comparing the foraging behaviours of smolt specialists with non-specialist seals revealed 4 different foraging strategies. One consisted of seals (17.6%) that fed on coho smolts and ignored Chinook in the river mouth, while a second group of seals (17.6%) appeared to target larger fish that preyed on Chinook smolts near the estuary. The two other seal groups did not feed in the estuary in association with the concentrated numbers of smolts, but either remained resident (52.9%) and fed near their main haulouts, or were transient (11.8%) and left the study area. My results suggest a high degree of individual foraging specialization—and show that a small number of seals were specialized in consuming coho smolts, but did not appear to respond to the large pulse of smaller bodied Chinook smolts during the outmigration. Such information concerning the fine-scale foraging behaviour of harbour seals in relation to pulses of out-migrating smolts can be used to design mitigation strategies to enhance coho and Chinook populations.

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Bioenergetic studies can quantify the conversion of chemical energy contained in food to biologically useful energy to understand how changes in diet quality and quantity affect overall energy budgets and nutritional status. However, chemical energy is intrinsically linked to the macronutrients contained in food (i.e., lipid and protein) in terms of energetic density and digestive efficiency. For northern fur seals (Callorhinus ursinus) it is unknown how efficiently they transform dietary gross energy to net energy. I fed six trained adult female fur seals eight experimental diets composed of four prey species (capelin, walleye pollock, Pacific herring, and Magister squid), alone or combined. I measured the fur seals’ digestive efficiency for energy and macronutrients across diets. I also investigated the effect of dietary intake on digestive efficiency, and tested the hypothesis that mixed-species diets provide a greater nutritional return than equivalent single-species diets. I quantified net energy uptake by measuring excreta energy loss and measuring heat increment of feeding. My results revealed significant differences between digestive parameters across diets. I found that digestible energy (95.9–96.7%) was negatively affected by both ingested mass and dietary crude protein. Furthermore, urinary energy loss (9.3–26.7%) increased significantly with increases in dietary crude protein. I also found that the heat increment of feeding (4.3–12.4%) increased with decreasing dietary lipid content. Overall, net energy gain (57.9–83.0%) was positively correlated with lipid content. I found that macronutrient digestibility differed across diets and that, overall, lipids were more digestible (96.0–98.4%) than crude proteins (95.7–96.7%). Also, dietary protein influenced the ability of fur seals to digest lipids and proteins. Overall, my results demonstrate that low lipid prey not only contain less gross energy, but result in proportionally lower net energy gain following digestion, partly due to decreasing digestibility of lipids in high protein diets. I also found that, counter to predictions, mixed-species diets do not provide fur seals with greater energetic or macronutrient gains than single-species diets. These findings contribute to understanding the nutritional ecology of northern fur seals and the impact that changes in diet can have on the fur seals’ nutritional state.

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Air-breathing divers, such as marine mammals, should adjust their diving behaviours in relation to the depth and density of their prey to minimize the energetic costs and maximize the benefits of foraging. However, there is little experimental data to test these predictions or to develop models to predict the responses of marine mammals to changes in prey availability. The objectives of my study were to 1) determine how changes in prey availability affect dive behaviour and foraging efficiency in Steller sea lions (Eumetopias jubatus) and 2) develop models with data from free-diving captive Steller sea lions to estimate foraging costs in wild animals and evaluate energetic trade-offs between different foraging strategies. I measured the diving metabolic rate, dive durations, and food intake of 4 trained sea lions diving in the open ocean on simulated prey patches of high- or low-densities at 10 m and 40 m. I also measured diving metabolic rates of sea lions performing 4 controlled dive types that allowed me to estimate the separate costs of different dive components (i.e., surface time, bottom time, and transiting to and from depth). I found that animals diving on prey patches with low prey density altered their dive behaviours and spent proportionally less time actively foraging, which ultimately decreased their foraging efficiency. I also found that making single, longer dives were less energetically costly than making multiple shorter dives in a bout, but that the sea lions replenished oxygen stores more efficiently when making a bout of dives. Finally, I determined the metabolic cost of transiting to and from depth (20.5±13.0 ml O₂ min₋¹ kg₋¹) was greater than the cost of foraging during the bottom portion of a dive (13.5±4.1 ml O₂ min₋¹ kg₋¹). With these values, I generated a predictive equation to estimate the diving costs of free-ranging animals. Overall, my results indicate that Steller sea lions do alter their dive behaviour in relation to prey availability and that different foraging strategies have different energetic costs. These results can be used to understand how changes in prey availability affect the overall energy balance and health of Steller sea lions.

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Fin whales are acoustically active year-round, particularly during the winter breeding season when males produce song as a reproductive display. Little is known about movement patterns and population structure of the fin whale (Balaenoptera physalus) in the North Pacific. I used passive acoustic data recorded using bottom-mounted autonomous recording instruments to answer questions about the winter distribution, behaviour and population structure of fin whales in a high-latitude area of the eastern North Pacific Ocean. I found that fin whales are present in British Columbia (BC) waters throughout the winter, and their acoustic behaviour suggests that they are engaging in reproductive and possibly feeding behaviour. The presence of these individuals indicates that not all fin whales migrate south in the winter to low-latitude breeding grounds. I found that the majority of fin whales recorded in BC sang a doublet song with alternating ~13 s and ~17 s intervals between alternating classic and backbeat notes (song Type 2). At the most northerly recording site, and the two offshore sites, I found a few instances of a previously described doublet song type that has longer internote intervals (song Type 1). This suggests that two spatially segregated populations of fin whales are using BC waters, with the population producing song Type 2 being more coastal. Through a literature search, I found evidence of song Type 2 occurring from northern BC to Oregon. Song Type 1 has been documented off Southern California, the Bering Sea and the central North Pacific. My results are consistent with evidence showing that two genetically distinct fin whale populations occur in the eastern North Pacific, and that the population that produces song Type 2 is more closely related to the Southern Hemisphere subspecies of fin whale, than to the other population of fin whales in the North Pacific. My results show that BC waters are important for fin whales year-round, rather than just during the summer feeding season―and that the fin whales in BC likely belong to two distinct populations (one of which may belong to the Southern Hemisphere subspecies) and may require separate plans for their conservation and management.

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Foragers with narrow dietary niches often exhibit specialized hunting behaviours that improve their efficiency for capturing preferred prey, but can leave them vulnerable if the abundance of this prey declines. I examined the specificity of foraging behaviour by a highly selective predator, the northern resident killer whale (Orcinus orca), which specializes on Chinook salmon (Oncorhynchus tshawytscha). Northern residents are undoubtedly well adapted to capture Chinook, however, their hunting tactics have never been described due to the challenges of quantifying underwater behaviour. To address this research gap, I deployed archival tags (DTAGs) on 32 killer whales to measure their acoustic and kinematic behaviour during foraging dives. Reconstructed 3-dimensional tag tracks indicated that foraging and non-foraging dives were kinematically distinct. While engaged in hunting behaviour, whales dove deeper, remained submerged longer, swam faster, increased their dive path tortuosity, and rolled their bodies more than during other activities. Maximum foraging dive depths reflected both the deeper vertical distribution of Chinook (compared to other salmonids), as well as the tendency of these fish to evade predation by diving steeply. Inferences from whale movements further revealed that salmon engaged in other anti-predation strategies, including increasing swim speeds and evasive manoeuvring. DTAG records also provided the first definitive link between echolocation and prey captures by resident killer whales, who displayed significantly higher clicking rates and spent proportionally more time echolocating prior to capturing a fish than they did afterward. Rapid ‘buzz’ click sequences were often produced before fish captures, which is consistent with their hypothesized function of close-range prey targeting. Furthermore, prey handling ‘crunches’ were usually detected following kills and, with buzzes, provide possible acoustic proxies for capture attempts and successes that could be used to estimate foraging efficiency. My thesis determined that northern resident killer whales possess specialized foraging behaviours for targeting Chinook salmon. The specificity of these behaviours may make the whales less effective at capturing other types of fish. If northern residents have limited flexibility to modify their foraging behaviour to successfully exploit other prey types when Chinook availability is reduced, prey capture efficiency (and thus per capita energy intake) could decline.

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The core breeding population of northern fur seals (Callorhinus ursinus) in North America has declined significantly since the 1980s on St. Paul Island (one of the Pribilof Islands) while the smaller nearby population at Bogoslof Island (eastern Bering Sea) has increased exponentially. Further south, the population of northern fur seals on San Miguel Island off the coast of Southern California has fluctuated between exponential growth and catastrophic declines associated with re-occurring El Nino events. The goal of my thesis was to assess the physiological status of these three breeding populations of northern fur seals in North America to determine whether nutritional differences could explain the different population trajectories. I collected fecal samples (scats) in July 2009 from these three islands and measured the fecal metabolites of two hormones — a glucocorticoid associated with the stress response, and triiodothyronine (T3), a thyroid hormone associated with metabolic rate. I also assessed feeding conditions using diet and foraging data. I found that sub-adult males and lactating females on St. Paul Island experienced poorer feeding conditions (lower energy content food and longer feeding trips for lactating females) than at Bogoslof Island, but that only the females were nutritionally stressed. I also found that the San Miguel Island population differed physiologically compared to the northern populations in Alaska in terms of stress and nutritional status. The San Miguel fur seals were the most physiologically stressed of the North American fur seal populations (based on elevated levels of glucocorticoid metabolites). However, the stress was most likely related to heat stress and not food (based on low concentrations of T3 metabolites). The available hormone, diet, and foraging data from northern fur seals in North America suggest that lactating females were nutritionally stressed on St. Paul Island and heat stressed on San Miguel Island, and experienced better conditions on Bogoslof Island.

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Habitat characteristics and mechanisms that enable predators to successfully forage are poorly understood in open marine ecosystems. I addressed this problem in the eastern Bering Sea using animal-born data-loggers carried by lactating northern fur seals (Callorhinus ursinus) from two populations breeding in distinct oceanographic zones — a declining population on St. Paul Island on the continental shelf, and an increasing population on Bogoslof Island over the oceanic basin. The data-loggers recorded water temperatures, dive depths and animal locations throughout foraging trips that lasted as long as 17 days and extended as far as 460 km from the islands. I contrasted tag-derived ocean temperatures with concurrent shipboard measurements and found that the fur seal data revealed finer-scale hydrographic processes with less estimated error than ship-derived data, particularly in dynamic oceanographic areas. I also identified probable foraging hotspots using first-passage time analysis of at-sea locations of individual females, and linked them to fine-scale hydrographic data using habitat selection models. I found that hot spots were related to thermoclines and surface fronts (although not with water temperature), and that the relationships differed between populations and among foraging strategies. St. Paul Island fur seals that mixed epipelagic and benthic dives focused their effort in areas with deeper thermoclines that may concentrate prey closer to the ocean floor, while strictly epipelagic foragers tended to use waters with shallower thermoclines allowing prey to migrate closer to the surface. Fur seals from Bogoslof Island foraged almost exclusively over the Bering Sea basin and appeared to hunt intensively along the fine-scale fronts that surrounded the island while fur seals from St. Paul Island extended their trips off-shelf to forage in areas with similar oceanographic features. It appears that lactating females rely on fine-scale boundaries in the open ocean to effectively concentrate prey, and that the relative distribution and accessibility of these oceanographic features account for the inter-island differences in foraging patterns. Collectively, my thesis shows that wide-ranging, diving animals such as fur seals can be used to produce detailed maps of marine habitat and demonstrates the importance of fine-scale habitat characteristics to top predators foraging in dynamic oceanographic environments.

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Management plans for North Atlantic right whales (Eubalaena glacialis) focus on preventing mortality from ship strikes and fishing gear entanglement. However, population recovery may also be limited by nutritional stress. I derived growth curves and quantified the food requirements of North Atlantic right whales by age, sex and reproductive state. I also compared their predicted needs with field estimates of prey consumption to evaluate the model predictions and consider whether different demographic groups of right whales might be nutritionally stressed. Energy requirements were estimated using a bioenergetics model that incorporated uncertainty in energy inputs and outputs. Consumption was estimated with prey samples taken near feeding whales in two critical feeding habitats—Cape Cod Bay (n=28 net collections) and the Bay of Fundy (n=19 optical plankton counts). Model predictions indicate that mothers invest heavily in their calves, which effectively double in size and attain ~73% of their mother’s length by weaning at one year of age. Calves gained an average of ~1.7 cm and ~34 kg per day while nursing during this rapid growth phase. Body growth was best described using a two-phased Gompertz model and could not be fit using any of the single continuous growth models commonly used for other mammals. Energetically, calves required the least energy (~1129 MJ/day) and lactating females required the most (~2934 MJ/day). Adult males and non-reproductive females fell in between at ~1140 and ~1217 MJ/day respectively. Estimates of energy requirements for juveniles, adult males, pregnant and non-reproductive females compared favorably with estimates of actual prey consumption in their winter habitat (i.e., they differed by ≤15%), suggesting that the model was reliable. However, lactating females appear to obtain considerably less (~45%) of their predicted daily energy requirements in Cape Cod Bay, and almost met their needs in the Bay of Fundy (obtaining ~87% of daily requirements). This suggests that lactating females may be experiencing an energy deficit, which may affect reproductive rates and slow population recovery. Nutritional stress may thus be limiting the recovery of North Atlantic right whales.

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South American fur seals (Arctocephalus australis) continue to survive in Peru in spite of commercial harvesting, periodic disappearance of prey (i.e., El Niño), and competition with the Peruvian anchoveta fishery. I investigated the ability of the Peruvian population of fur seals to recover from catastrophic declines at two temporal and spatial scales. The first analysis determined intrinsic rate of growth (r) and the potential carrying capacity (K*—the number of fur seals that could be supported in Peru in the absence of sealing and El Niño) from 1880–2010, and the second used pup counts from 1984–2010 to determine the relationship between prey abundance and the timing of pupping at an important fur seal breeding site in southern Peru. Model results indicated that South American fur seals in Peru have an intrinsic growth rate r of 0.20 and a potential carrying capacity K* of 115,000 seals. Recent counts (2007) show that current population is at 33% of the estimated mean numbers of fur seals alive from 1880-1925. Analysis of 25 years of counts of pups and adult females at the breeding site showed a correlation between anchoveta biomass and mean birth dates (r² = 0.59, P
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Pacific white-sided dolphins (Lagenorhynchus obliquidens) are one of the most abundant cetaceans in British Columbia and throughout the North Pacific Ocean. However, little is known about their seasonal distributions and energy requirements. I analyzed sightings of dolphins attained opportunistically by volunteer observers and from scientific surveys—and found that Pacific white-sided dolphins have been seen with increased frequency along the BC coast over the past 54 years, and seasonally over the past 8 years. The sightings data showed a southward range shift from the 1950s to 2010, and a seasonal movement from offshore to nearshore waters concurrent with the timing of the herring spawn on the BC coast. I deduced whether seasonal movements reflect seasonal shifts in energy requirements by measuring resting metabolic rates and total energy intake for three captive white-sided dolphins twice per month for one year. Open-circuit gas respirometry revealed relatively high resting metabolic rates (~30 MJ day⁻¹ or ~0.3 MJ kg⁻¹day⁻¹) suggesting that white-sided dolphins may need high-energy prey to fuel their energetic requirements. Average resting metabolic rates of the three dolphins were constant throughout the year despite an increase in food consumption in the fall (October to December). I used these average resting metabolic rates and other parameters associated with growth, activity and assimilation efficiency to inform a generalized bioenergetic model and estimate the food requirements of Pacific white-sided dolphins globally, regionally, and locally. My bioenergetic model predicted that wild dolphins require ~30 MJ day⁻¹ for calves, ~60 MJ day⁻¹ for juveniles, ~65 MJ day⁻¹ for adults and pregnant females, and ~90 MJ day⁻¹ for lactating females. These energy requirements are ~50% higher than observed for dolphins fed in captivity, and are generally higher than estimates for other similar sized small cetaceans inhabiting temperate waters. My model predicts that an average sized dolphin (78 kg) in the wild would consume ~10 kg of fish per day, or about 13% of its bodyweight. Pairing information about prey requirements and seasonal distributions of dolphins with fisheries data can be used to assess spatial overlap between dolphins and fisheries, and may assist in reducing entanglement, by-catch, and conflict over prey.

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A leading hypothesis to explain the decline of Steller sea lions (Eumetopias jubatus) in western Alaska is the reduction of prey abundance or change in prey distributions caused by commercial fisheries. We sought to improve on past studies that attempted to assess competition between sea lions and fisheries by estimating the local amounts of prey accessible to sea lions. We explored the relationships between sea lion population trends, fishery catches and the prey biomass accessible to sea lions around 33 rookeries from 2000-2008. We focused on three commercially important species that dominate the sea lion diet: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estimate the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass (estimated using our accessibility model and also within 10, 20 and 50 km of each rookery). Of the 304 statistical models we constructed to compare accessible prey biomass and catch to sea lion population trends, only three relationships were significant. These three suggest that sea lion population change rates increased (became less negative) with increasing accessible pollock biomass in the Aleutian Islands and with cod biomass in the Gulf of Alaska. No relationships were found between sea lion population trends and Atka mackerel biomass. Given that the majority of the relationships we explored were insignificant, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s. Sea lion population trends appeared to be affected by some unknown factor associated with regional differences. Removing fish catches or adding catch to our predicted distributions of groundfish abundances had no measurable effect on sea lion population trends. These observations suggest that sea lion populations were largely unaffected by fishery removals during this period.

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Determining diets of pinnipeds by visually identifying prey remains recovered in faecal samples is challenging because of differences in digestion and passage rates of hard parts. Analyzing the soft matrix of faecal material using DNA-based techniques is an alternative means to identify prey species consumed, but published techniques are largely non-quantitative, which limits their applicability. I developed and validated a real-time PCR technique using species-specific mitochondrial DNA primers to quantify the diets of Steller sea lions (Eumetopias jubatus). I first demonstrated that the proportions of prey tissue DNA in mixtures of DNA isolated from four prey species could be estimated within a margin of ~12% of the percent in the mix. These prey species included herring Clupea palasii, eulachon Thaleichthyes pacificus, squid Loligo opalescens and rosethorn rockfish Sebastes helvomaculatus. I then applied real-time PCR to DNA extracted from faecal samples obtained from Steller sea lions that had been fed 11 different combinations of herring, eulachon, squid and Pacific ocean perch rockfish (Sebastes alutus), ranging from 7-75% contributions to a meal mix (by wet weight). The difference between the average percentage estimated by real-time PCR and the percentage of prey consumed was generally less than 12% for all diets fed when percentages of prey consumed were corrected for differences in mtDNA density among the prey items. My findings indicate that real-time PCR can detect the quantity of prey consumed for a variety of complex diets and prey species, including cephalopods and fish.

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Despite its essential role in bioenergetic modeling, reliable measures of energy expenditure (i.e., oxygen consumption) associated with the different activities of wild animals have remained elusive. Oxygen consumption rate (VO₂) associated with activity can be estimated as a function of heart rate (fh), and the empirical relationship between the two has been determined for several aquatic vertebrates while fasting and resting. However, the simplified fh:VO₂ relationships established from such studies may differ under more complex physiological circumstances, such as when animals are foraging at depth or feeding on prey. I assessed the efficacy of using fh to predict VO₂ in 7 captive Steller sea lions, Eumetopias jubatus, while fasting and feeding at rest (on land or in water) and while diving (up to 40 m in the open ocean). Linear mixed-effects models revealed that environment, amount of food fed, and type of diving activity all altered the fh:VO₂ relationship. They also showed that different linear equations are needed to predict VO₂ from fh for sea lions fasted while on land or in water, but that a single equation can predict VO₂ on land regardless of whether fasted or feeding. When in water, feeding animals a 4, 6, or 12 kg meal changed the fh:VO₂ relationship compared to fasted animals. While fh can reliably be used to predict VO₂ in diving sea lions, the relationship differed between single dive cycles (one dive +surface interval) and dive bout cycles (multiple dives+surface intervals). However, the equation that predicted VO₂ for single dive cycles did not differ from that for sea lions resting on the surface. Neither dive duration, dive depth, nor food consumed significantly affected the fh:VO₂ relationships. Heart rate could be used to predict VO₂ in diving sea lions, but only over complete dive cycles or dive bouts where animals recovered fully from the O₂ debt incurred underwater. Based on these results, separate equations that distinguish among environmental, digestive, and diving states can be employed to accurately predict VO₂ from heart rate in wild Steller sea lions.

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