An important clue here is that seemingly all the fish are one species (carp) and one size class (large)...
We do not believe that there is evidence that this die-off is the result of pollution... If a pollutant were the cause, the effect would not be limited to one species and one size of fish...
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.
http://mysticriver.org/myrwa-blog/2015/6/18/whats-happening-to-carp-on-the-mystic.html