What happens to fish during floods?

[afishinado]

Good article found on the PFBC site this morning:

Trout during floods

 

[JackM]

So they don't drown?

 

[JackM]

It is rare for a wall of water to come surging downstream. Usually the water rises gradually and as maintaining position starts being labored for the fish, they will move to the slack areas and then keep adjusting to the same level of water velocity, hopefully zero. In my diagram, the fish are facing upstream, but in significant eddies behind large obstructions, they may face downstream.

Also side channels/braids may offer refuge. Sometime, as the water recedes, fish can get trapped if they don't have the wherewithal to recede as well-- stockies, probably, because the raceways never dry up.

 

[FiveWeight]

They get a room at the Holiday Inn Express until the water comes back down. The same thing happens to them that has happened for the past bazillion years. Nothing.

 

[salmonoid]

It's touched on in the article, but the science behind sticking to the stream bottom is covered by this:

https://en.wikipedia.org/wiki/Law_of_the_wall

If you're not mathematically inclined, this boils down to there being a logarithmic relationship between stream velocity and distance from the stream bed (the wall). While water might be ripping along at the top of the stream, where you can see it, near the stream bed, it is barely moving. It's a safe place to hang out, just like being downstream of a rock, or close to a bank. Of course, if the fish moves out of the low flow area, then it could potentially be washed downstream, until it either reaches another low flow area, or it succumbs.

More than "nothing" happens to them. The risk for injury or death certainly goes up, but the majority of them survive. Their habitat post-flood might be vastly different, though.

 

[Acristickid]

Fished a small stream in the Laurel Highlands that was hit by a major flood a month or so ago yesterday. It’s a class A rainbow stream. I saw one fish and my buddy missed one. We fished a mile of stream.

We were asking ourselves “What happened to The fish?” The stream doesn’t appear to be in good shape.

 

[troutbert]

acristickid wrote:
Fished a small stream in the Laurel Highlands that was hit by a major flood a month or so ago yesterday. It’s a class A rainbow stream. I saw one fish and my buddy missed one. We fished a mile of stream.

We were asking ourselves “What happened to The fish?” The stream doesn’t appear to be in good shape.

In severe floods, where the stream and floodplain really get shredded, the trout mortality rates can be high.

A study in West Virginia found that in the section of brookie stream they studied, the mortality rate was 95%.

So what happens to trout in a flood? The answer is:

It depends.

The mortality rate can be very low, nearly zero.

Or it can be very high, nearly 100%.

And it can be anywhere in between.

It depends on the landscape and the rates of flow.

The good news is that even where trout populations got knocked down hard, the trout populations usually rebound pretty quickly, within 4 years or so.

 

[troutbert]

Here's video of a debris flow in Colorado.

https://www.youtube.com/watch?v=8mKC3eID074

Another in Washington state.

https://www.youtube.com/watch?v=SrYqJlCuppA

 

[JeffK]

The other thing a flood can do is deplete the bugs in certain stretches of stream - they can't move away as well as trout. A guide I know in NE PA told me how to find fish in the Brodhead after the severe floods in the early 2000's, some which caused the stream to jump to a new bed. Seine for bugs. If they are there, go fishing. If they are absent, go elsewhere. This worked for me when fishing many flood ravaged streams.