At least some of the 656 fish found dead last month in Ashland Creek below the city's Waste Water Treatment Plant apparently were exposed to lethal amounts of chlorine, but investigators have been unable to trace the source.
Some of the wild juvenile steelhead found dead in the creek June 24-25 showed they were exposed to almost 10 times the amount of chlorine considered by the state Department of Environmental Quality to be toxic to fish, reports show.
Tests and inspections since then have not pinpointed the source, but no dead fish were found upstream of the waste water treatment plant's outflow into Ashland Creek about one-third of a mile from its confluence with Bear Creek, according to the Oregon Department of Fish and Wildlife.
"Those levels we measured were well above those acute toxicity levels," said Dan VanDyke, the Oregon Department of Fish and Wildlife's Rogue District fish biologist, who helped investigate the case.
"At least at this point, there's no obvious evidence of a release from the treatment plant," VanDyke said. "At least as it stands right now, we don't have a confirmed responsible party."
The treatment plant was initially the sole focus of the investigation into the cause of the fish-kill, and VanDyke said ODFW has no other suspected sources.
Mike Morrison, the city's public works superintendent, said his crews and DEQ officials have looked at all aspects of the plant and have found no way a toxic release could have come from it.
"If it came from our plant, we at this point don't know how it happened," said Morrison, adding that the city will continue to look into a potential source.
The area where the fish died is a public area, and Morrison said he could not rule out someone sabotaging the creek to make the treatment plant look bad.
"We hope that's not the case," Morrison said. "I can't say for sure. We're just looking at all the possibilities."
Of the 656 dead fish collected at the scene, 277 were juvenile wild steelhead, according to the ODFW report. Another 371 were sculpin, which are native to the Bear Creek Basin. Seven were juvenile wild chinook salmon, and one non-native red-sided shiner was among the dead.
The fish were in good health and actively feeding before they were killed, VanDyke said. Other tests showed no bacterial or viral causes, he said.
None of the dead fish were wild coho salmon, which are listed as threatened under the federal Endangered Species Act.
Tests on a portion of the dead fish were conducted at the ODFW's fish-health lab in Corvallis.
One set of steelhead had 0.1 to 0.2 parts per million of chlorine in their gills, while another group had 0.1 ppm. The DEQ standard for acute toxicity for a one-hour exposure to chlorine is 0.019 ppm.
Since the fish-kill, DEQ inspectors have noted live fish using that reach of Ashland Creek, but some riparian plants have died, VanDyke said.
No one investigating the die-off smelled chlorine or herbicide, and no dead fish were found in return inspections, VanDyke said.
"All indications are it was a very localized event," he said.
The steelhead loss was almost three times larger than the last known large fish-kill in the basin, which captures urban runoff from five cities. In 2007, about 100 wild juvenile steelhead and coho salmon were found dead in lower Griffin Creek from an apparent petroleum release into the creek, records show. The source of that die-off was never pinpointed, and no one was prosecuted.
Reach Mail Tribune reporter Mark Freeman at 541-776-4470 or firstname.lastname@example.org.