As Temperatures Get Warmer, Fish Are At Risk

8:17 minutes

Fish mortality increases as a result of increasing water temperatures.
Fish mortality increases as a result of increasing water temperatures. Credit: Shutterstock

Climate change is expected to have a big effect on a sensitive group of creatures: fish. A new study out of the University of Arkansas predicts that there is likely to be a six-fold increase in large fish mortality events between now and 2100, specifically in freshwater lakes in Minnesota and Wisconsin. 

Known as “summerkills” and “winterkills”, seasonal die-offs are a part of fishy nature. But summerkills in particular are expected to happen at a greater frequency as temperatures increase. That’s due to climate change-related factors like algal blooms, infectious disease, and oxygen deprivation. 

Joining Ira to talk about the future for freshwater fish is Simon Tye, PhD candidate in biology at the University of Arkansas in Fayetteville.

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Segment Guests

Simon Tye

Simon Tye is a PhD candidate in the University of Arkansas Department of Biological Sciences in Fayetteville, Arkansas.

Segment Transcript

IRA FLATOW: It’s not just the Bay Area suffering the effects of climate change. We’re now going to head to the upper Midwest, where new research predicts how warming temperatures will make the problem worse. Joining me to talk about this is Simon Tye, PhD candidate in biology at the University of Arkansas in Fayetteville. Welcome to Science Friday.

SIMON TYE: Thank you, Ira. Happy to be here.

IRA FLATOW: Can you set the scene for us, Simon? Exactly what were you analyzing in this study?

SIMON TYE: Yes, we compiled the fish mortality events across the lakes in Wisconsin and Minnesota and associated those events that were caused by warm temperature with concurrent temperatures so we could predict into the future how many events might happen.

IRA FLATOW: And I’m guessing that the results did not turn out well. What did you find in your analysis?

SIMON TYE: No, not particularly. We found that under the worst climate change scenario, RCP 8.5, we can expect probably a 600% increase in the frequency of these events by the end of the century.

IRA FLATOW: Six times, right? 600% by the end of the century.

SIMON TYE: 600%.


SIMON TYE: Yes, sir.

IRA FLATOW: Well, you talk about summerkills and winterkills. Are these normal? Are they routine things that happen in the fish world?

SIMON TYE: Yeah, summerkills and winterkills are common terms used to refer to mortalities caused by seasonal temperatures, specifically warm temperatures in the summertime and cooler temperatures in the wintertime. They are routine things, but their frequency and magnitude is probably changing quite a bit with warming temperatures.

IRA FLATOW: And these go out of the normal range in your predictions, or in the study predictions.

SIMON TYE: Yes, they do. With increasing temperatures we expect to see an increase with the amount of fish mortalities, such as we’re seeing in San Francisco, in part because fish aren’t expected to evolve higher thermal tolerances at the rate that the climate is changing.

IRA FLATOW: Simon, walk us through specifically why it is that these mortality events would happen so much more frequently.

SIMON TYE: There are several deleterious environmental conditions associated with a warming climate that can directly affect fish. What we’re seeing in San Francisco right now is due to a harmful algal bloom, which can either pass toxins up through a food web such that there’s toxic algae that’s eaten by the zooplankton and then the fish, which can cause harm to the zooplankton and fish, often resulting in mortality, or just the fact that the water is warming. That can cause direct thermal stress, and it is simply too hot for the fish to persist. And lastly, all of these conditions reduce the dissolved oxygen concentration within the water body, which can result in fish and other organisms literally suffocating in the water.

IRA FLATOW: Is it because the fish can’t survive in the hotter temperatures?

SIMON TYE: Yes. There are several reasons why they die, but one is direct thermal stress, just the actual warmer temperature. And then there’s also indirect effects associated with that, such as lower dissolved oxygen concentration that results in many of the aquatic organisms literally suffocating in the water.

IRA FLATOW: Suffocating in the water. Does this happen to all fish, or are some fish more susceptible than others?

SIMON TYE: Most of the documented events have affected warm-water fish or fish with high thermal tolerances. However, one of the important findings of our study is that similar temperature deviations can affect both warm and cold-water fish and–

IRA FLATOW: And fish of all ages.

SIMON TYE: Yes, fish of all ages. Mainly, what we record is the adults that are visible to the human eye. However, earlier life stages, such as juveniles and embryos, are particularly sensitive to warm temperature, and we may not be able to recover those and document them.

IRA FLATOW: And they’re the worst stage of life to die, right? You’re not going to get any new fish.

SIMON TYE: Right, right. Yeah. It can have severe effects on population growth over time.

IRA FLATOW: And if the fish are going to be dying so much more, how could that affect the rest of the ecosystem?

SIMON TYE: It will likely destabilize the system to a degree. Most ecosystems function better and are more stable when they have all pieces of the food web. And a lot of the fish dying will remove much of the predation in these systems.

IRA FLATOW: This is Science Friday from WNYC Studios. We’re talking to biologist Simon Tye about fish die-offs due to climate change. He’s based in Fayetteville, Arkansas. Are there any parts of the Midwest that are more susceptible than other parts?

SIMON TYE: Our research showed that there’s a appreciable latitudinal gradient, such that southern locations are more apt to have these events. However, another key finding of our paper is that fish mortalities occurred at similar temperature deviations, which implies that local heat waves, like much of what the US saw throughout this summer, can cause mortalities regardless of the location.

IRA FLATOW: And how much of a temperature heatwave does it take?

SIMON TYE: That’s a very good question. It could probably be around the critical thermal maximum for cold-water fish, which is the temperature that they can persist at in a water body. However, for warm-water fish, mortalities occurred at about 5 to 10 degrees below their critical thermal maximum. So in other words, if you have probably 5 to 10 degrees above average, that could lead to a mortality event.

IRA FLATOW: Are we talking about the Great Lakes here, also, or is that a whole different story?

SIMON TYE: The Great Lakes is a whole different story because there’s a lot more intricacies of modeling temperature dynamics in those large bodies of water compared to smaller bodies of water.

IRA FLATOW: What body of water would we know of that would be most affected?

SIMON TYE: They can happen all over the place. Maybe Lake of the Ozarks is a larger lake in Missouri, north of where we are. But I wouldn’t be surprised if these events happen throughout the US. There was a similar event to San Francisco that happened along the major river between Germany and Poland about a month ago, and there was tens of thousands or hundreds of thousands of fish. And that would be comparable to a mass die-off in the Missouri, relative to North America.

IRA FLATOW: As someone who studies this, I imagine this is really concerning for you.

SIMON TYE: It is concerning. The reason I chose to study this topic and similar topics is I want people to understand the implications of a warming climate and realize what the effects could be when their children grow older.

IRA FLATOW: Yeah, and as someone who is younger and is going to be living with the effects of climate change, is there anything that you think that could be done to reverse this prediction?

SIMON TYE: The most that we can do as a society is just enact more sustainable policies. I don’t know if there’s anything at an individual level you can necessarily do besides noticing these events and reporting them to the authorities. But it’s more of a societal issue that we need to address.

IRA FLATOW: So it’s not something that the fish could possibly evolve to handle.

SIMON TYE: No, from what we’ve learned so far about fish evolving higher thermal tolerances is that the rate’s much lower than the rate of increasing temperature. So they won’t be able to evolve fast enough, if you will.

IRA FLATOW: Yeah, so what’s happening, the big fish kills happening in the San Francisco Bay Area is sort of– well, portends what’s going to happen in other places around the upper Midwest.

SIMON TYE: Yes. Yes. The way I like to think about it is, past generations learned about the Chesapeake Bay and environmental degradation there. For my generation, I learned a lot about the dead zone in the Gulf of Mexico. And I think future generations are going to learn a lot about just the fact that fish are declining globally and not necessarily about a specific location, necessarily.

IRA FLATOW: Yeah. As someone who has lived through those, I think you’re right. And I want to thank you, Simon, for taking time to be with us today.

SIMON TYE: Thank you. It was my pleasure.

IRA FLATOW: Simon Tye, PhD candidate in biology at the University of Arkansas in Fayetteville.

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About Kathleen Davis

Kathleen Davis is a producer at Science Friday, which means she spends the week brainstorming, researching, and writing, typically in that order. She’s a big fan of stories related to strange animal facts and dystopian technology.

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