Using vocalisations to explore climate change impacts on bearded seals

Research notes

Male bearded seals produce elaborate sounds (resembling songs) to attract females and to compete with other males during the mating season. This makes it possible to monitor breeding populations of this species using passive acoustic monitoring.

By: Samuel Martínez Llobet, Heidi Ahonen, Christian Lydersen and Kit M Kovacs // Norwegian Polar Institute. Jørgen Berge and Rolf Ims // UiT The Arctic University of Norway

The Norwegian Polar Institute and partners within the Fram Centre maintain a passive acoustic monitoring (PAM) network, consisting of recording instruments attached to oceanographic moorings around Svalbard that record underwater sounds throughout the year. In this particular study, the Institute and UiT The Arctic University of Norway used recordings from:

1) Kongsfjorden on the west coast of Spitsbergen, which has undergone dramatic change due to warming, including major declines in sea ice coverage

2) Rijpfjorden in northeast Svalbard, which remains much more “Arctic” and

3) Atwain, a site out in the drift ice northeast of Svalbard

to explore possible climate-change related impacts on bearded seal distribution/abundance. We explored seasonal variation in singing intensity of bearded seals across the year at the various sites, which have very different ice conditions.

Daily vocal rates of bearded seals in relation to the ice cover from February to August in Kongsfjorden (a), Rijpfjorden (b) and Atwain (c) Figure from Llobet et al (2021) Polar Biology

Bearded seals vocalisations were detected for an extended period at Atwain (January-July), while the vocal season was shorter at Rijpfjorden (February-June) and shorter yet again at Kongsfjorden (April-June).

The intensity with which they produced species-typical breeding vocalisations also varied markedly at the various sites. Singing was most intense at Atwain where the seals sang their elaborate downward trill vocalisations 24 hours a day for a period that extended over four months, reaching peak rates close to 400 trills per hour.

PAM monitoring sites used in this study of bearded seal vocalisations

Rijpfjorden, 24-hour trill singing also took place over a period of months, but round-the-clock singing started a bit later and ended a bit earlier in the season than at Atwain. Singing rates at Rijpfjorden were intermediate, with peak rates of 300 trills per hour.

Singing during all hours of the day was also recorded in Kongsfjorden, but only during the month of June and peak rates were less than 100 trills per hour. Singing intensity appears to have declined markedly in Kongsfjorden when data from this study is compared to earlier studies conducted in the fjord in the 1990s, a time when Kongsfjorden had extensive sea ice and was a key site for breeding bearded seals.

Median hourly number of songs per week (February to July) in (a) Kongsfjorden (b) Rijpfjorden and (c) Atwain. Blue boxes (25-75th percentiles) and the vertical lines (1st-99th percentiles) show variation around the median values (horizontal lines within the boxes). Figure from Llobet et al (2021) Polar Biology

Interestingly, sea ice cover in the study years was available throughout the vocal season at Atwain and Rijpfjorden, while in Kongsfjorden there was a mismatch between the peak in vocal activity (June) and the time when sea ice was present (until April).

Sea ice has declined precipitously in Kongsfjorden, starting around 2006, and although the seals still seem to be doing breeding behaviour at the same time as in the past, there is no longer ice cover for them to give birth or rest on. Ongoing climate warming and concomitant sea ice reductions are likely to increase the incidence of such mismatches.

Bearded seal. Photo: Kit Kovacs and Christian Lydersen / Norwegian Polar Institute

We do not currently know if bearded seals in Kongsfjorden are less vocal because individual animals have shifted their distribution northward or whether there has been a population decline locally. In either context, it was interesting to discover that bearded seals in Svalbard do make use of the drift ice around the archipelago for breeding, an unexpected finding in this study that might provide the species with some respite from the reduction in shore-fast ice that is taking place because of increased influxes of Atlantic Water and warmer air and ocean temperatures.