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For four months, University of British Columbia master’s student Zachary Sherker tried to solve the case of the missing tracking devices.
From 2008 to 2018, researchers and salmon hatchery staff inserted more than 100,000 tiny passive integrated transponder tags into the abdomens of wild and hatchery-raised salmon smolts to follow their migration to the Pacific Ocean. But thousands of the tags mysteriously vanished.
Across the Pacific Northwest, wild salmon populations are in peril. As many as 50 percent of juvenile salmon in the Cowichan River on southeastern Vancouver Island, British Columbia, for example, die during their downstream migration. But what exactly happens to them, and which predators pick them off, whether birds, mammals, or other fish, has been the subject of ongoing—and sometimes quite heated—debate.
So Sherker searched for the missing tags, using technology rather than the naked eye. He hauled a 24-volt battery in a backpack, and scanned the ground using a custom-designed powered magnetic antenna array featuring a loop of cable at the end of a 1.2-meter-long pole. The array reactivated the dormant tags, alerting Sherker with an audible beep.
To little avail, he scoured harbor seal haulout sites and the forests bordering the Cowichan River in hopes of detecting any of the glass-encased tags—each about the size of a grain of rice—that had been pooped out by predators.
Sherker even investigated the communal “latrines” frequented by river otters. “Follow enough otter slides that come out from the river and eventually you’ll find a big pile of crap,” he says. “They defecate and urinate and basically roll around in each other’s scat.”
As a last resort, he ventured beneath a great blue heron nesting colony located close to the Cowichan River estuary. It was messy work. “You had to find the right quality of hat so that it wicked off a lot of the wet guano,” says Sherker.
“The smell is something you get used to. When you scan a whole summer, your smell receptors sort of adapt in your favor,” says Sherker, who slept in his wanting 1993 Nissan pickup truck during much of the field season.
But the malodorous working conditions yielded success. Sherker discovered a mother lode of more than 450 tags nestled in the heron guano. More searching eventually revealed about 1,200 tags when he also scanned heron nesting colonies at Big Qualicum River, also on Vancouver Island, and Capilano River, on the British Columbia mainland north of Vancouver.
Each of the tags Sherker pulled from the guano tells a story: of when it was inserted into a smolt, but also about the species of salmon, the location of its natal river, and the number of fish each was released with. The oldest tag Sherker and his colleagues found had sat dormant since 2008. “They were missing in action,” he says. “We were sort of able to revive them back to life.”
The finds offer new clues to help answer the long-standing puzzle of what is eating the salmon.
Based on the tracking tags they found, Sherker and his colleagues estimate that heron ate about 1.3 percent of all the tagged smolts. The rate increased to as high as 3.2 percent when the study included an estimate of unfound tags defecated outside the colonies.
One dry year proved exceptional. The study estimates that in 2016, when water levels in the Cowichan River were critically low and pools dried up, herons feasted on as many as six percent of the smolts.
The research supports the findings of an earlier study in the Columbia River on the west coast of the United States that showed how birds can have a considerable impact on juvenile salmon.
In a bid to reduce predation on millions of juvenile salmon, the United States government shot 5,576 adult double-crested cormorants and destroyed the eggs in 6,181 nests between 2015 and 2017. Starting in 2008, officials also employed rows of fences made of landscape fabric plus stakes, ropes, and flagging to reduce nesting habitat for Caspian terns. Management actions against both species occurred at or near their breeding colonies strategically located on East Sand Island at the mouth of the Columbia River.
Daniel Roby, a professor emeritus with the Department of Fisheries and Wildlife at Oregon State University, says bird predation of salmon smolts in the Columbia River is “orders of magnitude” greater than in the three British Columbia rivers highlighted in Sherker’s study.
While he is surprised by the high predation rates by a relatively small number of herons—there are only about 420 adults in the three colonies—the large number of hatchery-reared salmon released during the herons’ nesting season understandably benefitted the birds.
“Clearly, the birds have figured out there’s an abundance of food to be had when these smolts are released,” Roby says. “They’re just doing what they do—they’re eating fish. And with these hatchery releases we’re setting the table for them.”
However, Sherker notes that hatchery releases may be bolstering the salmon population to more closely approximate the number of smolts that historically migrated down these rivers.
Sherker, crediting the BC Conservation Foundation and Cowichan Tribes for assistance with his study, does not foresee a similar cull of herons in British Columbia. But on river systems where weirs are employed to regulate flows, he says managers might consider releasing more water in the springtime to improve smolt survival during their downstream migration.