Fin whales have been so heavily hunted during the commercial whaling era that only a small part of the population has survived in the southern hemisphere and marine biologists still know little about the life of the world’s second largest whales. All the more encouraging are the research results of scientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the Johann Heinrich von Thünen Institute of Sea Fisheries, according to which a large number of baleen whales regularly reside in the krill-rich waters around Elephant Island. Evidence for this is provided by underwater sound recordings from the region, on which so many fin whale calls can be heard in the peak month of May that the individual sounds merge into one sound carpet, the research team now reports in the journal Royal Society Open Science. In view of this, the marine biologists are now calling for protective measures for this important habitat, so as not to endanger the emerging recovery of the fin whale population.
Fin whales are still rare and, according to the textbook, occur in groups of three to a maximum of seven animals. AWI marine biologist Elke Burkhardt was all the more surprised when she counted more than 100 fin whales in the sea north of Elephant Island during an expedition of the German research icebreaker Polarstern to the Scotia Sea in late Antarctic summer 2012. Was this aggregation a chance find or do the world’s second largest baleen whales regularly converge here in such large numbers? And if so, why?
To find answers to this question, Elke Burkhardt and her team deployed a mooring with two underwater acoustic recorders and a device for determining the food supply in the coastal area northwest of the island in January 2013. For three years, from January 2013 to February 2016, the instruments recorded the soundscape of the underwater world as well as data on the food supply in the upper water column, thus helping to identify what is probably one of the most important habitats of southern fin whales.
“Our Polarstern observations were no coincidence. As our recordings show, the animals regularly stay in the waters around Elephant Island from December to August. Here, they not only hunt for Antarctic krill, but also start looking for mates. Most of the fin whale calls were recorded by our recorders exactly in the same season as the breeding season of the population in the southern hemisphere”, reports Elke Burkhardt.
The fin whales can be identified by a species-specific muffled sound: “Humans would probably only perceive it as a vibration in the pit of the stomach, because its central frequency is around 20 hertz and thus extremely low,” explains Elke Burkhardt. Bull fin whales that are ready to mate and want to attract females emit this bass sound in rapid, regular succession. “Their courtship may also explain why our recorders recorded so many of these calls in May that they merged into one sound carpet and could hardly be distinguished as individual sounds,” says the AWI marine biologist.
New arguments for a marine protected area around Elephant Island
She is delighted to see so many fin whales at Elephant Island: “If this aggregation is indeed a sign of a growing fin whale population, then it would be a respectable success for the international whaling moratorium that came into force 35 years ago,” says the marine biologist.
At the same time, however, the new findings also give cause for concern: “On the one hand, there is a lot of fishing for Antarctic krill in the Scotia Sea; on the other hand, this area, which is very important for fin whales, is frequently visited by cruise ships. For these reasons, it is now all the more important to comprehensively protect the marine area around Elephant Island and to regulate both krill fishing and tourism in such a way that damage to the fin whale population can be ruled out,” says Elke Burkhardt. For this purpose, the acoustic environment (soundscape) should be recorded regularly to document changes in the stocks.
Where do the fin whales of Elephant Island spend the winter?
While analyzing its underwater recordings, the research team came across another interesting detail. The 20 hertz call sometimes contains an accompanying sound with a frequency of 86 hertz. This, in turn, resembles fin whale sounds recorded by Chilean marine biologists off the central coast of Chile – especially during the time of year when the recorders at Elephant Island rarely recorded baleen whale sounds. Are the sounds in both regions possibly produced by the same whale population that migrates back and forth between the southern Shetland Islands, of which Elephant Island is a part, and the Pacific coast of Chile?
“It is believed that fin whales produce population-specific accompanying sounds that can be used to distinguish different populations from one another. If this assumption is correct, it is reasonable to conclude that those fin whales that spend the southern summer at Elephant Island give birth to their calves later in the year in the warmer waters off the Chilean Pacific coast, and that these whales regularly commute between the two areas,” says Elke Burkhardt.
However, further investigations are necessary to obtain certainty, for which the research team from Bremerhaven has anchored additional underwater recorders in the vicinity of the island. These are to be recovered in 2022. Currently, the marine biologists are evaluating their underwater recordings from the time after 2016. The first sound snippets are promising: The fin whale meeting point at Elephant Island was also very well visited in the summers after 2016.
Press release Alfred Wegener Institute for Polar and Marine Research
Link to the study: Elke Burkhardt, Ilse Van Opzeeland, Boris Cisewski, Ramona Mattmüller, Marlene Meister, Elena Schall, Stefanie Spiesecke, Karolin Thomisch, Sarah Zwicker and Olaf Boebel (2021): Seasonal and diel cycles of fin whale acoustic occurrence near Elephant Island, Antarctica. R. Soc. Open Sci. 8: 201142. 10.1098/rsos.201142
More on the subject:
