MYSTERY surrounds the appearance of more than a hundred dead catfish washed up at the Boyne River Mouth.
Residents have been finding the fish since Monday. Along the high tide line on Boyne Island, dead catfish were strewn every few metres along the high tide line from the morning before.
A spokesperson from the Department of Environment and Heritage Protection (DEHP) confirmed staff had inspected a section of the Boyne River on Monday and yesterday following reports of dead fish.
"Departmental staff have found over 100 dead catfish between the mouth of the Boyne River and the Bruce Highway bridge, approximately 20 kilometres south of Gladstone.
"The cause of death is currently unknown. Departmental staff are investigating and have conducted water quality monitoring and sampling.
"The water quality monitoring undertaken on 4 April 2012 identified reduced salinity levels due to freshwater inflow. All other monitoring results have found water quality is consistent with those of a healthy waterway."
Without any water quality issues being identified so far, the search for other explanations is on.
One resident at the river mouth suggested the fish might have spilled over Awonga Dam since the January rains.
That theory was rejected yesterday by the Gladstone Area Water Board (GAWB), which said no significant number of fish, catfish or otherwise, had spilled over the dam wall this year.
Last year, tens of thousands of barramundi flowed over the Awoonga Dam wall when it spilled over for the first time.
However, GAWB corporate services manager Gary Larsen yesterday told The Observer there had been almost no fish spill over the wall this year.
Mr Larsen said none of GAWB's monitoring could shed any light on where the dead fish had come from.
However, he said that since the dam spilled over on January 28, large numbers of catfish had been observed swimming upstream in the Boyne River.
He said GAWB had informed DEHP shortly after the spill about the catfish movements.
Mr Larsen said a large number of those catfish had swum upstream to the bottom of Awoonga Dam wall.
He said they had schooled there for some time, but GAWB's hatchery manager had regularly monitored the health of the fish and found no signs of poor health.
Mr Larsen emphasised no explanation was known for the dead catfish washed up recently, but one hypothetical possibility was that they had run out of food where they were schooling upstream.
The Awoonga Dam is still spilling over. It is currently at a level of 40.30 metres, which is 30 centimetres over the wall's spillway.
* DEHP was formerly the Department of Environment and Resource Management