In a new study published by Cell Press, experts have discovered that North Atlantic right whales have become significantly shorter in the last three to four decades. The experts report that, since the 1980s, the average length of the whales has declined by about seven percent.
The research suggests that even though whales are now largely protected from direct catch, other stressors are having an impact on their fitness.
“On average, a whale born today is expected to reach a total length about a meter shorter than a whale born in 1980,” explained NOAA scientist Joshua Stewart. “But that’s just the average – there are also some extreme cases where young whales are several meters shorter than expected.”
“Major impacts on life history like this have been documented in heavily exploited commercial species, especially fishes, but to our knowledge this is the first time these kinds of impacts are being recorded in a large mammal.”
In collaboration with an international team of scientists, Stewart set out to document the challenges faced by right whales as indicated by changes in their life history characteristics, including size. The experts analyzed aerial photogrammetry measurements collected over 20 years.
“We were able to build on our previous work that used conventional aircraft in the early 2000s by adopting new drone technology to extend the time series in recent years,” said John Durban at Oregon State University (formerly with NOAA). “In both cases, we were able to measure whales by flying a camera high above them, essentially giving them a health check without them knowing we were there.”
The whales have been consistently monitored since the 1980s, including detailed records of attached-gear entanglements, age, and size. Beyond the effects of entanglements, the experts examined the potential impacts of other stressors such as noise, ship strikes, and prey availability.
“Fishing gear entanglements in this population are unfortunately fairly common, and entanglements resulting in attached gear and severe injuries have been generally increasing over the past several decades,” Stewart said.
“Previous studies have shown that the increased drag from entangling gear requires right whales to spend a lot of extra energy just to go about their normal activities, and that is energy they might otherwise spend on growth or reproduction. In some cases, entanglements can be lethal, but it turns out that even sub-lethal entanglements can have lasting impacts on right whales.”
The findings show that entanglements are one of the stressors associated with shorter whales. According to the researchers, stunted growth in right whales may lead to a higher risk of life-threatening gear entanglements, as well as reduced reproductive success.
“The smaller you are, the less energetic reserves you have, and the harder it might be to survive a serious entanglement or sustained food shortage,” explained Stewart.
“So it’s possible that these life history changes could translate into population viability impacts. But this really makes me wonder about how large whales worldwide are being impacted by entanglements. This is by no means a problem unique to right whales – entanglements are a major threat for whales, marine mammals, and other marine species worldwide.”
“Because North Atlantic right whales have this incredibly detailed dataset with known ages, sizes, entanglement histories, and so forth, we could directly examine how these impacts are affecting growth rates. My guess is that many other species are being similarly affected, but we just don’t have the ability to detect it in less well-studied populations.”
The study is published in the journal Current Biology.