For the past few days, a multitude of silvery dead fish have littered the waters near the Charles River Dam.
The state Division of Fisheries and Wildlife began receiving reports of dead menhaden, known as “pogies,” earlier this week as the city endured its second heat wave of the year.
Division of Marine Fisheries biologists determined that the pogies, which are typically between 8 and 11 inches long, died of natural causes.
Fish kills that include only one or two species are “almost always a natural event,” Katie Gronendyke, spokeswoman for the state Executive Office of Energy and Environmental Affairs, said in a statement. If pollution were the cause, many more types of aquatic life would have been affected.
“With the recent heat wave, fish kills are not unexpected. As temperatures increase, the water cannot hold as much oxygen as when it is cold,” Gronendyke said. “Aquatic plants also increase their oxygen consumption, lowering the amount of available oxygen for fish and other aquatic life.”
Estimating that at most 1,000 fish have died, she said the water temperature in the river is peaking at its usual 81 to 83 degrees, and the number and size of fish kills has not been unusual this year.
A natural kill of menhaden in the Boston area last year was much bigger, she noted. It killed 40,000 to 50,000 fish along the Mystic River in Everett and Somerville.
Julie Wood, deputy director of the Charles River Watershed Association, said that in addition to the extreme heat, the fish suffocating may also be linked to the cyanobacteria — toxic blue-green algae — that bloom in the river every year.
The Department of Public Health issued an advisory last Thursday after finding levels of cyanobacteria that surpassed their guidelines in the Charles River, from the Boston University Bridge to the Museum of Science, spokeswoman Ann Scales said.
Wood said the fish kill seemed to be unusually large. “To see that many dead fish all in one area isn’t necessarily a typical occurrence on the Charles,” Wood said.
“People don’t think of temperature as a pollutant, but temperature drives so much in a native environment,” Wood said, noting that the water temperature has been above the water quality standards for aquatic life during the past three days. “Our summers are longer, hotter, and dryer — none of that is good for healthy river environments.”
The Mystic River in Everett and Somerville reeked as thousands of dead pogies rotted along the shore last summer.
@CityOfBoston @MassDCR seen at the Charlestown locks/ Charles River dam this morning - the result of hundreds of fish caught in the locks. so awful. who can we notify about this in order to prevent it? 😔 @BostonTweet @universalhub @boston25 pic.twitter.com/jaJy1MXL1v— Emily Patricia (@emilyyshea) August 1, 2019