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In killer whales, fish- versus mammal-eating ecological differences are regarded as key ecological drivers of sociality, but the potential influence of specific target prey characteristics remains unclear. This thesis aimed to study the social patterns and dynamics of Icelandic killer whales feeding upon herring, a schooling prey that undergoes frequent changes in distribution and school size. I used a multi-disciplinary approach combining photo-identification and genetic data to understand the sociality, role of kinship and genetic differentiation within the population. Individuals sighted in summer-spawning and overwintering herring grounds during at least five separate days (N = 198) were considered associated if photographed within 20 seconds of each other. Photo-identified individuals were genotyped (N = 61) for 22 microsatellites and mitochondrial DNA control region (611 bp). The population had weak but non-random associations, fission-fusion dynamics at the individual level and seasonal patterns of preferred associations. The society was significantly structured but not hierarchically. Social clusters were highly diverse and, whilst kinship was correlated with association, it was not a prerequisite for social membership. Indeed, some cluster members had different mitochondrial haplotypes, representing separate maternal lineages. Individuals with different observed movement patterns were genetically distinct, but associated with each other. No sex-biased dispersal or inbreeding was detected. This study revealed that the Icelandic population has a multilevel society without clear hierarchical tiers or nested coherent social units, different from the well-studied salmon- (‘residents') and seal-eating populations in the Northeast Pacific. In the Icelandic population kinship drives social structure less strongly than in residents. These findings suggest effective foraging on schooling herring in seasonal grounds promotes the formation of flexible social groupings which can include non-kin. Killer whale sociality may be strongly influenced by local ecological context, such as the characteristics of the specific target prey (e.g., predictability, biomass, and density) and subsequent foraging strategies of the population.

The northern Norwegian killer whale (Orcinus orca) is an important predator but little is known about its population dynamics, particular in response to changes in its main prey, the highly dynamic Norwegian spring spawning (NSS) herring (Clupea harengus). The main aims of this thesis were to estimate killer whale population parameters, to explore the future viability of the population, and to explore the response of this predator to changes in distribution and abundance of its main prey over the last 25 years. Population size was estimated as ~ 700 individuals, taking heterogeneity of capture probabilities into account and correcting for unmarked animals. Apparent survival rates of 0.974 (SE = 0.006) for adult males and 0.984 (SE = 0.006) for adult females were estimated accounting for temporary emigration, transience and trap-dependency. Temporary emigration was greater for males than females. Calving intervals ranged from 3 to 14 years (mean = 5.06); equivalent to 0.197 calves per mature female per year. Future viability of the killer whale population was evaluated under various plausible scenarios. The baseline scenario using the best available information predicted a viable population and indicated that the population may be increasing size. Analysis of data on naval sonar activity, killer whale sightings and herring abundance showed that naval sonar activity appeared to have a negative effect on killer whale presence during a period of low prey availability. A time lag of four years was found between the first sign of NSS herring changing its distribution and reduced killer whale presence inside the fjord system. Analysis of energy budgets showed that killer whales spent more time travelling/foraging in 2005/06 than the 1990s. The fjord system was inferred to be a preferred habitat for killer whales when there was a higher density of NSS herring in this area compared to offshore area.

For the majority of social species, group composition is dynamic, and individuals are interconnected in a heterogeneous social network. In this study I investigate the mechanisms underpinning social structure in the endangered southern resident killer whale (Orcinus Orca) population using a long term dataset, and explore the consequences of these. My results demonstrate that resource availability may be an important determinant of social network structure. A significant relationship between the connectivity of the social network and salmon abundance occurred, with a more interconnected network in years of high salmon abundance. As networks are non-random, highly connected individuals may play a key role in population processes such as information and disease transmission. While associations occurred both within and between matrilines, females had a significantly higher number of associates than males, as did older individuals of both sexes. Older males played a more important role in interconnecting the network. The attributes of group leadership were then investigated in matrilines and in individuals. Leadership was not a factor of size or mean age of matriline. However, there was a significant relationship between leadership score and the matriline sex ratio. Individually, females had higher leadership scores than males, and there was a positive correlation between leadership score and age in both sexes. I suggest that the oldest females have the highest 4 leadership scores due to increased ecological knowledge that comes with a prolonged lifespan. Using multi-generational records for two populations of killer whales, I show that both reproductive and post-reproductive mothers increase the survival of offspring, particularly in older male offspring. This is consistent with theoretical predictions, and may explain why female killer-whales have evolved the longest post-reproductive lifespan of all non-human animals. Given the role that individuals of high network centrality can play in population processes, understanding the driving forces behind social network structure is vital when designing effective conservation and management plans.

In this thesis I take a multi-disciplinary approach to identify and characterise ‘Evolutionary Significant Units’ (ESUs) and ‘Management Units’ (MUs) of killer whale in the eastern North Atlantic. Several markers and traits including mitochondrial DNA control region, 15N stable isotope values, tooth wear, tooth count, total body length and pigmentation pattern indicated sympatric lineages could be categorized in to two ecologically & morphologically disparate types, which should be considered as distinct ESUs. One type (type 2) appears to be a specialist and may predate cetaceans. The other type (type 1) appears to be a generalist, although the total niche width of the population appears to be more dependent upon between-individual variation than within-individual variation in dietary composition. However, the other indicator of long-term diet, apical tooth wear, was found in all type 1 individuals, suggesting some overlap in the components of the diet. There are shallow genetic differences between the two types based on mtDNA control region, however analysis of bi-parentally inherited nuclear DNA markers are needed to see if the two types are reproductively isolated. The data above on diet is further supported by observational data of individuals moving between the Icelandic herring grounds to the seal pupping haul-outs around the Northern Isles, Scotland. However, large scale movement of individuals is mostly correlated with the movement of large predictable prey stocks such as the Icelandic and Norwegian stocks of Atlantic herring. Therefore there is the potential for prey choice to cause intrinsic isolation through temporal or spatial isolation even when the prey type is similar, e.g. mackerel, Icelandic herring or Norwegian herring. The microsatellite data are consistent with this pattern and taken together the genetic and mark-recapture data identify four demographically independent MUs of type 1 killer whales. We only identify one community or MU of type 2 killer whales, which is small (10 individuals) and appears to be suffering from demographic stochasticity. Our results suggest intrinsic isolation through temporal and spatial isolation as the most parsimonious mechanism for reducing gene flow between populations of the same type. Further work is needed to determine if there is gene flow between types or if adaptive divergence feeds back to reduce gene flow through mate choice.

Echolocation click features are compared for three different killer whale ecotypes in the Northeast Pacific off the coast of Washington. A dataset consisting of manually labeled killer whale encounters identified to ecotype based on pulsed call matching was used to generate echolocation click archetypes for resident, transient and offshore killer whales. Results indicate that resident and offshore killer whales can be distinguished acoustically based on the spectral shape and inter-click intervals of their echolocation clicks. Efficient, semi-automated identification will allow expanded use of passive acoustic monitoring for management of these populations.

Food availability has been identified as a critical factor influencing the growth, individual health, and population dynamics of killer whales (Orcinus orca) in the coastal waters of the eastern North Pacific Ocean. Between 2014 and 2019 we conducted 113 hand-launched unmanned aerial system flights over mammal-eating Bigg’s killer whales around Vancouver Island, resulting in 20,545 aerial photographs of 95 individually identified animals. I conducted photogrammetric measurements from high-quality images in 91 individuals, a sample that ranged from first-year calves to mature adults of both sexes; this is the first study to estimate size, growth, and health of Bigg’s killer whales. Individual lengths ranged from a 2.4m neonate to an 8.3m 38-year-old male. Using a Richard’s growth curve model, I estimated asymptotic adult length at 6.4 ± 0.1m (standard error) in females and 7.3 ± 0.2m in males, as well as age of inflection at 14.2 ± 2.8 years in females and 18.4 ± 2.3 years in males. Comparison with sympatric salmon-eating Southern Resident killer whales found that both sexes of Bigg’s killer whale measured significantly longer than Southern Residents (female z-test P = 0.003; male z-test P = 0.093) but there was no significant difference in age of inflection between males (z-test, P = 0.53) or females (z-test, P = 0.45) between the two populations. Analysis of eye patch ratio (a proxy for body condition) revealed that all age/sex classes of Bigg’s killer whales were more robust than Southern Resident killer whales, and the difference was most significant when comparing calves (z-test, P < 0.0001) and juveniles (z-test, P < 0.0001) between the two populations. I propose that in the absence of major discrepancies in growth trends, morphometric divergences between the two populations are largely a function of prey availability.

Killer whales produce pulsed calls, which are used for communication. Calls are highly stereotyped and repertoires are unique to individual pods. Discrimination amongst these calls and comparison of call repertoires between pods can help determine population structure in killer whales and can be used to track pod movements. Calls were detected in underwater acoustic recordings in August and September 2017 in the Arctic waters of Eclipse Sound, in Nunavut, Canada. We present a repertoire of killer whale calls recorded. Eleven stereotypic call types, three biphonic and eight monophonic, were identified using manual call organization and manual whistle contour extraction. A higher diversity of calls was detected in the hydrophone located in the known narwhal aggregation site in Milne Inlet, than at the second hydrophone deployed at the mouth of Eclipse Sound which is the proposed entrance and exit point for the killer whales. The potential for increased killer whale presence and magnitude of predation on narwhals is a source of concern for management of the population and by Inuit subsistence hunters who rely on narwhals for food and economic benefit. Describing the acoustic repertoire of killer whales seasonally present in the Canadian Arctic may help understand their behavior and seasonal movements. The results presented may provide a basis for future acoustic comparisons across the North Atlantic and aid in characterizing killer whale ecotypes making seasonal incursions into Arctic waters.