Pacific Great Blue Herons Are Snacking on Thousands of Salmon

University of British Columbia doctoral candidate Zachary Sherker used a specially designed magnetic device to scan the forest floor under heron rookeries to find Passive Integrated Transponder tags placed inside juvenile salmon by scientists to study their migration patterns. Many of those tagged fish were eaten by Pacific great blue herons, who excreted the tags. Photo courtesy of Zachary Sherker.

A University of British Columbia study published in the Canadian Journal of Zoology estimates that thousands of juvenile wild and hatchery reared Chinook salmon and hatchery Coho salmon in the Salish Sea region of British Columbia are being eaten by Pacific great blue herons.

The study, led by UBC doctoral candidate Zachary Sherker, is the first to estimate the abundance of juvenile salmon being preyed upon by an understudied bird species.

Sherker had been looking for evidence of salmon being preyed upon by freshwater predators recorded in western scientific literature, like racoons, otters, king fishers and mink, but found nothing. Then on a ride to seal haul-outs with Cowichan Tribes biologist Tim Kulchinksy, said Sherker, Kulchinksy observed a bunch of herons foraging at the outflow of the river and asked if Sherker had ever thought of herons.

That day, Sherker scanned the forest floor of heron rookeries and recovered from scat beneath the nests about 100 Passive Integrated Transponder tags that had been placed inside juvenile salmon by scientists and salmon hatchery staff between 2008 and 2018 to study their migration to the Pacific Ocean.

Those tags are about the size of a grain of rice, Sherker said. When heron swallow the fish whole, the tag passes straight through the heron’s digestive tract and out in the scat. Considering the number of tags deposited outside of rookeries, Sherker and his co-authors estimate that heron predation may account for up to 3% of all juvenile salmon deaths and could be as high as 6% in some years with low water flow.

“It should be noted that the relatively small size of juvenile salmon compared to other heron prey items suggests that they may be a particularly important food source for chicks during early rearing, when young herons are gape-limited and at high risk of choking on larger fish prey,” Sherker said.

Knowing where young salmon are dying has become more critical as salmon stocks decline. Researchers believe that herons preying on smaller, weaker salmon may even be beneficial.

According to Sherker, such predation could benefit salmon stocks by weeding out the weak and allowing for less competition and higher growth among other fish in these critical juvenile life stages.

The great blue herons are stalk-and-strike hunters who locate their prey by sight, and so need to hunt by day. They belong to a non-migratory and marine-oriented subspecies that range from Alaska to southern Washington state, with the largest concentration occurring in northwestern Washington and southwest British Columbia. During the non-breeding season great blue herons are widely dispersed in Puget Sound, foraging and roosting in coastal and lowland areas.