The Environment Protection Authority has investigated a fish kill in the River Derwent in Windermere Bay.
EPA Director Alex Schaap said a large number of dead juvenile barracouta had been observed in the bay area near Claremont Primary School late last week.
A similar fish kill of juvenile barracouta was reported the week before between the New Norfolk Bridge and just south of the Norske Skog premises at Boyer. Mr Schaap said it is likely that the two events are related.
"It appears only one species had been affected as there were reports of other fish, such as whitebait and trout, alive in the area" he said.
"While testing of samples taken from the earlier event is not yet complete, it is likely that in both incidents the barracouta ventured into the upper reaches of the estuary and became exposed to low salinity water when venturing into the shallows."
"Barracouta are a marine species and, while juveniles are abundant in estuaries, they are relatively intolerant of low salinity."