When the first spindly, armour-clad carcass was spotted in the fast-flowing Nechako River in early September, Nikolaus Gantner and two colleagues scrambled out on a jet boat, braving strong currents to investigate the grim discovery.
Days later, the remains of 10 others were spotted floating along a 100km stretch of the river in western Canada.
In total, 11 endangered white sturgeon have mysteriously died in a short period of time, blindsiding biologists, who are trying to save a fish teetering towards extinction.
The species has remained relatively unchanged in 200m years: toothless apex hunters that glide gracefully in a handful of British Columbia’s rivers. To navigate the murky waters, sturgeon gently brush whisker-like barbels that hang from their snout along the gravelly bottom.
White sturgeon, with a torso clad in five distinct bony plates called scutes, look every inch a prehistoric fish. The largest ever recorded reached 20ft long and another, believed to be 104 years old, weighed nearly 1,800 lbs.
“When you see a massive head appearing through the murky water and the eyes look at you, it’s just incredible to see this majestic animal alive,” said Gantner, a senior fisheries biologist with the British Columbia government. “And you gain respect for it, knowing that most fish we see are older than us.”
The rapid succession of deaths has taken an unexpected emotional toll on Gantner and his colleagues.
“I’m deeply saddened. These last couple of weeks, I feel like I’m going through grief,” he said. Each time he and colleagues tenderly move the hulking carcasses of the fish from the shore to the freezer and on to the necropsy table, he feels a pang of sorrow. “I don’t think I felt like that from other fish that I’ve worked with.”
So far, there are no obvious answers. The team hasn’t found any sign of trauma nor evidence of chemical exposure, disease, or angling-induced death.
“Whatever it is, it affects larger sturgeon, not other species. It’s constrained to a place in time and space. So that gives us some clues,” said Steve McAdam, a biologist with the province’s ministry of land, water and resource stewardship. “In a way, it’s easier to rule a bunch of stuff out than to rule some things in.”
The deaths in the Nechako are particularly painful for McAdam, who studied a similar die-off in the lower Fraser River in 1993 and 1994, when the region lost 36 fish in two years.
A battery of tests that followed that die-off was inconclusive, said McAdam; the events occurred in differing ecosystems, hundreds of kilometres apart, offering limited clues to investigators.
Because the team investigating the current episode has a narrow window of time to recover dead sturgeon before decomposition sets in and destroys valuable clues, they have appealed to the public for help. In a region where the fish have deep cultural ties to First Nations and are part of the curriculum in local schools, residents have paid close attention to the phenomenon.
A range of theories have been suggested, including a belief that elevated water temperatures are to blame. But McAdam said previous hot summers had not triggered similar die-offs.
“There’s no end to the ideas. There are some partial explanations, but we’re really trying to keep an open mind and not veer too far down one path,” he said.
Before the mysterious die-offs, white sturgeon, which are listed as a federal species at risk, were already in trouble.
Over the last century, the numbers in the Nechako River have dropped from more than 5,000 to only 500. Soon after a dam was built on the Nechako River in 1957, the species experienced what biologists call “recruitment failure” – new fish weren’t being added to the population.
It is from within that ageing population, already missing an entire generation of fish, that the 11 have died.
Overfishing, drainage projects and dam construction have all contributed to the collapse. On all the rivers in the province where sturgeon once thrived, dams have crushed their populations. Only the Fraser River, the largest without a dam, has a relatively healthy sturgeon population in the tens of thousands.
British Columbia has worked since 2001 to help the species recover, drawing teams of provincial and federal biologists, First Nations groups and the industries tied to sturgeon habitat loss, like hydroelectric dam operators.
Efforts include using hatcheries, a “stopgap measure” to help the population recover, as well a longer-term goal of restoring habitat.
But the sudden death of 11 members of a species already spiralling towards demise mirrors a trend all over the world: sturgeon have become the most threatened genus of fish.
All of the 26 remaining species of sturgeon are now at risk of extinction. They are the victims of overfishing; in some species, like beluga sturgeon, the roe is prized as caviar. And the habitats they have persisted in are disappearing.
“They are a quite a charismatic species and it’s a fish that has been around for millions of years. So you don’t take it lightly when it’s in danger,” McAdam said.
The abruptness with which the fish have died has puzzled biologists in part because white sturgeon have been closely studied and monitored for the last three decades, precisely because of their precarious situation.
“And then within a week, this happens. We have a new huge question mark,” said Gantner. “It’s really blindsided us.”
Both Gantner and McAdam were hopeful that the deaths will serve a broader end, providing valuable insight to biologists into what might have happened – and how a similar outcome can be prevented in the future.
Because the other option – that they have already reached some kind of a tipping point – is too bleak to consider.
“We’ve never done the experiment of eliminating them fully and seeing how truly important sturgeon are to an ecosystem,” said McAdam. “And personally, I don’t think we ever want to.”