Blue herons preying on young salmon
Evidence under Stanley Park rookery indicates birds are predators of fish
The excrement of great blue herons in Stanley Park has produced the equivalent of scientific gold.
The findings are part of the first study to show herons eat so many young salmon, or smolts, that they should be considered predators of the fish in the wild.
“The most interesting part is to confirm that herons are a significant predator of migrating salmon smolts,” said Zachary Sherker, a masters of science student at the University of B.C.
In the summer of 2017, Sherker went to the heron rookery by the tennis courts in the park with a mobile detector looking for passive integrated transponders, or PIT tags. Each tag is a thin glass tube with a microchip about 12 mm in length. They’ve been placed in the stomach of salmon in the Capilano River since 2008.
Sherker found about 600 tags underneath the heron nests in the bird’s excrement.
Sherker wasn’t so much surprised as excited: it confirmed earlier findings in the Cowichan River that herons were eating Chinook smolts.
“Herons were one of the predator species we looked into and ended up finding a considerable amount of tags and identifying them as a major predator,” he said.
PIT tags remain dormant until a low frequency magnetic signal activates them and sends a unique code back to the detector.
The study is Sherker’s thesis project looking at the role played by herons in the decline of salmon. He’s studying under supervisor Andrew Trites in the Marine Mammal Research Unit.
Sherker is working in collaboration with the B.C. Conservation Foundation and Cowichan Tribes. The project is funded by the Pacific Salmon Foundation.
Sherker said after about 400 tags were found in droppings in the Cowichan heron rookery, he decided to check other rookeries near river systems.
“To find the number of tags in the heronry in the Cowichan was a big surprise,” he said.
“We didn’t know if this heron predation would translate to other river systems.”
In analyzing the data from Cowichan, he found that larger smolts are more likely to be eaten by herons. He said this may be due to the larger smolts swimming closer to the centre of the river while smaller ones swim closer to the edge.
When magnetic arrays are installed along a river, they trigger PIT tags so that the smolt’s movement can be tracked as they swim by. When there is low water depth, for example, data collected from PIT tags can show how far the smolts make it and how many survive.
When the Cowichan River had low water flow in 2016, predation by herons almost doubled.
“We suspect that this may be due to the fact that herons are limited to foraging by water depth,” he said.
“When the river goes down and the flow goes down, it may be expanding the heron foraging grounds by making new areas accessible.”
In periods of low water depth, salmon move more slowly, which would also increase their exposure to predation, he said.
The hatchery in Cowichan River releases about 600,000 smolts a year. In a normal water flow year, heron consume one to 1.5 per cent of that number. In a year when there is a low water flow, that can increase to about three per cent.
But he cautioned that predation rates may be higher. Herons, he pointed out, don’t defecate exclusively in the rookery, so not all tags would be deposited underneath their nesting spot. Plus, not all tags are detected in the excrement in a rookery.
“This one to three per cent, depending on flow, is a minimum predation estimate,” he said.
To find the number of tags in the heronry in the Cowichan was a big surprise.
ZACHARY SHERKER