What’s Causing the Fish Kill at Lake Carnegie in Princeton?

Dead fish at Lake Carnegie. Photo: Monika Reimitz.
Dead fish at Lake Carnegie. Photo: Monika Reimitz.

Princeton residents who have lived near Lake Carnegie for more than 25 years have never seen anything like it — hundreds of dead, rotting fish floating in the lake and scores of fish washed up on the shore.

Most of the fish are gizzard shad, but residents have also taken several photos of dead carp. Many of the dead shad have red blotches on their heads. Other fish can be seen in the lake gulping for oxygen.

Residents began seeing dead fish washed up on the shore last week. Photo: Monika Reimitz.
Residents began seeing dead fish washed up on the shore last week. Photo: Monika Reimitz.

Several residents and fishermen contacted the state last week and this week to alert officials about all the dead fish. The state Department of Environmental Protection Division of Fish and Wildlife first responded to a fish kill report at Carnegie Lake on April 13, a state spokesman said.

The conservation officer who responded observed about 50 dead gizzards shad, but he also observed schools of hundreds of other fish that were all in good shape. The conservation officer did not note any other fish gulping at the surface, so water quality probably is not an issue, DEP Spokesman Larry Hajna said.

dead fish
A dead fish in the lake. Photo: Steve Hiltner.

Gizzard shad are sensitive to temperature changes, but there are also common bacteria such as Aeromonas that can be a problem for fish already stressed due to spawning activity, Hajna said. Spring die-offs are not uncommon with the gizzard shad, he said.

The Department of Environmental Protection sent a crew to Lake Carnegie this week. The group saw 40 to 50 dead shad, collected them and sent them to the DEP’s fish pathologist to confirm whether bacteria is responsible for the die-off, Hajna said. The results have not come back yet. More fish will be collected today, April 21.

“Again this is not an uncommon occurrence with this species which can tend to overpopulate an area, especially a smaller water body, and become more susceptible to disease transmission,” Hajna said. “The disease is not transmissible to people, but we would not recommend anyone taking a handful of these home for a fish fry.”

Hajna said the outbreaks tend to run their course, and that the die-off should begin to diminish by next week. If bacteria killed the shad, he said it is possible that the carp died of the same bacteria.

Gizzard shad are very sensitive to cold water temperatures and their inability to acclimate causes mortality at low temperatures. As water temperatures rise, populations of disease-causing organisms such as bacteria and parasites increase. Viruses can also kill fish. Experts say that diseases in lakes seldom kill all the fish,  and are likely to affect only one or two species.

Many fish kills result from low concentrations of dissolved oxygen in the water. Fish need oxygen to survive, and they get theirs in the form of oxygen gas dissolved in the water. Warm water holds less dissolved oxygen than cold water. Other organisms in the water consume oxygen, including algae and bacteria. The dissolved-oxygen concentration sometimes drops too low for the fish, and a die-off can occur. The die-off can occur as a result of purely natural conditions, or because of human activity that results in adding fertilizers, manure, nitrogen and phosphorus to water systems. Low dissolved oxygen can result from other factors such as poor flushing or circulation, dredging, or a sudden rain after a dry spell.

Fish kills also can occur as a result to toxic compounds released into a body of water. In order for this to occur, the toxic compound must be highly concentrated.


  1. CSI Princeton: Special Aquatic Unit! (No, but thanks — interesting and a little concerning!)

  2. I bicycle on the D&R tow path 3 to 5 times a week and Wednesday (4-20-16) was the worst for the fish die off. I bicycle from Kingston into Princeton and noticed the dead fish from just above the damn up to those pedestrian bridges. There were no dead fish in the canal, the dead fish were all on the lake side, concentrated near the shore. I have bicycled along the D&R towpath for decades and I have never seen anything this bad. Most of the fish were very small but there were some big fish mixed in with the smaller fish. In some areas, there were great masses of dead fish. This is an exceptional occurrence for this area, not at all common. So far, it does not seem to be affecting the turtles or the birds. Along that whole route I saw no birds, turtles or other scavengers feeding on all these dead fish. Strange.

  3. Most communities in the P area applied lawn chemicals on 4/14 and 4/18 … a short analysis of application dates, upstream and downstream connections, could lead to some answers.

  4. On Sunday 4/24 there were hundreds of dead fish in the section of the Millstone River between US 1 and the canal and hundreds more lining a small nearby section of Lake Carnegie’s shore. Heaviest concentration of dead fish in the river were near where it flows under the canal into the lake.

Comments are closed.