Midwest Fish Kills Exacerbated By Record Heat

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August 6, 2012

This summer, extreme heat and drought have brought on larger than normal “fish kills” throughout the Midwest. Fish are dying by the tens of thousands. All Things Considered host Audie Cornish speaks with Aaron Woldt, Fisheries Program Supervisor for the U.S. Fish & Wildlife Service Midwestern Region, about what’s happening in these waters.

Copyright © 2012 National Public Radio. For personal, noncommercial use only. See Terms of Use. For other uses, prior permission required.


Contemplate this temperature for a moment. The Des Moines River, 97 degrees. Similar temperatures have been measured in other bodies of water across the Midwest this summer and that’s spelled trouble for many fish. The combination of extreme heat and drought has led to worse than usual fish kills, a widespread die-off of fish.

Tens of thousands of fish, including sturgeon, bass, catfish and carp, have been reported dead. To help us understand what’s happening, we’ve called on Aaron Woldt. He’s the Fisheries Program supervisor for the U.S. Fish and Wildlife Service, Midwestern Region.

Welcome to the program, Aaron.

AARON WOLDT: Thank you.

CORNISH: So, to start, we should be clear that fish kills actually do happen every summer, but this year we’re reading that they’re unusually bad. Can you explain what normally causes a fish kill?

WOLDT: Sure. And you’re absolutely correct. You know, fish kills are common in the summertime. They’re also common in the wintertime. It’s part of the natural cycle, but things like extreme heat and drought conditions can cause fish kills to be more frequent or, in some cases, worse.

And, you know, the main thing behind a fish kill is, you know, simply lack of oxygen in the water and that can be made worse by extreme heat because water, when it gets hot, simply just physically can’t hold as much oxygen as it does when it’s colder. So, if the water heats up, it holds less oxygen and, you know, fish will start to die from lack of oxygen.

CORNISH: How much worse are things this year?

WOLDT: It’s hard to say with any certainty. Certainly, we at the Fish and Wildlife Service, as well as our state partners, are getting more frequent calls this year about fish die-offs and, when you get calls about fish die-offs, it’s hard to say what exactly caused it because there’s other factors other than, you know, heat that can cause fish die-offs, but it seems like they are more widespread this year and more frequently occurring.

CORNISH: What kinds of fish are most vulnerable?

WOLDT: It really varies from case to case. You know, certainly, with fish kills, cool and cold water fish are most vulnerable, so you know, when you’re thinking cool water fish, those are, you know – a good example of that is Northern pike and for cold water fish, you know, those are usually your trout and salmon species. But it’s even hitting some of the warm water fish and, you know, warm water fish, in general, are more tolerant of warm temperatures than low oxygen.

But we’re seeing things like common carp show up in die-offs this year. When common carp are dying off, that tends to tell you that it’s a pretty significant year for summer die-offs. I wouldn’t say it’s the – you know, the worse one on record that we’ve had, but you know, when you see warm water fish starting to succumb to summer kills, that’s a sign that things are a little bit, you know, on the worse end of the spectrum.

CORNISH: Aaron, give us a picture of what happens when some body of water, a lake or a pond, hits, you know, what we’re hearing, 97 degrees. What’s going on there?

WOLDT: Well, what happens is – and when a body of water gets that warm – and it’s somewhat case-specific, but the real thing that starts happening is the water in that body of water can not – it’s simply not capable physically of carrying a high amount of oxygen, so the oxygen levels in the water starts to go down and it starts to stress the fish.

Now, it can happen very, very quickly where, you know, large numbers of fish will suddenly succumb to low levels of oxygen and just start to float and that’s usually what draws, you know, people’s attention. When they look at body of water and they suddenly see a bunch of fish floating, it can happen quickly simply just from lack of oxygen.

CORNISH: Do we have any idea on what kind of effect that these fish kills have on the ecology of the water? I mean, once you have all these dead fish around, what does that mean for that ecosystem?

WOLDT: Well, fortunately, when we have fish kills, they’re usually not a total fish kill. I mean, it looks like a lot of fish. It always does. There’s always lots of fish on the surface, but very rarely is it a complete fish kill of every fish in the body of water. Usually, the biggest fish – because they have a higher oxygen demand, those bigger fish will die first and then, usually, some of the smaller ones survive because they have less of a biological oxygen demand. So the smaller fish that do survive will repopulate that body of water.

In the cases where there is a total fish kill, though, those fish species will disappear from that lake and it might require some management action to bring them back.

CORNISH: Aaron Woldt is Fisheries Program supervisor for the U.S. Fish and Wildlife Service, Midwestern Region. Aaron, thank you.

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