For the third time in five years, a major fish die-off has struck Puget Sound's beleaguered Hood Canal, adding to evidence that the fjord...

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For the third time in five years, a major fish die-off has struck Puget Sound’s beleaguered Hood Canal, adding to evidence that the fjord remains sick.

Tuesday, carcasses of flounder, ling cod, rockfish, wolf eels and shrimp were found washed up on the shores of the southern part of the canal, in the western flank of the Sound. Some usually deep-water fish were gasping at the surface for oxygen, officials reported.

“It’s alarming,” said Wayne Palsson, a researcher for the state Department of Fish and Wildlife who spent part of Tuesday diving in the canal to document the damage. “It’s emotional seeing this happen.”

The latest episode is but a visible illustration of a problem that has been afflicting the canal all summer — lethally low levels of oxygen. Until now, the problem was confined to deeper water, so most of the fish could survive by staying in shallow water, said Jan Newton, a University of Washington scientist who has been leading research on the canal.

But around midnight Monday, the low-oxygen water shifted to the surface so quickly that fish apparently had nowhere to go and suffocated, she said. Dead fish were surfacing throughout the southern third of the canal.

“It does appear to be substantial and widespread,” said Greg Bargmann of the state Fish and Wildlife Department.

Winds from the south or cold ocean currents are two possible reasons for a sudden rise of low-oxygen water, Newton said. Wind could push oxygen-rich water on the surface of the canal north, causing deeper, oxygen-starved water to rise up in the south. Colder ocean water could also be pushing the deep, low-oxygen water to the surface.

After major fish kills in 2002 and 2003, scientists, politicians and government agencies united to try to identify the source of the trouble and figure out what to do about it.

The lack of oxygen could come from changes in how the canal is flushed by the currents, or excess nitrogen and sunny weather fueling algae blooms, which then die and decompose, consuming the water’s oxygen.

Warren Cornwall: 206-464-2311 or wcornwall@seattletimes.com