If you’ve braved the heat and walked around the Inner Harbor recently, you might have noticed dead fish floating on the surface. What’s going on?

This is a common summer phenomenon, experts said. As the water gets warmer, there is less oxygen. Couple the heat with algae blooms, and even less oxygen is left, leaving fish no place of refuge.

Dead catfish and eels might be seen floating on the surface of the water because where they usually dwell — in the bottom of the harbor — has little to no oxygen due to the algae blooms, said Jack Cover, the general curator at The National Aquarium.

Cover said the dead fish that people have seen is part of a “minor mortality,” and not a fish kill.

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Many fish are typically good at adapting to quick changes in water temperature, Cover said. There is a tipping point, though, and if the temperatures become too hot, many of the fish will swim out farther into the Patapsco River where the water is cooler.

Adam Lindquist, vice president of the Healthy Harbor Initiative at the Waterfront Partnership, said the harbor’s ecosystem is under a lot of strain because of the weather, and there is currently a large algae bloom in the harbor, which passersby can tell just by looking at the unusually brown water.

“When that algae dies, which happens very quickly, it sucks oxygen out of the water and creates dead zones where there’s not enough air for fish and crabs to survive,” he said.

A dead catfish in the water of the Inner Harbor. (Justin Fenton)

Algae blooms are caused by heat warming up water mixed with runoff that contains fertilizers, creating a good environment for phytoplankton, which is a type of algae, to grow quickly, said Eric Schott, an associate research professor at the University of Maryland Center for Environmental Science.

Lindquist said the runoff water also contains sewage and even fecal matter from pets, birds and rodents. But sewage is probably less of a factor in algae blooms appearing, Lindquist and Schott said.

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What can happen next, is something called a turnover event, where after a rainstorm with colder temperatures, the top layer on the harbor will mix with the bottom layer that is already devoid of oxygen. That leaves the entire harbor deoxygenated, Schott said.

Although it is typical to see this happen once or twice during the summer, Schott said he has noticed an uptick in dead fish floating in the harbor this week.

On Wednesday evening, there was a thunderstorm with light rain and Friday morning had just over an inch of colder rainfall, according to the National Weather Service. These storms could have contributed to a turnover event.

The Maryland Department of Natural Resources tracks water quality in the harbor around the National Aquarium, and for the last week, it has shown low levels of oxygen in the water, Lindquist said.

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On Friday at noon, the dissolved oxygen level was at 3.67 mg/L, according to the DNR Aquarium East Surface chart. Lindquist said healthy water should have a dissolved oxygen level at 6.5 to 8 mg/L.

The chart also tracks algae blooms, and Thursday from 2 p.m. to around midnight, there was a larger algae bloom compared to earlier in the week, according to the chart.

When conditions like this continue, and there are algae blooms and more dead fish, sometimes the water at the bottom of the harbor can carry hydrogen sulfide, which can leave a rotten egg smell, Schott said.

“We’re not experiencing that [the rotten egg smell] this time and I think that’s why the dead fish caught us all by surprise,” he said.

So, how can algae blooms be prevented?

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“There’s simple things like not using fertilizer,” Lindquist said. “But what we really need is green infrastructure throughout Baltimore City that captures storm water runoff and allows it to soak into the ground instead of sending it into our storm drains, which go right into our streams.”

Green infrastructure includes rain gardens that allow storm water to soak into the soil, bumpouts in roads with plants that can absorb storm water, and rain barrels.

Cover echoed Lindquist about adding more green infrastructure. He said people should also be cognizant of cleaning up after their pets.

“There’s so many things we can do to improve that water quality,” Cover said. “People love steamed crabs; do it for the blue crabs.”

Lindquist said the Healthy Harbor Initiative has built rain gardens around the Inner Harbor to help lower the amount of stormwater runoff that goes into the harbor.

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Despite hearing about dead fish floating in the harbor, Lindquist said he saw dozens of menhaden swimming in the water Friday, and that it was an encouraging sight.

“That’s part of the reason we need to restore and protect the harbor not because it’s a dead waterway we want to bring back to life, but because it’s actually full of life, and we want that life to flourish,” he said.