Hungry during migration? Humpback whale movement from the Barents Sea to a feeding stopover in northern Norway revealed by photo-ID analysis

Abstract

In the last nine winters, a humpback whale feeding aggregation has been observed in the nearshore waters and fjords of northern Norway. This seasonal humpback whale occurrence is presumably driven by a shift in the overwintering distribution of Norwegian spring spawning (NSS) herring into these coastal areas. Eastern North Atlantic humpback whales feed during summer in relative remote regions in offshore waters. Their new proximity to a populated coast allowed to monitor this fairly understudied population in Norwegian waters. The North Norwegian Humpback Whale Catalogue (NNHWC) was established in the first winter of their nearshore occurrence and contains identification-photographs from both dedicated surveys and public submissions. The Norwegian Sea is thought to be used as a migration corridor during the seasonal movement of eastern North Atlantic humpback whales between the Arctic summer feeding ground in the Barents Sea and wintering grounds at low latitudes. The fjord systems of northern Norway are located roughly en route of this movement and could constitute a feeding stopover area for whales feeding in the Barents Sea. The main aim of this study was therefore to investigate the migratory connectivity between these two regions by comparing photo-identification data from both areas. The photo-identification material further allowed for a detailed description of this new Norwegian winter-feeding area with respect to site fidelity and the length of stay. A total of 342 individual humpback whales from the Barents Sea and 866 individuals from northern Norwegian coastal waters have now been identified between 2010-2019. The photographic comparison between these two regions yielded 39 individual matches. Among these were 17 within-season re-sightings, which demonstrate that the Norwegian fjords indeed constitute a stopover early in the southward breeding migration. The seasonal return (site fidelity) to this new feeding stopover area progressively increased throughout the nine years of study and remained between 60 and 80% in the last four years. Re-sightings within a season in northern Norway revealed that individuals occupied the area between two days and 15 weeks, on average 27.5 days (SD = 11.5). Although it is not clear whether humpback whales foraged on NSS-herring offshore before they entered the fjord-systems, the results indicate that humpback whales are capable of a flexible migration strategy, integrating feeding stopovers when prey is available.