|Team Sawfish 2015. Dr David Morgan not pictured. Photo: Jeff Whitty|
Our adventures began in early August, the early dry season, in the lower estuarine pools of the Fitzroy River. This time of the year is often ideal as the days are warm, nights are cool and rain is absent. By August, river discharge has fallen and the once flowing river is transformed into a chain of isolated pools. The small amount of river flow that remains and the relatively cold air temperatures keep the estuarine pools fresh and cool, with only the occasional spike in salt when the tidal waters intrude from nearby King Sound.
|The lower estuarine pool where we captured Dwarf Sawfish in August 2015. Photo: Jeff Whitty|
|A Dwarf Sawfish tagged and released by Team Sawfish in the Fitzroy River in 2015. Photo: David Morgan|
|A sawfish cake. One of the few Largetooth Sawfish we saw on this trip.|
In early December, our team found itself on the river once again, but this time amongst an unfortunate situation. Our team was informed by local residents that there had been a large die-off of sawfish and Bull Sharks in the upper reaches of the Fitzroy River.
|Some of the Largetooth Sawfish and Bull Sharks killed by the low dissolved oxygen event in December. Photo: Jeff Whitty|
Unable to determine the cause of death from the animals themselves, we turned to the environment to see if there was any evidence to suggest what happened. We deployed multiple sensors throughout the entirety of the affected pool to detect any abnormalities in the temperature, oxygen levels and pH of the water column; it was not long before we had identified the silent killer. There was very little oxygen below a depth of 1 meter and no oxygen below 2 m in this 10+ m deep pool. From interviews with local residents, we learned that a small rain event had washed oxygen-hungry organic matter into the pool. Without the additional input of freshwater, this organic sludge became concentrated within the single pool and likely absorbed the dissolved oxygen in the water, killing all bottom dwelling species. Similar occurrences have taken place in other seasonally flowing rivers in Australia but thankfully, residents said that this is a rare event in the Fitzroy River.