LEILA FADEL, HOST:
A few years after a warm water event killed off sea life in the North Pacific, humpback whales are now rebounding in Alaska. Researchers are watching this population to gauge the greater health of the ocean. From member station KTOO in Juneau, Claire Stremple reports on what they found.
CLAIRE STREMPLE, BYLINE: The reflection of Mount Wright ripples on the glassy water of Glacier Bay National Park. Then, suddenly, a humpback breaks the surface for a huge mouthful of herring.
(SOUNDBITE OF MARINE LIFE AMBIENCE)
STREMPLE: Chris Gabriele snaps a few photos.
(SOUNDBITE OF SPLASH)
CHRIS GABRIELE: Wow.
STREMPLE: She's a biologist with the park service and has been observing whales here for decades. But the sight of one of the mammals up so close still wows her.
GABRIELE: I did get a photo of his dorsal fin.
STREMPLE: Whales are recognized by their fins and flukes - that's the tail.
GABRIELE: We'll try and get close one more time to get the other side.
STREMPLE: The photos and a log of the whale's coordinates are the two newest entries in a data set that stretches back to the early '70s, among the longest-running humpback studies in the world. Some whales have returned here for more than 40 years.
GABRIELE: And they're raising their kids here. And the kids come back year after year after year, and so it's an important place to them.
STREMPLE: Researchers could track the severity of a recent marine heat wave, known as The blob, because the whale population sank by more than 70%, then stayed low. But lately, there's a glimmer of hope in the data. The humpback whale population is growing.
GABRIELE: It's been really encouraging, last year and this year, to start to see the number of calves we've been seeing.
STREMPLE: From 2014 to 2016, consistent record-high ocean temperatures in the Northern Pacific decimated the humpback population that visits the preserve. The year before The blob, Gabriele counted an all-time high of 163 whales. After The blob, only 45 of them came back.
GABRIELE: So the whales were kind of a sentinel that showed us what happened. Otherwise, we would not really have known.
STREMPLE: The humpback population stayed low until last year. This study reveals how deeply the marine ecosystem was affected.
GABRIELE: I think it's important to keep doing this work because we can come back in five years, 10 years, 20 years and look at what the whales are telling us.
STREMPLE: The numbers aren't quite up to where they used to be, but Gabriele expects the population to stay healthy as long as ocean conditions stay stable. But that's not a guarantee.
JOHN WALSH: It wouldn't be surprising if there's another blob event in the next 10 or 20 years.
STREMPLE: John Walsh is the chief scientist at the International Arctic Research Center at the University of Alaska Fairbanks. He's anticipating another oceanic heat wave because his lab has attributed events like the blob to climate change. He views the blob through the lens of arctic sea ice, and he found that like whales, ice recovered somewhat a few years after the warming.
WALSH: But we'll have these modest recoveries after the big events. And then there'll be another big extreme somewhere down the road.
STREMPLE: Each time the ecosystem takes a hit, it bounces back, but each time a little less.
(SOUNDBITE OF SEABIRDS COOING)
STREMPLE: Park service biologist Chris Gabriele says the return of the whales is a sign of the ecosystem righting itself - for now.
GABRIELE: It really gives me hope that when the conditions are good, they're really very resilient.
STREMPLE: There's a sign of that hope back on the bay. A mother whale and her calf come up to breathe together.
(SOUNDBITE OF WHALES BREATHING)
STREMPLE: Their backs are slick, dark arches on the water.
For NPR news, I'm Claire Stremple in Glacier Bay, Alaska. Transcript provided by NPR, Copyright NPR.