Was up on the Normanby River a few weeks ago and only managed to catch one barra. Walking along the bank (and dodging crocs) I saw dozens of dead barra, some up to 1 metre in length.
Read this today, which explains a lot. The water was warm to me, but apparently very cold to the locals I spoke to.

Queensland's Department of Environment and Resource Management says the mysterious death of thousands of fish in a Cape York River may have been caused by a fungal attack.

There were reports last week of around 1,000 dead fish, mostly barramundi and jewfish, seen floating in the Normanby River near Rinyirru (Lakefield) National Park in far north Queensland.

DERM spokesman Andrew Buckley says departmental officers have visited the site to take water quality and fish samples to example environmental factors and eliminate other causes of fish death such as contamination.

“Initial samples showed water quality parameters such as dissolved oxygen, pH and salinity were within the expected normal ranges. Water samples were also sent to a laboratory for chemical analysis,” Mr Buckley says.

“Early examination indicates the fish may have died from a fungal attack commonly known as red-spot disease; however this will need to be confirmed by a government veterinarian.”

According to the Department of Agriculture, Fisheries and Forestry, red-spot disease has been found commonly in many waterways across Australia. It says the disease has often been associated with acid water run off and can appear after heavy rains.

DERM says fish deaths can sometimes also occur naturally if there is a rapid change in the water temperature. “Fish will commonly become stressed with sudden changes to water temperature caused by cooler weather,” Mr Buckley says.

“Lower water temperatures can depress the immune system of fish, particularly barramundi, and allow fungal or viral infections to invade the fish’s skin.”

It’s understood test results from samples of the fish are expected next week. DERM says it will continue to liaise with Cook Shire Council and Rinyirru (Lakefield) National Park staff on the outcome of the tests.

People are being warned not to eat sick or dead fish which they have caught in the region.