Study discovers more questions around mysterious Ross seal
Spotting animals in the vastness of the Antarctic pack ice always requires a certain amount of luck, as the region has incredible dimensions. This makes it very difficult for researchers to find out more about the lifestyles of such species, which reside either deep in the pack ice or far out in the Southern Ocean. Among these is the mysterious Ross seal, which should actually be found all around Antarctica. A study by a team of German and South African researchers set out to find out more about the seal’s habits and instead discovered even more questions.
Ross seals appear to dive between 100 to over 700 meters for 5 to often over 20 minutes, following no particular diurnal rhythm, at least during the Antarctic winter. But at least during the dawn and dusk hours they dive deeper, but again not necessarily longer or more frequently than during the rest of the day. That’s one of the few findings that Dr. Mia Wege and Marthán Nieuwoudt Bester of the University of Pretoria’s Institute of Marine Mammal Research and Dr. Horst Bornemann of Germany’s AWI have discovered in their work with the mysterious and rarely studied Ross seals. The three researchers have now published the results of their work in the latest issue of the journal Antarctic Science.
In order to get the data of the dives at all, it had been necessary to keep a lookout for Ross seals on several expeditions to East Antarctica, which was not an easy undertaking given the vastness of the area and the seal’s size, which is not exactly huge at just under 2 meters in length. If one was discovered, it was caught with a net and fitted with a satellite-based transmitter, which was supposed to measure and transmit its diving and resting data, and after a year at the latest it would fall off again as it moulted. But besides the fact that Ross seals are difficult to spot in the pack ice, the technology didn’t play along as the research team would have liked. The team was only able to collect sufficient data for their work from ten seals, which is too little for a statistical evaluation and thus a reliable statement. The researchers reached their limits especially when it came to the question of resting behavior, as there was considerable variation between individual animals. After all, the team was able to show that Ross seals like to rest on ice floes in the middle of the day from spring to summer. Some of the animals even showed resting for days, interrupted only by short dives. Since these were usually females, they may have given birth to their young during that time and gone through a nursing period of about 13 days, Dr. Wege and her team speculate. But they could not record any data on this.
Despite the small number of animals that have been delivering data and the functioning transmitters, the researchers were able to show that at least the previous assumption that the animals increasingly go on prey hunts at night is not entirely correct. Rather, it was during the twilight hours that the animals mostly went diving and thus did not quite follow the diurnal rhythms of their prey, fish and squid. Interestingly, however, the animals seem to adapt this behavior to the factor of the substrate: if they are in the open ocean in autumn, they follow the rhythm, but on the ice they do not. However, the researchers do not have an explanation for this either.
Overall, the team said, the study shows that there are still many unanswered questions that would actually be necessary to understand the lifestyle of Ross seals. For example, there is the question of the timing and duration of the reproductive season. “This limitation results from the commuting behavior of Ross seals to and from pelagic foraging areas north of the pack ice, where they are inaccessible in the open ocean for most of the year,” Dr. Wege and her colleagues write. That’s why they’re calling for a more coordinated, multinational comprehensive investigation into the mysterious seal. Because considering the fact that the Antarctic world is also changing in the meantime, the seal could suddenly come under pressure and threaten to disappear before it was really known.
Dr Michael Wenger, PolarJournal
Link to the study: Wege, M., et al (2023). The nightlife of a Ross seal: Diving and haul-out behavior from the eastern Weddell Sea. Antarctic Science, 1-12. doi:10.1017/S0954102022000438.
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