Last updated May 3, 2018 at 12:19 pm
Humpback whales numbers are bouncing back, with most females in the west Antarctic pregnant most years.
Back from the brink
The commercial exploitation of Humpback whales was banned by the International Whaling Commission in 1966. It the intervening half-century, whale populations have recovered well, to the point that the species was listed as under “Least Concern” in the most recent 2016 review by the International Union for Conservation of Nature.
One of the regions that whales are repopulating is the Western Antarctic Peninsula, which is experiencing some of the most rapid rates of regional climate change on Earth, with temperature rises of 7C since the 1950s. Scientists don’t know a lot about the migratory movements of humpback whales in this region because they were nearly wiped out in the early 1900s. Humpacks were abundant, easy to catch, and floated when killed, making them an attractive target. This means we don’t know much about the lives of whales in this region of the Antarctic before they were reduced to a tiny remnant population.
A team headed by Logan Palklin from the University of California Santa Cruz set out to find out more about the breeding cycle of whales along the Western Antarctic Peninsula to help provide a baseline against which to measure future climatic changes in the region.
How do you know if a whale is pregnant?
In the past trying to figure out whether a whale was pregnant or not was a matter of observation. Researchers would study mature females and simply count whether or not they were observed with calves. Earlier still pregnancy rates were figured out by analyzing carcasses from whaling.
While it’s difficult to get a whale to pee on a pregnancy test stick, there are now much better biochemical methods for determining pregnancy. Progesterone, a hormone associated with pregnancy, can be measured in other marine mammals like dolphins by taking samples from the milk of lactating females, or urine or saliva samples. These methods are not practical for whales, which are too large to be captured or handled. Hormone levels are instead measured from samples that can be sampled in the ocean, namely whale poo, skin and blubber biopsies, or blowhole excretions.
Females reproducing annually
538 whales were sampled around the Western Antarcitc Peninsular between 2010 and 2016. The research team found that the pregnancy rate was 63.5%, although it varied a lot from year to year.
Surprisingly, the team also found evidence that females were reproducing annually, with over half of the females with calves found to be pregnant again the following year (54.5%)
This pregnancy rate indicates that that humpbacks are recovering well, but the rate is much higher than expected given that the total population is only growing at 3.4% per year. So either there has been an error in measuring humpback whale population growth, or there are a lot of fetal or newborn whale calves not surviving to the next migration season. This is something the team are looking to answer with future work, as they continue to monitor the whales recover and response to future environmental challenges.