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

Abstract

Studying movement patterns of individual animals over time can give insight into how they interact with the environment and optimize their foraging strategies. Humpback whales (Megaptera novaeangliae) undertake long seasonal migrations between feeding areas in polar regions during summer/early winter and breeding areas in more tropical areas during late winter/spring. The Norwegian Sea is a part of the humpback whale migration route between the Barents Sea and their southern breeding grounds, and during the last decade several individuals have had up to a three month stop-over period around some specific fjord areas in Northern Norway. Here they feed on Norwegian spring-spawning (NSS-) herring (Culpea harengus L.) to seemingly re-fuel before continuing their southwards breeding migration. Their smaller scale movement and individual variation in behavioral patterns during this stop-over period are not well understood, including why some whales have been observed to leave the fjords and then later return within the same season.

Therefore, this study used data from 12 satellite tagged humpback whales and for the first time segmented their tracks into five distinct movement modes; ranging, encamped, nomadic, roundtrip and semi-roundtrip. This was done by using a behavioral change point analysis (BCPA) to pick out homogeneous segments based on persistence velocity at relatively small scales, and then modeled the net squared displacement (NSD) over time 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.

The most common movement mode was ranging behavior (54%), particularly seen during the start of their southwards migration and in areas outside the fjord systems in late winter, indicating when the whales moved over larger distances in the offshore habitat. Inside the fjord systems, encamped, nomadic, round and semi-roundtrip modes were more prevalent in December-January, suggesting the whales are mainly foraging on overwintering NSS-herring in this area. Half of the whales left the fjords and came back again during the same winter seasons, and these trips lasted from 4-22 days and were conducted in late December or January. During these trips, 60% of their behavior consisted of ranging behavior, sometimes split by shorter periods of encamped, roundtrips or nomadic behavior. We hypothesize that these trips may serve as “searching trips” where the whales search for better feeding opportunities outside the fjord systems, and if better foraging conditions are not found, they return to the fjords to continue their feeding. This study serves as a baseline for future studies investigating both this theory and humpback whale behavior in general, and confirms that the method is useful to analyze smaller scale movement patterns of satellite tagged whales.