Recent weather has been blamed as the "likely" cause of dead fish in Mooroopna's Craigmuir Lake.
On Sunday, many yellowbelly were seen dead on the shores of the popular walking and fishing spot.
In a letter supplied to the News by a concerned resident, a Greater Shepparton City Council staff member said the matter was investigated on Monday with an Environment Protection Authority representative.
The letter said initial indications showed low dissolved oxygen caused by weather patterns.
“... a significant rain event causing inflows of bio matter followed by hot days and cold nights, and the long-term climate of drought conditions,” the letter read.
“This means that it is likely (although not confirmable) that the lake has ‘flipped'.
“The usual conditions of a lake are that it is warmer on top due to sun heating up and colder below (stratification), but these climatic conditions can reverse this, and along with the excessive inflows of leaves which then break down, can cause deoxygenation, mainly affecting large fish which have a higher oxygen demand.
“EPA have taken fish for laboratory testing for disease and water samples onsite to determine dissolved oxygen and other water-quality factors and poisons, and will contact me once they have some results (which may or may not confirm or rule out any of the above).”
It was noted all dead fish were 40 cm or larger and no birdlife seemed to be affected.
The letter said the water was "relatively clear and there is not a lot of floating matter or biomass".
“However, there are some patches of filamentous algae (not blue-green algae) and the water level is on the low side, though not excessively."