Characterizing movement and searching behavior of humpback whales along the North-Norwegian coast

Abstract

Background: Studying movement patterns of individual animals over time can give insight into how they interact with the environment and optimize foraging strategies. Humpback whales (Megaptera novaeangliae) undertake long seasonal migrations between feeding areas in polar regions and breeding grounds in tropical areas. During the last decade, several individuals have had up to a three month stop-over period around specific fjord-areas in Northern Norway to feed on Norwegian spring-spawning (NSS -) herring (Culpea harengus L.). Their behavioral patterns during this period are not well understood, including why some whales seemingly leave the fjords and then later return within the same season. Methods: Here we for the first-time classified humpback whale tracks into five distinct movement modes; ranging, encamped, nomadic, roundtrip and semi-roundtrip. A behavioral change point analysis (BCPA) was used to select homogeneous segments based on persistence velocity. Then, net squared displacement (NSD) over time was modeled to differentiate movement modes. This study also visually identified longer roundtrips away from the fjords that lasted several days and examined movement modes within these. Results: Inside the fjord systems, encamped mode was most prevalent in December-January, suggesting the whales were mainly foraging on overwintering NSS-herring in this area. During the same winter seasons, half of the whales left the fjords and then returned. We hypothesize that these trips serve as 'searching trips' during which the whales seek better feeding opportunities outside the fjords. If better foraging conditions are not found, they return to the fjords to continue their feeding. The overall most common mode was ranging (54%), particularly seen during the start of their southwards migration and in areas outside the fjord systems, indicating that the whales mainly moved over larger distances in the offshore habitat. Conclusion: This study serves as a baseline for future studies investigating both the searching trip theory and humpback whale behavior in general, and confirms that this method is useful to analyze smaller scale movement patterns of satellite tagged whales.