Cyclone Gabrielle: State of Hawke's Bay's ecosystems concerning for scientists

Scientists are worried about the state of Hawke's Bay's ecosystems after Cyclone Gabrielle destroyed fish and organism habitats.

Some rivers are showing no signs of life and could take many years to recover - a process that's being hindered by a constant flow of silt.

The Esk River is in a sad state - its banks are scoured out and blanketed in silt. The muddy waters are lined with flood debris after a terrifying torrent tore through the entire valley in February. 

The Hawke's Bay Regional Council is now surveying 40 sites monthly, instead of annually, to examine the impact of the cyclone. It's a level of monitoring that hasn't been done by regional councils until now. 

Its senior freshwater scientist, Dan Fake, said while some smaller rivers are bouncing back, larger rivers like in Esk Valley and the Tutaekuri River are really suffering.

"We're seeing some pretty devastating impacts," he said.

The region's rivers dumped millions of tonnes of silt on its floodplains.  

Cyclone Gabrielle: State of Hawke's Bay's ecosystems concerning for scientists
Photo credit: Newshub.

Since then scientists have been searching for macroinvertebrates, which are the little critters that fish feed on. But in the Esk River, they're struggling to find signs of life. 

"Usually from a sample like that, you'd find hundreds of little things scooting around. So little mayflies and stone flies, all kinds of trout and fish food. But it doesn't look like there's any in there. And that's kind of what we've been seeing since the cyclone," said Fake.

They're also using EDNA testing and electric fishing, which temporarily stuns the fish so they can be scooped up and inspected. But the nets are mostly empty.

"We haven't found many, if any," said Fake. 

Although Fake said the things that live in New Zealand rivers are well adapted to floods, the cyclone was a major disturbance in some areas where the floodwaters were over 10 metres higher than usual. So it could take the ecosystem many years to recover, especially in the hardest hit areas where any surviving fish are now surrounded by silt which gets in their gills and hides prey and predators. 

Cyclone Gabrielle: State of Hawke's Bay's ecosystems concerning for scientists
Photo credit: Newshub.

"It doesn't take much rain for the waters to erode the sediment and get it into the river and dirty it up again. And it's just going to be another stressor on those fish communities," said Fake.  

But because they're migratory fish, he's hoping fish eggs from healthier areas will be washed out to sea then the young will swim up the Esk and Tutaekuri rivers in spring.

"What's a worry is the food supply for them, so that's those invertebrates. If they don't bounce back then they'll be some hungry fish."

It's something he's concerned about.

"My biggest fear, to be honest, would be the bugs not bouncing back as quick as we might like."

And it's not just the freshwater habitats hurting, but the region's estuary and ocean environments too. The council's senior scientist for marine and coasts, Becky Shanahan, told Newshub a massive issue is the huge volume of sediment. 

"That's our biggest concern. Yeah, definitely, I would say," said Shanahan.  

Cyclone Gabrielle: State of Hawke's Bay's ecosystems concerning for scientists
Photo credit: Newshub.

To find out how marine life has fared, scientists are taking core samples and the findings are more encouraging than in the rivers. 

"It's not just empty shells which you expect to see, but also lots of live stuff," said Shanahan. 

But Shanahan's worried about the constant pulse of sediment from land into the estuaries.

"They don't have time to recover in between and they're just getting these massive dumps of sediment. It's a problem for the estuary to actually function and do the ecosystem services that it does in order for the estuary to work."

However, the sediment is a challenge for their scientific monitoring. Drop cameras are usually deployed to survey the sea floor - but since the cyclone, it's a murky mess.

"The fish struggle to see, it can affect their gills and just their ability to function well," said Shanahan.

Just how long the marine ecosystem will take to recover is unknown. 

"The most stressful is that we don't have a sense of yet what the recovery trajectory and time frame really looks like," said Shanahan.   

Despite the challenges, scientists will continue to monitor rivers, estuaries and oceans in the hope of getting a clearer picture of the damage and what the pathway to recovery looks like for Hawke's Bay's ecosystems.