This Oct. 17, 2016 photo shows exposed cottonwood tree roots after European nightcrawler earthworms and two other earthworm species ate through the upper soil layers near Stormy Lake in Nikiski, Alaska. (Photo courtesy Matt Bowser/Kenai National Wildlife Refuge)

This Oct. 17, 2016 photo shows exposed cottonwood tree roots after European nightcrawler earthworms and two other earthworm species ate through the upper soil layers near Stormy Lake in Nikiski, Alaska. (Photo courtesy Matt Bowser/Kenai National Wildlife Refuge)

Coho salmon adapt to chow down on earthworms

Baby coho salmon have to last all winter in Kenai Peninsula streams without much to eat, but in recent years, they’ve been getting a new menu item in the spring — invasive earthworms.

A team of researchers from the Kachemak Bay Research Reserve, which is part of the Alaska Center for Conservation Science at the University of Alaska Anchorage, have found that juvenile coho salmon in the Anchor River watershed have been filling up on a type of earthworm in the spring. The earthworms, not native to the area, have helped the salmon regain all the weight they lost in the winter of 2012–2013, and then some, said Coowe Walker, one of the researchers on the project.

“They gain all that weight back and then some in (roughly) two weeks,” Walker said. “They’re just gorging on earthworms.”

Coho salmon, also known as silver salmon, populate many of the streams on the Kenai Peninsula, laying their eggs between July and November. The eggs hatch in the early spring and the baby salmon emerge from the nest in May or June, spending between one and three winters in streams and up to five winters in lakes before migrating to the sea, according to the Alaska Department of Fish and Game. The juvenile coho feed on aquatic insects and plankton as well as salmon eggs deposited in their streams.

The earthworms are a new staple, though. Walker said she and the other researchers observed the trend in other studies and decided to investigate as a side project. When they pumped the stomachs of some of the juvenile salmon they sampled, they were stuffed with two types of worms, neither native to Alaska. In fact, there are no known native earthworms to the state.

Dolly Varden trout also prey on the earthworms, Walker said. The Dolly Varden they studied, taken on the same day as the coho, were from a different stream system but also showed earthworms in their stomachs.

Invasive species are often needled as bad and needing to be eradicated. Exotic earthworms do eat a lot of plant matter in forest ecosystems that would otherwise decay and become nutrients for the forest. But in this case, they benefited young salmon by providing a nutrient-dense food. The extra nutritional boost may also improve their chances at sea, according to a presentation Walker and her team assembled on the project.

The earthworms the juvenile coho are eating are much more packed with nutrients than some of the other prey available to them. For example, one earthworm is equal to about one kilojoule of energy. It takes 419 Baetis mayfly larvae or 239 midge larvae to equal one earthworm. It’s about the same as one sockeye salmon egg, according to the presentation.

Though there are no known earthworms native to Alaska, they are all over the peninsula now, said Matt Bowser, an entomologist with the Kenai National Wildlife Refuge. Bowser has authored and assisted with several studies on different types of earthworms on the refuge since 2010. The worms are introduced in three main ways, depending on the type of worm, he said.

“They can get spread by tires and people will dump their fishing bait,” he said. “We have a couple species on the refuge that were probably dumped that way. Nightcrawlers and red marshworms — you can buy these in tackle shops … The other way they get around is plantings, anytime you move around soil.”

In some cases, the worms haven’t made a huge impact yet, as in the case of octagonal-tailed worms. However, , the effect is visible in the area around Stormy Lake. European nightcrawlers, so named because they burrow deep and emerge at night, haul in leaf litter from the forest floor and eat it in their burrows. In some areas around the lake, between the nightcrawlers and two other worm species, so much of the upper soil layer is gone that the tree roots are exposed, Bowser said.

“When you walk over there by Stormy Lake, you’ll see the leaf litter is just gone,” he said. “You’ll see there was leaf litter in the past, and you’ll see old moss lines on the tree.”

The primary impact earthworms have is on soil composition. As engineers of their environment, they cycle nutrients much faster than Alaska’s forest ecosystems naturally would. That may lead to the propagation of plants that do better in those soils, such as some of the plants native to European soils that are invasive here, but it’s hard to say what will happen here, he said. Their presence likely does have a positive impact for salmon and other fish species, like rainbow trout, he said.

Worms are limited in range. By themselves, they’ll only move about 30 feet a year, so in the case of the European nightcrawlers near Stormy Lake, they’re almost exclusively located next to the boat ramp, where they were likely introduced, he said. The smaller worms, like the octagonal-tailed worm, have been able to get around in vehicle tires and are now fairly widespread on the refuge, he said.

Survival also varies based on species. The worms that most people buy for their compost bins, known as red wigglers, are fairly harmless because they can’t survive low temperatures and are used to having a readily available food supply, Bowser said.

Kyra Wagner, the district manager for the Homer Soil and Water Conservation District, said red wigglers are safe for people to have in their compost bins because they’re not likely to become invasive, she said.

“They’re like the household pets of the worm world,” she said. “They’re totally used to having high intense food ratios. They’re put in worm bins … there’s a lot of food going in there. If you were to let them loose in the real world, they are used to getting food scraps every day … and they wouldn’t survive very well.”

The worms that Walker and her team studied are unusually aquatic — most worms are water-averse, but that type has actually been known to be in the water, Bowser said. They haven’t been spotted on the central Kenai Peninsula yet. But the refuge scientists are keeping their eyes out for earthworms — they did more survey work this year, he said.

“We were working on a monitoring project … we added earthworms into it as part of our protocol,” he said. “… I’m always just poking around in the leaf litter.”

Walker said she and her team don’t have the funds to continue working on the worm project at the moment but would like to do more research on it in the future.

“We’re documenting something that’s going on, and what we’re learning raises questions we would like to pursue,” she said.

Reach Elizabeth Earl at

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