I know what you did last winter: Bowhead whale unusual winter presence in the Beaufort Sea

Nikoletta Diogou – niki.diogou@gmail.com

Twitter: @NikiDiogou
Instagram: @existentialnyquist

University of Victoria
Victoria, BC V5T 4H3

Additional authors: William Halliday, Stan E. Dosso, Xavier Mouy, Andrea Niemi, Stephen Insley

Popular version of 1aAB8 – I know what you did last winter: Bowhead whale anomalous winter acoustic occurrence patterns in the Beaufort Sea, 2018-2020
Presented at the 184 ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0018030

The Arctic is warming at an alarming pace due to climate change. As waters are warming and sea ice is shrinking, the arctic ecosystems are responding with adaptations that we only recently started to observe and strive to understand. Here we present the first evidence of bowhead whales, endemic baleen whales to the Arctic, breaking their annual migration and being detected year-round at their summer grounds.

Whales, positioned at the top of the food web, serve as excellent bio-indicators of environmental change and the health of marine ecosystems. There are more than 16,000 bowhead whales in the Bering-Chukchi-Beaufort (BCB) population in the Western Arctic. The BCB bowheads spend their winters in the ice-free Bering Sea, and typically start a journey early each spring of over 6000 km to summer feeding grounds in the Beaufort Sea, returning to the Bering Sea in early fall when ice forms on the Beaufort Sea (Figure 1). But how stable is this journey in our changing climate?

Figure 1. Map showing migration route of BCB bowhead whales and the wider study area.

The Amundsen Gulf (Figure 1), in the Canadian Arctic Archipelago of the Beaufort Sea, is an important summer-feeding area for the BCB whales. However, winter inaccessibility and harsh conditions year-round make long-term observation of marine wildlife here challenging. Passive acoustic monitoring has proven particularly useful for monitoring vocal marine animals such as whales in remote areas, and offers a remarkable opportunity to explore where and when whales are present in the cold darkness of Arctic waters. Figure 2 shows examples of two types of bowhead whale vocalizations (songs and moans) together with other biological and environmental sounds recorded in the Amundsen Gulf.

Figure 2. Examples of spectrograms recorded in the Amundsen Gulf of bowhead whale songs on the left, and bowhead whale moans on the right. Spectrograms are visual representations of sound, indicating the pitch (frequency) and loudness of sounds as a function of time. Spectrograms on the left include bearded seal calls (trills) interfering with the bowhead songs. Spectrograms on the right include other ambient sounds (ice noise) that interfere with the bowhead moans. Image adapted from authors’ original paper.

Examples of characteristic calls of bowhead whales recorded during 2018-2019 in the southern Amundsen Gulf.

In September of 2018 and 2019 we deployed underwater acoustic recorders at five sites in the southern Amundsen Gulf and recorded the ocean sound for two years to detect bowhead whale calls and quantify the whale’s seasonal and geographic distribution. In particular, we looked for any disruptions to their typical migration patterns. And sure enough, there it was.

A combination of automated and manual analysis of the acoustic recordings revealed that bowhead whales were present at all sites, as shown for 3 sites (CB50, CB300 and PP) in Figure 3. Bowhead calls dominated the acoustic data from early spring to early fall, during their summer migration, confirming the importance of the area as a core foraging site for this whale population. But surprisingly, the analysis uncovered a fascinating anomaly in bowhead whale behavior: bowhead calls were detected at each site through the winter of 2018-2019, representing the first clear evidence of bowhead whales overwintering at their summer foraging grounds (Figure 3). This is a significant departure from their usual migratory pattern. However, analysis of the 2019-2020 recordings did not indicate whales over-wintering that year. Hence, it is not yet clear if the over-wintering was a one-time event or the start of a more stable shift in bowhead whale ecology due to climate change. The variability in bowhead acoustic presence between the two years may be partly explained by differences in sea ice coverage and prey density (zooplankton), as summarized in Figure 4.

Figure 3. Number of days with acoustic detections per month for bowhead whales for sites CB50 (blue), CB300 (green), and PP (red) in 2018-2019. The yellow shaded areas represent time periods at each station when the ice concentration was below 20% (“ice-free”), grey areas when ice concentration was 20%-70% (“shoulder season”), and white areas when ice concentration was greater than 70%. Image adapted from authors’ original paper.

Figure 4. Graphical summary of the objectives and major results of the study.

The findings of this study have important implications for understanding how climate change is affecting the Arctic ecosystem, and highlights the need for continued monitoring of Arctic wildlife. Passive acoustic monitoring can provide data on how whale ecology is responding to a changing environment, which can be used to inform conservation efforts to better protect Arctic ecosystems and their inhabitants.