Why did I see a pile of dead fish in the Providence River?

Megan Hall: Welcome to Possibly, where we take big problems like the future of our planet and break them down into small questions with unexpected answers. I’m Megan Hall.

Today we answer a question from our listener Alexandra Hahn, who observed something unusual:

Alexandra Hahn: I wrote to you because last spring I saw a pile of dead fish in a river in the city center. And I wondered what they were doing there, what kind of fish they were? How did they die?

Megan Hall: Reporter Will Malloy spoke with Possibly co-founder Professor Stephen Porder to tell us what’s going on. Hi Will!

Will Malloy: Hi Megan!

Megan Hall: So why were there so many dead fish in the river?

Will Malloy: There is actually a name for the phenomenon that Alexandra noticed: fish kills. A fish kill occurs when a large number of fish die in the same area in a short period of time.

Megan Hall: That sounds bad! But why does this happen?

Will Malloy: Like humans, fish need oxygen to live. However, unlike us, they do not get it from the air; instead, they breathe in oxygen gas dissolved in the water.

Stephen Pounder: So they actually breathe oxygen underwater, just like we breathe oxygen out here in the air.

Will Malloy: And basically, fish deaths can occur when the oxygen content in the water drops and is no longer sufficient for the fish to survive.

Stephen Pounder: If you use up all the oxygen in the water, the fish will die. Just like if you use up all the oxygen in the air, we will die.

Megan Hall: OK, that makes sense. But why would there suddenly be less oxygen in the water?

Will Malloy: The process is a little complicated, but one thing that can remove oxygen from water is actually the decay of dead plants and algae.

Stephen Pounder: When a dead piece of seaweed falls to the bottom of the bay, it decomposes and consumes oxygen.

Will Malloy: You can think of it as the opposite of photosynthesis – when plants do photosynthesis, there is a reaction that produces oxygen. When dead matter decays, there are reactions that consume oxygen.

Megan Hall: OK, sure, but doesn’t something die in the bay all the time?

Stephen Pounder: So there is always algae and seaweed in the water. But to cause a fish kill, you need some kind of growth spurt followed by a die-off.

Will Malloy: So when there’s an increase in life – more kelp, more algae – after a short period of time you’re actually going to see more death because some of those plants die, which uses up a lot of oxygen.

Megan Hall: What causes this “life boost”?

Will Malloy: It can have many causes, but often occurs after a heavy rain shower.

Megan Hall: Why does rain lead to more algae and seaweed in the water?

Will Malloy: I wasn’t kidding when I said it was complicated! During a heavy rainstorm, the rain can wash fertilizer from people’s lawns into the sewers.

Stephen Pounder: If it rains too much, our sewage system cannot handle it.

Will Malloy: This means that the sewers are overflowing. And when some sewers are overflowing, they can flow into bodies of water like rivers, bays or even the sea. And that means that all of this fertilizer and the rest of our wastewater is in in water.

Stephen Pounder: And so you get a sewage flush that consists of nutrients. And these nutrients ensure the emergence of life and growth in the water.

Megan Hall: And after this fertilizer encourages the growth of other plants in the water, they eventually die, depriving the fish of the oxygen they need to survive?

Will Malloy: Exactly! It doesn’t happen every time it rains. I mean, the conditions have to be just right, but generally it’s a big chain reaction that starts with a lot of rain and ends with a lot of dead fish.

Megan Hall: So what can we do to prevent this?

Will Malloy: Try planting things in your own garden that don’t need fertilizer. And in general, cities and towns can create more green space to break up paved areas. This way, rainwater can go into the soil instead of the bay.

Megan Hall: Thanks, Will! That’s it for today. For more information or questions about the impact of your choices on our planet, visit thepublicsradio.org/possibly or subscribe to us wherever you get your podcasts.

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Possibly is a co-production of The Public’s Radio, Brown University’s Institute for Environment and Society, and Brown University’s Climate Solutions Initiative.

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