Sea otters disappeared in the Aleutian Islands. Then the full effects of climate change manifested

(Credit: J. Tomoleoni/USGS)
Published: Sep. 25, 2020 at 11:37 AM AKDT
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ANCHORAGE, Alaska (KTUU) - In the 1970s, the waters of the Aleutian Islands were a beautiful hue of golden brown with lush kelp forests and abundant sea otters chowing down on sea urchins.

But when researcher Doug Rasher visited the island chain in 2014, that’s not what he saw. The marine environment was pink and blanketed with sea urchins. The kelp forests were gone and so were the sea otters.

“It looks like a barren ghost town,” he said.

Rasher, a senior research scientist at the Bigelow Laboratory for Ocean Sciences in Maine, and several colleagues visited six different Aleutian Islands. They boarded an oceanographic vessel and traveled from Adak to Attu, a 717-kilometer distance, to study the changing habitat in Southwest Alaska’s waters. Their research, published this month in the journal Science, details how climate change has pushed the environment past a critical tipping point.

Sea otter populations in Southwest Alaska have been declining for decades, and researchers wanted to learn how the loss of the key urchin consumer would impact the ecosystem.

James Estes, now an emeritus professor at the University of California Santa Cruz, has been studying the Aleutian Islands since 1970. When he first visited Amchitka Island, he could count anywhere from 50 to 300 sea otters from the shore.

“In the early years, it was a very different place,” Estes said. “Kelp was abundant, otters were abundant, the whole coastal ecosystem was a very different looking place than it was 30 to 40 years later after the otters collapsed.”

The leading hypothesis for the disappearance of sea otters is that killer whales have begun preying on the animal as great whales, Steller sea lions and harbor seals have become less abundant. When the sea otter population dwindled, sea urchin numbers grew.

The urchins devoured entire kelp forests until they were essentially gone. With no more kelp to feast on, the urchins turned to one of the oldest structures in the ecosystem: coral-like algae with a skeleton made of limestone like a “living rock.”

Sea urchins have devoured the kelp forests of the Aleutian Islands and are now feasting on the coral-like algae.
Sea urchins have devoured the kelp forests of the Aleutian Islands and are now feasting on the coral-like algae.(Credit: J. Tomoleoni/USGS)

“Honestly, it’s devastating because the minute you get in, you see that these urchins are wreaking havoc on the near-shore ecosystem and are rapidly grinding away this long-lived reef,” Rasher said.

Warming oceans have further compromised the living rock because its strong, calcified structure is being weakened by acidifying water.

“It’s always been considered this super stable habitat because it’s been there forever. Nobody ever thought it would go away, and recently we have noticed, it has gone away,” said Brenda Konar, an author of the study and professor of marine biology at the University of Alaska Fairbanks.

Otters were able to delay the degradation of the living rock caused by climate change by keeping urchin populations in check. Now that otters no longer serve that function, Rasher is concerned the living rock will disappear before scientists can fully understand its role in the system.

The living rock is a habitat for fish, sea stars, sponges and clams that only associate with the structure, Rasher said. Kelp forests used to grow off of the living rock and house coastal fish, which are similarly depleted due to loss of habitat. This further impacts the food chain since consumers like eagles, which would normally prey on those fish, have turned to feed on seabirds.

This isn’t the first time Alaska otter populations have been depleted. In the 19th century, otters were hunted near extinction in Alaska by Russian fur traders. Sea otters have rebounded and even expanded their territory in many parts of Alaska, but from Castle Cape to the end of the Aleutian Islands, otters are almost absent.

“When sea otters were hunted to near extinction during the maritime fur trade, these reefs were there,” Rasher said. “The difference is now, we not only have the loss of sea otters but also the emergence of climate effects, and that’s what has really changed the game.”

The impacts of climate change and urchin grazing are even more obvious in areas where otter populations never returned after the Russian fur trade, the study found. Those areas have a further eroded living rock.

“Unless we curb our global emissions, the loss of these long-lived reefs is likely to accelerate in the coming decades,” Rasher said.

There is a silver lining in their research. Rasher said restocking sea otters could buffer some of the near-term damage caused by urchins and ocean acidification. However, there are concerns that reintroduced otters could end up as orca food, placing the Aleutian environment in the same situation it faces now.

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