Occurrence and diet of killer whales in northern Norway: seasonal patterns relative to the distribution and abundance of Norwegian spring-spawning herring

Our objectives were to investigate the seasonal occurrence of photoidentified killer whale pods in relation to the of Norwegian spring-spawning herring and whether or not pod-specific differences in the occurrence or diet of whales could be demonstrated. In a 4-year study, the killer whales occurred in different areas during the summer and fall–winter, and these areas coincided with the distribution areas of herring. Killer whales were encountered most during October–January when the whole adult and part of the adolescent stock of spring-spawning herring in the study area. Thirty-nine killer whale pods were identified, and generally the same pods were encountered each of the study. Differences in the timing and areas of occurrence of pods could be demonstrated. Adult and adolescent seemed to be the main prey of the killer whales during both fall–winter and summer. Six new species were added to list of known prey species of Norwegian killer whales. The present pattern of seasonal occurrence of killer whales in the waters of northern Norway is expected to change as a result of growth in the Norwegian spring-spawning herring.