By TODD McLEISH/ecoRI News contributor
Just three North Atlantic right whales were born this winter, a precipitous decline in the species birth rate that has scientists concerned for the future of one of the rarest whales on Earth.
With four right whales killed by human causes last year, the birth rate is now below the mortality rate, signaling a population decline from which the animals may have difficulty recovering.
The endangered whales give birth off the coast of Georgia and northern Florida, and the three calves born this winter is the lowest total since 1999. An average of 24 calves were born each year during the 2000s, and the average for the 2010s had been 13.
“We had an increasing trend from 1982 to 2009, when we had a record 39 calves born, but since then it’s been going in the other direction steeply,” said Robert Kenney, a marine mammal expert at the University of Rhode Island’s Graduate School of Oceanography who manages the sighting database for the North Atlantic Right Whale Consortium. “I’m more worried about the animals than I was the first time we had a drop in calf numbers in the 1990s.”
The prior decline quickly reversed itself, but Kenney doesn’t see the present decline in birth rate improving any time soon.
“The most obvious reason for the decline is that something has disturbed the predictability of their food supply,” Kenney said. “There’s something about the warming water or the timing of the spring plankton bloom or something else — the food is just not where the whales expect it to be in the abundance and concentrations they expect. They still go to their traditional feeding grounds, but they don’t stay because the food isn’t there.
“They’re spending more time hunting for food, and looking for food is energetically expensive because they have to travel. The more they travel, the more chance they have of running into fishing gear and becoming entangled.”
Fishing-gear entanglement is the leading cause of mortality for right whales, followed by ship strikes.
A healthy female right whale gives birth every three years, according to Kenney. They are pregnant for a year, they nurse their calf for a year, and they take a year to recover and regain their fat stores so they can become pregnant again.
“But if she can’t get find enough food to put on that fat, she’ll skip a year,” Kenney said. “So that resting period between pregnancies gets longer as they become more and more energy stressed.”
In recent years, female right whales have doubled the interval between pregnancies from 3-4 years to 6-7 years, which lowers the species’s overall birth rate.
“Survival and mortality haven’t changed,” Kenney said. “The change in their population trajectory is because of a decline in the birth rate. Not enough babies are being born to replace those that are dying.”
Scientists believe that only about 524 right whales are known to exist, up from about 400 a decade ago.
“With the way the climate and oceanography is changing, we don’t know if the population can adapt to it and rebound,” Kenney said. “They’ve adapted multiple times through their history, so they might be able to do so again. But before, they weren’t getting drowned in fishing gear and run over by ships with the same frequency.”
Mortality from ship strikes is no longer increasing, despite significant growth in the shipping industry, thanks to regulations imposed in 2008 requiring ships to decrease their speed to 10 knots in areas where the whales are known to spend time during certain periods of the year. Just one right whale per year, on average, is killed by being struck by a ship.
About four or five right whales are known to die annually as a result of becoming entangled in fishing gear. However, it’s likely that others die but their carcasses aren’t recovered.
“If a healthy right whale is killed by a ship, it floats and is apt to wash up on a beach, so we know about it,” Kenney said. “But when a whale becomes entangled, it often takes a long time to die — they starve to death or eventually succumb to their injuries — so they are much more likely to have lost much of their fat and they sink, and we never know about it.”
Despite fishing regulations aimed at limiting whale entanglements, mortality rates haven’t declined. Four out of every five right whales have scars from being entangled at least once.
“There is nothing we can do in the short term about the changes in the ocean affecting the whale’s food supply,” Kenney said. “We can only stand by helpless and watch it happen. Where we can make a difference is on the human mortality side of the equation. We really need to get a handle on entanglements. It’s happening way too frequently.”
Unfortunately, he said, the future looks bleak for right whales.
“Given the expectation that changes in the ocean are going to be continuous and are going to get worse, the handwriting could be on the wall,” Kenney said.
Rhode Island resident and author Todd McLeish runs a wildlife blog.