SICK redfin caught at Burrendong Dam in April may have been affected by parasites and bacteria.
However, tests on the fish were inconclusive as to the exact nature of the cause of the marks.
A number of anglers at Burrendong Dam, near Wellington, had caught redfin that were covered in red blotches, prompting concerns.
Samples were provided to the NSW Department of Primary Industries by recreational fishermen.
The aquatic biosecurity team had laboratory testing carried out.
The testing was inconclusive.
It has been suggested the cause could be environmental.
However, without further laboratory testing, the exact cause will not be established.
The Department of Primary Industry has requested anyone who catches a redfin exhibiting red blotches to provide it for further investigation.
It has also advised people not to eat fish that have severe ulcers.