The Southern Right Whales that visit Cape waters every year from about July to November have been having... well, a whale of a time, as usual.
This is apparent from the latest available survey figures that confirm that this particular whale population is still increasing at a rate of about 6.8 percent a year, very close to the biological maximum, with the mature cows producing a calf every three years on average.
“That’s been very consistent, although the new model Professor Butterworth and colleagues at UCT have adopted now (to compute the population) suggests there may have been a slower rate of increase during the early 1970s,” says whale expert Dr Peter Best, now officially retired but still closely involved with cetacean (whale and dolphin) research through Pretoria University’s Mammal Research Institute.
He suggests that this slower rate of increase was probably attributable to the illegal Soviet catches of right whales through the 1960s in particular, which eventually ended in the early years of the following decade.
“They were probably holding the rate of increase down a bit,” Best says.
In 1997, previously classified true data on the Soviet whaling operation in the Southern Hemisphere between 1947 and 1972 was revealed, showing that official data given to the International Whaling Commission had been heavily falsified and that the killing of more than 9 000 blue whales, about 46 000 humpback whales, 3 350 right whales and more than 21 000 sperm whales had been unreported.
Best, who works from the Iziko-South African Museum in the Gardens, started an annual aerial survey of the southern rights between Nature’s Valley and Muizenberg way back in 1971, using a fixed-wing aircraft. In 1979 he switched to a more manoeuvrable helicopter for the surveys which have continued ever since, although fixed wing aircraft continued to be used as well through to 1987, making it one of the longest continuous surveys of a whale population in the world.
Although he no longer flies himself, he is still involved in analysing the data from the annual surveys which are based on photographs of cow-calf pairs.
Southern right whales are individually identifiable from the unique pattern of the callosities on their heads: whitish, wart-like, roughened patches of skin on their heads.
But many of these whales are also identifiable from their unique pigmentation.
“We’re very fortunate that a very high proportion of our animals are born with white or grey markings on their backs, so that provides an additional means of identification. And some of those markings are sex-linked, so you know whether it’s a male or female, which is also very useful,” Best explains.
The post-first-year survival rate of southern right whales is “incredibly high” at 98 percent to 99 percent, he points out. “Of course that figure may be skewed by the fact that the population is increasing so strongly, meaning that there are relatively few really old animals. I expect when the population stabilises, the rate will be a bit lower.”
Scientists don’t know how long southern right whales live, but they do have confirmation of one individual that was photographed as a calf from the cliff path in Hermanus in 1966 that was identified 39 years later as an adult – the oldest yet identified.
“And we have some whales that were first photographed in 1974 that are still having calves.
“They would have been mature when they started having calves – six or seven as a minimum – so they’re probably also all around 40 years old, and we think they can go on for more than that. Most baleen whales show very little sign of falling reproductive activity with age and they keep going for much of their lives.”
There have been several surprises in the survey data, including that the whale population – both cow-calf pairs and other individual animals – has moved towards the west over time, Best reveals.
“There are still animals in the east, but the bulk of the population has been shifting almost to the west of Agulhas – there are masses of them between Agulhas and Pearly Beach, for example, where we’ve never had them before.
“And interestingly, while numbers of cow-calf pairs have continued to rise, there’s been a marked fall-off in the number of other whales without calves during the last few years,” he adds.
But he and his research colleagues are not overly concerned about this trend, because they’ve started identifying an increasing number of southern rights off the Namibian coastline.
“Dr Jean-Paul Roux has been running aerial surveys there and has got about 80 images, and we’ve matched 17 of these with animals from the Republic (South Africa),” says Best.
“Bear in mind that our (photographic) catalogue is based entirely on cow-calf pairs. We’ve ignored all the other animals and most of the males are not identified, so we’ll never get a 100 percent matching with the Namibian animals.
“But what’s very interesting is that of the 13 animals with distinctive grey or white markings that we’ve seen in Namibia, we’ve identified 12 of them as having been born in South Africa. So we feel there’s a move towards Namibia, although very few of them are cow-calf pairs.
“Historically, of course, there were a lot of right whales off Namibia, so maybe this is just a re-occupation of old territory.” - Cape Argus