Populations of humpback whales living in the Southern Ocean near Antarctica could be rebounding, new research suggests.
During the late 19th and early 20th century, commercial whalers hunted humpback whales (Megaptera novaeangliae) to near extinction. But global humpback whale populations, which fell by about 90 percent, have been recovering following a ban on the commercial hunting of humpback whales in 1966.
Humpback whales living around the Western Antarctic Peninsula (WAP) could also be making a comeback, indicated by females showing high pregnancy rates, researchers report in a study published in the journal Royal Society Open Science.
Humpback whale. Image by the National Oceanic and Atmospheric Administration (public domain).
To determine details of the humpback population living around the Western Antarctic Peninsula, the researchers used darts to collect 577 small samples of skin and blubber from whales between 2010 and 2016. The team sequenced DNA and found that the samples came from 239 male and 268 female humpback whales.
The researchers then tested samples of 244 individual females for progesterone, and found that, on average, 63.5 percent of the females were pregnant. “This overall rate is higher than reports of the pregnancy rate of female humpbacks taken in Antarctic whaling areas … from 1950 to 1956,” the authors write in the study.
The researchers also observed calves accompanying 44 female humpbacks. More than half of these lactating females were pregnant when the researchers sampled them, indicating the occurrence of annual pregnancy within this population.
The presence of both high pregnancy rates, and a high proportion of females that are both lactating and pregnant, is a sign that the humpback whale population along the Western Antarctic Peninsula is growing rapidly, the authors write in the study.
“We believe that the existence of annual reproduction in this population represents a response to favourable ecological conditions, as humpback whales along the WAP recover from past over-exploitation,” the authors write.
One of these favorable conditions is the lack of considerable competition from other baleen whale populations in the Southern Ocean that haven’t recovered as well from past depletion as the humpbacks.
The Western Antarctic Peninsula has also been experiencing rapid climate change, with an increase in temperature of nearly 7 degrees Celsius (12.6 degrees Fahrenheit) since the 1950s. This has resulted in the collapse of ice shelves, retreat of glaciers, and 80 more ice-free days per year on average than four decades ago, the authors write.
More ice-free days mean more open ocean where whales can access food, especially stocks of little shrimp-like crustaceans called krill that form the bulk of humpbacks’ diet, co-author Ari Friedlaender of the University of California, Santa Cruz, told The New York Times.
But while prey availability will likely increase in the short term, long-term trends may be more problematic, the researchers say.
“Our intent here is to document the current demography of this population and establish a baseline with which to assess the impact of future climatic trends,” the authors write. “We will continue to monitor this feeding aggregation of whales to document its recovery and response to future environmental changes.”
Pallin, L. J., Baker, C. S., Steel, D., Kellar, N. M., Robbins, J., Johnston, D. W., … & Friedlaender, A. S. (2018). High pregnancy rates in humpback whales (Megaptera novaeangliae) around the Western Antarctic Peninsula, evidence of a rapidly growing population. Royal Society Open Science, 5(5), 180017.