CAUSE OF DEATH?: Numerous dead gizzard shad and a few dead carp were seen floating in Lake Carnegie during the past week, prompting an investigation by the New Jersey Department of Environmental Protection. A spring die-off of gizzard shad is not uncommon, but the fish pathologist of the Fish and Wildlife Division of the DEP is evaluating specimens for possible bacterial diseases. (Photo by Donald Gilpin)

Is it an environmental crisis or just an annual rite of spring? 

Local residents have observed large numbers of dead and dying fish in Lake Carnegie over the past two weeks. The Division of Fish and Wildlife (DFW) of the New Jersey Department of Environmental Protection (DEP) has responded, inspecting the situation and taking specimen samples, which the DEP fish pathologist is in the process of evaluating for common bacterial diseases.

“A die-off of gizzard shad is not uncommon in the spring and not uncommon in Lake Carnegie,” explained Lisa Barno, chief of Fresh Water Fisheries for DFW. Ms. Barno noted that “the stress of spawning and the temperature changes together may have affected this particular fish population.”

Preliminary testing results for bacteria should be available by the end of this week, but “there is nothing we’re worried about environmentally,” she added. “If there were an environmental problem — run-off into the lake, pesticides or anything like that — we’d have seen a greater range of species affected.”

Early last Thursday afternoon dozens of dead gizzard shad floated alongside two dead carp close to the banks of Lake Carnegie near the Harrison Street Bridge. There were no additional dead fish visible there on Monday morning.

Streamwatch coordinator for the Stony Brook-Millstone Watershed Association Erin Stretz observed that water quality in Lake Carnegie, as measured on April 16, was normal, diminishing the likelihood of low dissolved oxygen (DO) levels as a cause for the die-off. When water warms up, she explained, it cannot hold as much DO, and fish can die as a result.

The Watershed monitoring did reveal a somewhat elevated pH level — 8.5 rather than the normal 7, and “Fish are sensitive to changes in pH level,” Ms. Stretz said. She suggested the possibility of bacterial infection, possibly caused by fertilizers, pesticide, or road salt washing into the lake. Ms. Stretz expects the DEP test results to provide answers to questions about the stressors that are causing the fish die-off.

Jonathan Kennen, ecological water coordinator at the U.S. Geological Survey in Lawrenceville, supported Ms. Barno’s preliminary assessment of the problem. This “die-off,” he said, “can simply be explained by the presence of low dissolved oxygen in the water column, cold stress or disease.”

“Lake Carnegie,” according to Ms. Barno, “has a prolific population of gizzard shad,” as Mr. Kennen confirmed. “No worries,” he said, “what gizzard shad lack in stamina, they make up for in fecundity. Each female is capable of producing between 280,000 and 400,000 eggs in a spawning season.”

Mr. Kennen added that Lake Carnegie’s dead-fish phenomenon is not unique. “Die-offs of gizzard shad happen in lakes and river systems throughout the northeast United States during late winter or early spring every year due simply to cold stress.”

Ms. Barno mentioned that fresh water fish are sometimes also affected by a bacterium called aeromonas, “but we don’t expect a high presence of that.”

Gizzard shad, a medium-sized member of the herring family, are sensitive to changes in water temperatures and have difficulty acclimating to temperature changes. Silver with bluish upper sides and back, they average 10-14 inches.