Steelhead out-migrating from North Sound rivers appear to have better marine survival than steelhead smolts from South Sound, and researchers are studying salmon poisoning disease as a potential cause.

Salmon poisoning disease, or Nanophyetus salmincola, is best known as the parasite that can make dogs sick when they eat raw salmon. It also has been found to affect the swimming performance of infected salmonids, potentially reducing their marine survival.

Fish pathologist Martin Chen is studying steelhead smolts for the NWIFC as part of the Salish Sea Marine Survival Project. Chen is sampling steelhead from the Tahuya, Skagit, Snohomish, Green and Nisqually rivers.

“We thought Skagit and Snohomish would be negative for the parasite based on historical records, and that was true,” Chen said. “We found lots of heavily infected fish in the Green and Nisqually rivers. I can see the parasite even as I’m dissecting them. Parasite numbers are as high as 18,000 per gram of kidney tissue.”

Both of those results were expected, but Chen was surprised not to find the parasite in any of the 24 steelhead sampled from the Tahuya River in Hood Canal. The Tahuya is across the canal from the Skokomish River, which has Nanophyetus in at least three tributaries.

South Sound fish also have their marine survival challenged by swimming through the Tacoma Narrows and Point Defiance, and facing more predation from seals. To make a more even comparison, researchers placed infected South Sound fish and uninfected North Sound fish in the same saltwater environment to compare survival. After 90 days in saltwater tanks at the USGS Marrowstone Island Laboratory, both groups had the same high survival rate.

“The fish from the Green River are still heavily infected,” Chen said. “It’s possible that infected fish have some disadvantage and they’re less able to escape predators. They don’t just hit the salt water and roll over and die.”

Chen also is testing drugs that could eliminate the parasite from infected fish. Two drugs are being tested on small numbers of coho at the Nisqually Tribe’s Clear Creek Hatchery, with some coho getting a 24-hour bath of each drug, and other fish having a drug mixed in their food. None of the fish from this initial test will be released or eaten.

“If the drugs were effective and you were rearing fish in a parasite-positive environment, you could clean up a group of fish before releasing them,” Chen said. “You would no longer have to compare survival of fish between two river systems, making our studies more valid. In addition, being able to eliminate Nanophyetus before release could have a practical application for Northwest hatcheries.”

For more information, contact: Martin Chen at marchen101@aol.com