Many big carp have died recently on the Mystic. Dead carp have been sighted all up and down the river, from Lower Mystic Lake all the way to the Amelia Earhart dam. We have reported the event to state agencies including the Department of Environmental Protection (Mass DEP) and had conversations with fish experts at the Division of Fisheries and Wildlife.
The state experts' best understanding of this is that it is a natural die off resulting from stresses from spawning in combination with some environmental factor (changes in water temperature, bacteria or virus that specifically affects carp, etc.).
An important clue here is that seemingly all the fish are one species (carp) and one size class (large). This leads the fish scientists to ask what might affect a group of fish at once like this, in this pattern. Spawning is a huge energy cost to fish, and the stress of spawning leaves them vulnerable to infections and other stresses from the environment. 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. 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.
We are keeping an eye on the situation for sure. We will report any new information in our website. The Pennsylvania Fish and Boat Commission has a web posting that has information that covers similar ground.
Please let us know if you see significant mortality among other kinds of wildlife, of course. Massachusetts Division of Fisheries and Wildlife has a web page on fish kills, and a number to call to report information.
For more information on fish that seem to be doing extremely well this season, see our River Herring Monitoring page.
Posted by Andrea Ritter