Several hundred fish have been found dead in a pond on the National Mall, officials said, and they were likely killed, in part, by the extreme heat in recent days.

Authorities with the National Park Service said they did not have an exact number of how many fish have turned belly-up in the pond at Constitution Gardens, but they estimated that it is “at least several hundred fish,” according to park spokesman Mike Litterst. The fish include a mix of bluegill, sunfish and bass, officials said.

Park officials have spent much of the week removing the dead fish from the pond, which sits just west of the World War II Memorial. Authorities said they have tested the pond and found “no indication of a hazardous material or pollutant in the water.”

But there is certainly quite a smell.

Litterst said he was at the pond Tuesday and “there was still an odor in the air.”

The large fish kill, Litterst said, is “not a rare occurrence.” The last time a fish kill of this magnitude happened on the National Mall was in 2013. The causes of the latest fish kill are similar to those in the kill three years ago, park officials said.

[Fish kill discovered in Mall pond, Park Service says]

One reason is the pond area’s “poor design and construction as a closed system, which makes maintaining an ecological balance difficult,” Litterst said.

Another issue, park officials said, is the recent extreme heat.

The pond is not very deep — only about two to three feet on average — and the water temperature quickly rises to a level that fish can’t tolerate, “and the dissolved oxygen levels in the water drop,” Litterst said.

Combine that with the extreme hot weather, and that creates a bloom of algae. That can “further rob the water of oxygen needed by the fish when the algae dies off,” Litterst said.

Officials said they have measured the low dissolved oxygen levels and found that they are likely the cause of the fish kill.

There have been discussions about whether the pond should even have fish, park officials said. In the past, the pond has been stocked. Park officials said they have longer-range plans to redo Constitution Gardens and make improvements to the pond that would make it a better ecological environment.