Q.
Dear Stream Doctor,
Although this question doesn’t directly have to do with
fishing for salmon, I was hoping that you (or your contacts)
may have an answer for me.
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.
Originally published November 7, 2005 on Fly Anglers Online by Bert.