A rarely seen species of whale is at risk because they are “homebodies” who stay and feed in the same waters all year round, warns new research.
Pygmy right whales (Caperea marginata) are the smallest and probably least studied of all baleen whales, according to scientists.
They say the “elusive” species is infrequently spotted in the wild, perhaps because of their relatively small size – 6.5 meters (21.3 ft) long and weighing up to 3.5 tons – and their “inconspicuous” behavior compared to boisterous humpback whales.
Historically, whalers rarely bothered hunting them.
What little was known about pygmy right whales until now was mainly based on beached animals.
The new study shows that they stay and feed in waters off southern Australia year-round – a behavior pattern that may endanger them.
Scientists say that in the region the whales inhabit south-eastern winds drive cold, nutrient-rich water from the depths to the surface between November and May, providing a food bonanza for seabirds and marine life.
The research team measured the ratio between stable nitrogen isotopes 15N and 14N and between stable carbon isotopes 13C and 12C in the baleen plates of 14 adult pygmy right whales, to infer their diet and habitat use.
These included both females and males that had stranded between 1968 and 2019 on the Tasmanian or southern Australian coast. The baleen plates were on loan from the South Australian Museum in Adelaide.
Study lead author Adelaide Dedden, a doctoral student in Rogers’ research group, said: “Baleen is made of keratin, like our fingernails, and grows throughout the life of the whale.
“As a stable tissue, baleen provides an ideal long-term signal to look at their diet and habitat use.”
She explained that because animals derive their nitrogen and carbon exclusively from food, the isotope ratios in their tissues reflect those of their prey.
Isotope ratios rise in a regular pattern across trophic levels within the food web, with phytoplankton having the lowest values, and apex predators the highest.
Baleen of pygmy right whales, on loan from the South Australian Museum. (Adelaide Dedden via SWNS)
By comparing the values measured in baleen to those published for a range of possible prey, the researchers could deduce which species are on the menu of pygmy right whales.
The results, published in the journal Frontiers in Marine Science, showed that the isotope ratios in the baleen of pygmy right whales closely match those of copepods and krill species from regions rich in zooplankton off Australia.
In contrast, there was no correspondence with isotope ratios of Antarctic krill, which means that pygmy right whales don’t make seasonal migrations to the Antarctic as many other species of baleen whales do.
Likewise, because the isotope ratios didn’t match those of pelagic fish, the research team deduced that unlike larger species of baleen whales, pygmy right whales don’t feed on fish.
But they warned that the restricted, mid-latitude range of pygmy right whales and their reliance on specific prey puts them at risk.
Rogers said: “As large-bodied mammals who feed on tiny prey, pygmy right whales need to consume vast quantities of food.
“This makes them vulnerable to changes in their local environment.
“Their home, the temperate oceans of the southern hemisphere, is warming at an alarming rate.”
He added: “We plan to study next how they will respond to this change.”
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