PINNIPED PREDATION
With chinook salmon on the decline and some policy makers blaming harbor seals, marine scientist Austen Thomas collects seal scat samples to track their consumption.
Story by Hayley Deti | Photos by Hannah Gabrielson
Off the coast of Vancouver Island, British Columbia, marine scientist Austen Thomas carefully examines the rocky shoreline. He crouches down and with the reassurance of rubber gloves and a disposable wooden tongue depressor, scoops up fresh scat left behind by a harbor seal. After collecting a few more steaming samples, he makes his way back to the boat.
Chinook salmon in the Salish Sea have been on an alarming decline and some blame the harbor seal. Decreased salmon populations pose a challenge for many species competing for this diminishing food source. With growing enmity from communities of the Pacific Northwest, lethal removal of harbor seals has become a subject of discussion.
The Salish Sea Marine Survival Project, a joint effort between American and Canadian scientists, is studying salmon decline to find possible solutions.
As a researcher for the project, Thomas spent two years collecting seal scat samples in the Strait of Georgia — the Canadian portion of the Salish Sea. He focused on rivers that carried a considerable amount of salmon. Carefully picking through the scat to find salmon vertebras, he and his colleagues measured the size differences between adult and juvenile salmon bones. Then, they amplified the fish DNA found and with some quick math, the team calculated what percentage of harbor seal diets come from chinook salmon.
Harbor seals, according to Thomas’s research, are individually eating around two kilograms of fish per day. Most of it is juvenile chinook salmon.
“A harbor seal would love to eat all day long on adult chinook salmon, but they have trouble catching them,” Thomas said.
Harbor seals are a species of pinnipeds — a group of marine mammals that can spend time on land, but are fully adapted to live in water. These seals live in the Salish Sea year-round and are the most abundant marine mammal in the area. But this hasn’t always been the case. Until the Marine Mammal Protection Act passed in 1972, seals were heavily hunted due to the belief they competed with fisheries. Their population has since grown and is now believed to be the largest this ecosystem can support. With their success story comes a larger number of hungry pinnipeds and a population of salmon, already battered by habitat loss, toxic chemicals and warming rivers, that can’t keep up with how fast they’re being consumed.
One adult chinook salmon can provide enough food for a single harbor seal, but in place of that one adult, they would need to consume several dozen juvenile salmon, said Alejandro Acevedo-Gutierrez, a marine mammal ecology professor at Western Washington University.
The chinook salmon decline is directly affecting endangered Southern Resident orcas. The fatty, big salmon are the orcas’ food of choice. In 1995, there were 98 orcas and as of December 2017, there were 76 alive. This group of orcas is protected under the federal Endangered Species Act, but the population shows little sign of recovery, according to the Center for Whale Research.
“When it comes to the Southern Resident orcas, I personally believe that the dependence on chinook is a bummer,” said Acevedo-Gutierrez.
Since his time spent collecting samples in Canadian waters, Thomas has moved back to Washington state, and is continuing his research- this time collecting scat from various locations within the Puget Sound. With these samples, he has come to the conclusion that the consumption of chinook salmon by marine mammal predators has increased substantially within the last forty years.
Such findings are helping to put seals in the cross-hairs. In March of 2018, Governor Jay Inslee signed Executive Order 18–02, establishing a task force to aid in the recovery and future sustainability of Southern Resident orcas. The task force, discussing possible long-term actions, has suggested methods of dealing with the seals, including lethal removal.
“In my sense, removing the pinnipeds,” Acevedo-Gutierrez said, “would just create a situation where something else will move in and pick up the slack.”
Thomas said that if the community does pursue exterminating seals in the Puget Sound, they have to do it correctly and keep numbers in mind.
“It has to be dramatic, meaning we would need to remove close to 50 percent of that population, and you would need to maintain that decreased number of animals for a long period of time to even be able to see a response,” he said.
