Killer whales (Orcinus orca) feeding on herring (Clupea harengus) were observed both from the surface and underwater. We refer to one of the feeding techniques used by killer whales as the carousel method, whereby whales cooperatively herded herring into a tight ball close to the surface. During herding and feeding, whales swam around and under a school of herring, performing much lobtailing and porpoising. When the herring were gathered into a tight ball whales often swam with the white underside of their body towards the fish and emitted large bubbles close to the surface. While feeding, whales spent more time circling around the ball of fish than eating. Ball formation is a known defence mechanism used by schooling fish, and the effort by killer whales appeared to be directed towards keeping the ball very dense and close to the surface. The whales stunned their prey by slapping the edge of the school with the underside of their flukes and then ate the stunned fish one by one. The tail slaps created a loud banging sound which could have been either a by-product or an aid to stunning the prey.

Population genetic structure of North Atlantic killer whale samples was resolved from differences in allele frequencies of 17 microsatellite loci, mtDNA control region haplotype frequencies and for a subset of samples, using complete mitogenome sequences. Three significantly differentiated populations were identified. Differentiation based on microsatellite allele frequencies was greater between the two allopatric populations than between the two pairs of partially sympatric populations. Spatial clustering of individuals within each of these populations overlaps with the distribution of particular prey resources: herring, mackerel and tuna, which each population has been seen predating. Phylogenetic analyses using complete mitogenomes suggested two populations could have resulted from single founding events and subsequent matrilineal expansion. The third population, which was sampled at lower latitudes and lower density, consisted of maternal lineages from three highly divergent clades. Pairwise population differentiation was greater for estimates based on mtDNA control region haplotype frequencies than for estimates based on microsatellite allele frequencies, and there were no mitogenome haplotypes shared among populations. This suggests low or no female migration and that gene flow was primarily male mediated when populations spatially and temporally overlap. These results demonstrate that genetic differentiation can arise through resource specialization in the absence of physical barriers to gene flow.

Our objectives were to investigate the seasonal occurrence of photoidentified killer whale pods in relation to the distribution of Norwegian spring-spawning herring and whether or not pod-specific differences in the occurrence or diet of killer whales could be demonstrated. In a 4-year study, the killer whales occurred in different areas during the summer and the fall-winter, and these areas coincided with the distribution areas of herring. Killer whales were encountered most frequently during October-January when the whole adult and part of the adolescent stock of spring-spawning herring wintered in the study area. Thirty-nine killer whale pods were identified, and generally the same pods were encountered each year of the study. Differences in the timing and areas of occurrence of pods could be demonstrated. Adult and adolescent herring seemed to be the main prey of the killer whales during both fall-winter and summer. Six new species were added to the list of known prey species of Norwegian killer whales. The present pattern of seasonal occurrence of killer whales in the coastal waters of northern Norway is expected to change as a result of growth in the Norwegian spring-spawning herring stock. Résumé : Nous avions comme objectifs d'étudier l'apparition saisonnière de groupes d'épaulards photo-identifiés relativement à la distribution des harengs norvégiens se reproduisant au printemps, et de déterminer s'il y avait des différences spécifiques aux groupes dans le nombre ou le régime alimentaire des épaulards. Lors d'une étude de 4 ans, les épaulards ont été observés à différents endroits à l'été et pendant la période automne-hiver, et ces endroits correspondaient à l'aire de distribution des harengs. On observait surtout des épaulards durant la période allant d'octobre à janvier lorsque la totalité du stock d'adultes et qu'une partie du stock des adolescents de harengs se reproduisant au printemps passaient l'hiver dans l'aire d'étude. Trente-neuf groupes d'épaulards ont été dénombrés, et ce sont surtout les mêmes groupes qui étaient observés chaque année de l'étude. On a fait état de différences dans les moments et les endroits où les groupes étaient observés. Les harengs adultes et adolescents semblaient constituer la principale proie des épaulards, tant pendant la période automne-hiver qu'à l'été. Six nouvelles espèces ont été ajoutées à la liste des proies connues des épaulards norvégiens. La distribution actuelle des épaulards dans les eaux côtières du nord de la Norvège changera, prévoit-on, à la suite de la croissance du stock de harengs norvégiens se reproduisant au printemps. [Traduit par la Rédaction]

Our objectives were to investigate the seasonal occurrence of photoidentified killer whale pods in relation to the distribution of Norwegian spring-spawning herring and whether or not pod-specific differences in the occurrence or diet of killer whales could be demonstrated. In a 4-year study, the killer whales occurred in different areas during the summer and the fall-winter, and these areas coincided with the distribution areas of herring. Killer whales were encountered most frequently during October-January when the whole adult and part of the adolescent stock of spring-spawning herring wintered in the study area. Thirty-nine killer whale pods were identified, and generally the same pods were encountered each year of the study. Differences in the timing and areas of occurrence of pods could be demonstrated. Adult and adolescent herring seemed to be the main prey of the killer whales during both fall-winter and summer. Six new species were added to the list of known prey species of Norwegian killer whales. The present pattern of seasonal occurrence of killer whales in the coastal waters of northern Norway is expected to change as a result of growth in the Norwegian spring-spawning herring stock.Résumé : Nous avions comme objectifs d'étudier l'apparition saisonnière de groupes d'épaulards photo-identifiés relativement à la distribution des harengs norvégiens se reproduisant au printemps, et de déterminer s'il y avait des différences spécifiques aux groupes dans le nombre ou le régime alimentaire des épaulards. Lors d'une étude de 4 ans, les épaulards ont été observés à différents endroits à l'été et pendant la période automne-hiver, et ces endroits correspondaient à l'aire de distribution des harengs. On observait surtout des épaulards durant la période allant d'octobre à janvier lorsque la totalité du stock d'adultes et qu'une partie du stock des adolescents de harengs se reproduisant au printemps passaient l'hiver dans l'aire d'étude. Trente-neuf groupes d'épaulards ont été dénombrés, et ce sont surtout les mêmes groupes qui étaient observés chaque année de l'étude. On a fait état de différences dans les moments et les endroits où les groupes étaient observés. Les harengs adultes et adolescents semblaient constituer la principale proie des épaulards, tant pendant la période automne-hiver qu'à l'été. Six nouvelles espèces ont été ajoutées à la liste des proies connues des épaulards norvégiens. La distribution actuelle des épaulards dans les eaux côtières du nord de la Norvège changera, prévoit-on, à la suite de la croissance du stock de harengs norvégiens se reproduisant au printemps. [Traduit par la Rédaction]

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