Chances are it is due to the hot summer with little rain. many waterways have had a problem with low dissolved oxygen levels leading to less hardy fish dying. Please keep your eye on it if possible and if you notice a significant amount or continued death contact the Masswildlife offices and they will send out biologists to check, which is what I sadly had to do at the Blackstone this year. We had a major fish kill, with roughly 3000 fish dead by wildlife's count, due to low dissolved oxygen levels
Chances are if it is due to that the recent rains and cooler weather will help put the waterway back on track. It is also possible that the fish could have gotten separated from the main body of water recently in a smaller pool, cut off from the main body due to low water and died. The with the recent raise in water levels been floated back into the main body.
This is just my thoughts. It always sucks to have this sort of thing happen, but sometimes nature can be mean. Sadly the fish kill on the Blackstone was due to human error, hopefully this one is not. If your still concerned by all means contact the Mass wildlife offices. they are easy to deal with and it is always better to be safe then contaminated
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