Ecologist Dr Keith Bishop says the situation in the Hastings River is grim and getting worse.

Dr Bishop has been working on the Hastings River since 1997 and has spent the last 22 years studying how river flows influence the river's ecology.

In the 1980s, Dr Bishop studied the ecology of freshwater fish for over 10 years in the Kakadu National Park, Northern Territory. In that time he surveyed fish along the length of streams draining the Arnhem Land escarpment.

He now surveys the upper Hastings estuary, an eight kilometre length of the waterway between Wauchope and Beechwood, to help understand how diminished river flows allow oceanic saltwater to infiltrate the estuary and how this impacts valuable fish habitats - the beds of aquatic plants which provide important nursery, sheltering and feeding environments.

This work commenced with Port Macquarie-Hastings Council in 2000, but since 2014, Dr Bishop has continued his important work independently and self-funded. He conducts similar assessments of the Manning River to our region's south.

The most recent survey, conducted in the Hastings River on December 16, showed salinity intrusion was at record levels, and the upper estuary was in a very sad state.

"The Hastings River last stopped flowing during an intense drought with peaked in the spring of 2002," Dr Bishop explained.

"At that time, salinity in surface waters in the upper estuary near Beechwood, which is 34km upstream of the ocean mouth at Port Macquarie, reached 16 parts per thousand (ppt).

"This was a little less than half-strength ocean water."

Dr Bishop said ocean water is typically measured at 35 ppt.

"On the recent survey in mid-December, the surface salinity was 20 ppt, a little less than two thirds-strength ocean water.

"It is clear that the salinity is rising very rapidly given that the survey done in November 2019 revealed the salinity had reached only 12 ppt."

At these high-salt levels, the environment is at risk.

Dr Bishop said at 20 ppt salinity, an unsafe limit for even the toughest freshwater-associated plant species,all aquatic plants have now been eliminated from the upper estuary.

As a result, there are now very few nursery, feeding or sheltering habitats for fish species in the Hastings.

"Juveniles of the recreationally-important Australian bass rely greatly on beds of aquatic plants - so they are now in great trouble," Dr Bishop said.

Read more: Tougher pumping restrictions for the Hastings River

Riverbank vegetation is less exposed to the estuary's salt water so the impacts are usually less drastic in comparison to aquatic plants. However Dr Bishop said the plants close to the water's edge and further up the bank, including Matrush (Lomandra) which is important for holding the banks together, are now visibly stressed.

"In areas where pasture grasses reach the water's edge there is a clear increase in bank collapses," Dr Bishop said.

"This is most probably caused by salt-affected roots dying off then rotting.

"The roots would normally hold the underlying soil together, providing stability to the banks."

And it isn't just the vegetation feeling the impacts.

Precious aquatic creatures, unique to our local waterways, are dying. In the 128 surveying runs undertaken since 2000, Dr Bishop has never once recorded dead turtles in the estuary.

"With salinity at 20 ppt we saw them commonly, all freshly dead," he said.

Read more: 137 days and Hastings is in a water emergency

"All were short-necked turtles from the Dharra subspecies group which are unique to the Hastings and Macleay rivers."

Dr Bishop said while there has been limited research into the salinity tolerance of short-necked turtles, one study showed that exposure to just 15 ppt salinity for a period of two weeks was enough to stop the turtles from feeding and significantly increase their blood salts.

One of the most visible side-effects of the salinity intrusion has been the widespread reddening of certain areas of the estuary.

Dr Bishop explained this reddening is caused by the iron in the substrate which makes up the bed of the estuary.

"Saltwater is naturally denser than the iron-rich fresh groundwater. This would allow the saltwater to push into the surrounding substrates forcing out and displacing the less-dense groundwater. An iron leachate then deposits on the bed of the estuary," Dr Bishop said.

Read more: Water carting commences for Gloucester community

Dr Bishop hopes his ongoing work will help refine environmental river flows into the future to support the work of local authorities and landholders to better protect the important aquatic habitats that make the Hastings River system unique.

"Extreme events such as the current one provide very valuable large steps forward in understanding these processes," he said.

Thanks for subscribing and supporting local journalism. While you're with us, you can now receive updates straight to your inbox from the Port Macquarie News. To make sure you're up to date with all the news, SIGN UP HERE.