The dead salmon, which had been used to produce eggs and baby fish for the Pleasant River, were discovered by hatchery staff around 8 a.m. Wednesday. While the exact cause is still under investigation, staff said something caused the water flow into the giant tank to drop by half, resulting in low oxygen levels in the water.
But Paul Santavy, manager of U.S. Fish and Wildlife Service hatchery, said the loss would not affect the facility’s production of young fish and eggs for the Pleasant River. The 142 salmon that died were 5 years old and were slated for release later this year after being spawned for three years.
“All of these fish had produced progeny that are already in the population,” Santavy said. “We were mostly just holding onto them as a backup.”
Craig Brook is one of two federal facilities in Maine for spawning and rearing of Atlantic salmon, which are protected as an endangered species. The Craig Brook hatchery artificially spawns salmon from seven Maine rivers — the Penobscot, Machias, East Machias, Pleasant, Dennys, Narraguagus and Sheepscot — as part of a multimillion-dollar effort to restore the fish to Maine’s waterways.
Millions of young and juvenile salmon reared at the two facilities are released into Maine rivers annually.
The salmon population from each of the seven rivers has a separate area, or module, at the Craig Brook hatchery in order to protect the genetic diversity of each river’s strain of fish. The Pleasant River module was the only one affected by the malfunction.
The problem so far has stumped hatchery staff and an engineer brought in to investigate. Santavy said there is speculation that air may have built up in one of the lines, thereby blocking the water flow.
“This facility has been operational for 10 years and nothing like this has ever happened before,” Santavy said. The Pleasant River’s system has since been restored to normal, he added.
This is the second mysterious incident this year that resulted in deaths at the hatchery.
During the winter, about 50 percent of the eggs for the Penobscot River inexplicably died. But despite the loss of almost 800,000 eggs, the facility still was able to produce a normal contingent of salmon hatchlings — known as fry — because the other USFWS salmon facility at Green Lake near Ellsworth keeps a backup line of eggs.
Fish experts at three different facilities looked for causes in the mass die-off of eggs but were unable to come to a conclusion.
Unlike most of the adult brood stock used for spawning at Craig Brook, the Pleasant River fish that died this week had never spent any time in the wild. Instead, they were hatched at the facility and raised to adulthood.
Brood stock from the Penobscot are composed of adult fish that are captured while returning to the river to spawn. Brood stock from most of the other rivers — where annual salmon returns are small to negligible — are gathered by recapturing young salmon released more than a year earlier and then raising them to adulthood at the hatchery.
The Penobscot remains the only river in the U.S. with a sizable run of spawning salmon.
More than 1,900 salmon have been caught at the Veazie Dam trap on the Penobscot so far this year, which is slightly less than last year but still well above the 30-year average. Of those, 679 were taken to become brood stock at Craig Brook while the remaining fish were released above the dam.