I'm a Trout Unlimited volunteer in Washington State and have been involved with urban stream restoration projects for the past 20 years. The good news is that we have Coho & Chum Salmon returning to urban streams that were essentially barron during the 50's/60's. The bad news is, we've been experiencing "pre-spawn mortality" for the past 10 or so years with our returning Coho.

Pre-spawn mortality is a condition where apparently healthy salmon enter a stream on their spawning run and within a day or so in the stream, die for no apparent reason before their able to spawn. When the fish die, they have gaping mouths (appearing like they can't breathe), try to jump out of the water (like the water is toxic) and die while in pristine looking condition. What's even more puzzling is that Coho fry and Cutthroat that are in the stream at the same time don't seem to be affected. We have seen "kills" of fry in the stream, but not at the same time of the Coho pre-spawn mortality.

Testing hasn't revealed problems with the water, autopisies haven't indicated problems with the fish, but yet they die. Many reasons have been speculated, but I haven't found anyone with definative proof or a solution.

Do you know why? Andy, Seattle, Wa.

A. Dear Andy:
From your concluding sentence, it appears that others have been involved with this problem (water chemists, biologists), and it seems that these folks, who are much closer to the situation than I am, would be in a better position to come up with answers. You've got a lot of resources in your area (state fisheries biologists, the UW School of Fisheries, USFWS) and I would suggest that you pursue this with these folks, if you haven't already. You've likely already done this, but that's about the best I can offer.

I did, however, contact 3 former colleagues (fisheries biologists) at Battelle in Richland (I spent 35 years there before retiring) to see if they could offer anything; here are the responses from 2 of them:

One said: "It may be a cse of "metabolic overload" in that adult salmon have greater metabolic requirements than juveniles. They could get stacked up, be stressed due to low water (and possibly disease), and "freak out," thus putting them over the top with respect to oxygen requirements."

The second offered: "Pre-spawning mortality is a prescribed fate of fish that die before they spawn. There are many reasons attributed to this particular fate and it is not attributed to only 'apparently healthy' fish. Sick and injured fish can also die before they spawn. I believe pre-spawn mortality only applies to fish that have made it to the spawning area. Fish that are harvested by man or beast are counted differently wen modeling to estimate recruitment or survival. Dam fall back and straying are also considered differently than pre-spawning mortality.

This is the first article that comes up on Google https://www.pnly.gov/ecology/pubs/PDFs/PO4_00216.pdf

He continues: "This webpage (https://www.washingtontrout.org/WFRnov03.shtml#RESEARCH) provides a similar definition as the one I provided but includes predation: 'Pre-spawning mortality occurs when adult salmon returning to streams and rivers die before they are able to spawn. To some extent, this is a natural phenomenon. It is known that predation by dogs, bears, otters, heron, and other animals that prey upon adult salmon contributes to pre-spawning mortality. Similarly, when stream flows drop quickly after a storm event, adult salmon may die from stranding before they are able to spawn.' "

He concludes: "Googling prespawning mortality will give you much more about dead salmon than most anyone could possible want to know."

Well, I don't know how much any of the above is new to you or could possibly be helpful in your situation, but it's about all I can come up with. I wish you luck in your endeavors.

Best wishes, ~ Bert

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584
Email: streamdoctor@aol.com

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.