Mystic River carp die-off probably some natural thing, advocacy group says

The Mystic River Watershed Association ponders all the dead fish floating to the top of the Mystic of late, concludes it's probably a natural phenomenon, unrelated to either possible oil spills or low amounts of oxygen in the water:

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. But the fish die-off phenomenon extends all up and down the river and affects only large carp. If a pollutant were the cause, the effect would not be limited to one species and one size of fish. So this event calls for another kind of explanation.

We also do not believe that the cause is low dissolved oxygen levels (another common cause of fish kills). There is no evidence of dramatically low dissolved oxygen levels along the length of the Mystic, and carp are, in any case, among the most resilient fish in the face of low oxygen.

So the evidence suggests that is a rare, but essentially “natural” die-off, affecting only one species. It’s a dramatic event. These are large fish. But we do not believe that it is the result of water quality issues in the Mystic, and we do not think that wildlife in general or people are at risk from whatever is causing it.