More than 30,000 fish rushed up an inlet on the Mystic River behind Costco last Wednesday, July 25, and died of natural causes in a massive fish kill related to several unique circumstances.
It was actually the second large fish kill in that area over one week, with about 2,000 fish dying of natural causes near the Amelia Earhart Dam on July 19 as well.
In both fish kills, the species that died were all Menhaden fish, commonly called “pogies.”
The Department of Environmental Protection (DEP) and the Executive Office of Environmental Affairs (EOEA)/Division of Marine Fisheries said they had both done testing to make sure it wasn’t caused by a pollutant.
“We took water samples on July 19 and July 25 and there was no oil or hazardous materials found that could have caused the fish to die,” said Ed Coletta, a spokesman for DEP. “We did see very little dissolved oxygen in the water and the temperature on July 19 was 75 degrees, which is pretty warm. When we were called out on July 25, we also found low dissolved oxygen and the water temperature was up to 83 degrees, which is really high.”
Katie Gronendyke of EOEA said they have determined that it wasn’t pollution related, but rather a rush due to spawning or a predator. Some have speculated it could have been Striped Bass chasing them.
“The Division of Marine Fisheries has determined that this kill of Menhaden was likely due to natural causes, including aggregations forming for spawning and/or predator avoidance, localized oxygen depletion in a shallow area, and rising water temperatures,” she wrote. “DEP’s samples showed water temperatures at 75 degrees two weeks ago and 83 degrees last week.”
In essence, having so many fish in one area where the water temperature was so hot simply sucked out the oxygen and they died.
The problem now comes to cleaning it up, and that’s going to be Mother Nature’s job.
“The fish are there,” said Coletta. “There is no way to remove them and it’s an issue that will be left to sink to the bottom.”
Fish kills are more common in inland rivers, especially in the southern United States, but are rather uncommon here – especially in brackish water such as in the Mystic.