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In Southcentral Alaska, salmon declines are pinned on a toothy invader

 

Earlier this month, I wrote about the Yukon River’s chinook salmon runs, which have lately plummeted for reasons that remain murky. While researchers are years from cracking that mystery, the Yukon isn’t the only Alaskan river losing its salmon. In the state’s Susitna River basin, which courses through Southcentral Alaska near Anchorage, the mighty fish has also crashed. And unlike in the Yukon, scientists may have pinpointed the culprit: Northern pike, a rapacious invader with an appetite for young salmon.

That pike are invasive in the Susitna is a curious artifact of Alaska’s rugged geography. The fish — snaky creatures whose long jaws are spiked with needle-like teeth — are native throughout the American Midwest and Canada, and they also naturally inhabit the entirety of northern Alaska. But the Alaska Range, a 400-mile-long band of mountains that curves from Lake Clark to the Canadian Yukon, historically blocked the pike from the state’s southern reaches.

In Southcentral Alaska, invasive Northern pike can grow to more than 30 pounds. Photo courtesy of the US Geological Survey.

That changed in the 1950’s. Scientists aren’t sure how pike ended up in southern Alaska, but the most likely explanation, says Adam Sepulveda, aquatic biologist with the U.S. Geological Survey, is that an ice-fishing aficionado airlifted them by floatplane.

Although it took a few decades for the pike population to grow large enough to harm salmon returns, the nosedive, when it came, was swift. In a Susitna tributary called Alexander Creek, for instance, the number of chinook escaping to their spawning grounds plunged from nearly 4,000 fish in 1999 to less than 200 in 2010. Anglers grew despondent; once-vibrant fishing camps were boarded up. "Pike are the only species where fishermen will buy me a beer when they find out I'm working on them,” Sepulveda says.


Nonetheless, while anecdotal evidence of pike gluttony abounded, scientists weren’t sure if the behemoths were truly to blame for salmon collapses. Researchers suspected that other factors, like overharvest, bycatch, and climate change, were also responsible. Were there really enough pike to explain the disappearance of salmon?

The answer, says Sepulveda, is yes. He and some colleagues spent 3 years gillnetting pike along the length of Alexander Creek, cutting the fish open, and examining the prey in their stomachs. The conclusion, published earlier this month: Pike in Alexander Creek are capable of eating as many as 553,000 young salmon each summer — potentially every single salmon that hatches in a given year. What’s more, the predators sought out salmon even when there weren’t many to be had, suggesting pike have a taste for salmonid morsels.

At this point, attentive readers might be thinking: Wait a second, pike are native in Northwestern Alaska, yet places like Bristol Bay still have salmon out the wazoo. How do those fish manage to avoid becoming a pike’s lunch?

The difference, explains Sepulveda, is that salmon that have evolved in the company of pike favor deep-water refuges and steep streams, habitats that pike avoid. Salmon in Alexander Creek and other Southcentral systems, by contrast, didn’t face the same evolutionary pressure, and so tend to inhabit shallow, weedy areas — ideal conditions for a predator lying in ambush. If you wanted to create the perfect pike playground, you couldn’t do much better than Alexander Creek.

Pike are almost certainly in the Susitna Basin to stay, then; the question is whether their populations can be kept under control. The solution might be an intensive gillnetting campaign, like the one the National Park Service is currently waging against Yellowstone’s invasive lake trout. But while the Alaska Department of Fish and Game has reported some success in reducing pike numbers via gillnet, a potential full-scale assault would prove awfully tough to stage: While the Susitna drainage is the size of Indiana, it has all of two roads.

“From an ecological perspective, removing pike doesn’t look very promising,” Sepulveda acknowledges. “But because it's close to Anchorage, Alexander Creek is a very important fishery. From a social standpoint, it would be a great place to try something positive.”

Ben Goldfarb is a correspondent at High Country News. He tweets @bengoldfarb13.