MONTPELIER, Vt. — Vermont health officials said Tuesday a radioactive substance had been found in a fish sample taken from the Connecticut River nine miles upstream from the Vermont Yankee nuclear plant.

The substance, strontium-90, is a byproduct of nuclear fission that has been linked to cancer and leukemia.

William Irwin, the state’s chief radiological health officer, said that the sample was unusual in that the strontium-90 was found in the fleshy, edible portion of the smallmouth bass sample. Irwin said the substance more often turns up in fish bones. Nine of 13 bone, head and scale samples checked also turned up strontium-90, he said.

“It is to be expected to find strontium-90 in the bone, head, etcetera, because strontium is in the same chemical group as calcium,” Irwin said. “It is not as likely to be found in muscle tissue, yet the literature does describe results where they have found strontium-90 in the edible portion of fish. It’s just not as likely.”

Irwin said the Health Department would do further investigation to try see if more evidence could be gleaned tying the radioactivity to the Vermont Yankee plant. He said the finding could have been related to background levels in the environment resulting from above-ground atomic bomb testing in the 1960s and ’70s or from the Chernobyl accident in 1986.

Gov. Peter Shumlin, a frequent critic of Vermont Yankee and a supporter of closing the plant when its initial 40-year license expires next March, appeared to discount any source aside from Vermont Yankee.

“Today’s troubling news from the Vermont Department of Health is another example of Entergy Louisiana putting their shareholders’ profits above the welfare of Vermonters,” the governor said in a statement.