Large Dead Fish in Mystic River Likely Dying of Natural Causes

Spawn so hard.


Courtesy of Mystic River Watershed Association

In recent weeks, there’s been a growing number of large dead carp floating through the Mystic River. Fortunately, it looks like this is the result of a natural phenomenon and not the doing of some toxic blowback.

Today, the Mystic River Watershed Association explained that these goldfish-looking beasts, which are known to devour creamed corn and hot dogs, are apparently spawning themselves to death. The group notes in a blog post that spawning is a “huge energy cost” that leaves the fish “vulnerable to infections and other stresses from the environment.”

Because all the dead fish are of the same general size (large) and the same species, it’s unlikely that pollution is the culprit. Moreover, the die-off runs throughout the river and isn’t confined to a single spot, which further supports the idea that this isn’t due to pollution.

All the fish we are seeing may have been in the same physiologically stressed condition and all intolerant of whatever led to death. If these were spawning fish subject to stress or infection that has spread through their community, this would explain both why we see one size class and why they are dying more or less all at once… We do not believe that there is evidence that this die-off is the result of pollution. In the past two weeks, two slicks were reported to DEP at specific locations in the lower stretches of the river, something MyRWA has also been tracking.

Patrick Herron, deputy director for the Mystic River Watershed Association, calls it a “modest blight,” and notes that other species are thriving in the waterway. He points to the river herring; last year approximately 239,000 such fish made their way through the Mystic Lakes dam as part of an annual run. This year he expects the number to be even larger.

Herron says the group will keep monitoring the carp die off and working with state environmental authorities to make sure it is indeed a natural process.