Saturday's Pacific tsunami may have played a role in a perfect storm of events that triggered a mass fish kill in Throsby Creek, an environmental scientist believes.
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Dr Margaret Platell from the University of Newcastle's School of Environmental and Life Sciences said it was plausible that even though the tsunami's surge was tiny, it could have contributed to a sudden drop in oxygen levels when combined with other factors.
These factors included recent hot temperatures and a large quantity of decaying organic material deposited by revegetation, which was mobilised by a sudden influx of nutrient-rich stormwater entering the channel following Saturday evening's heavy downpour.
"Tsunamis have very long wave periods that come in for a long time and then go out for a long time. It is possible that it raised the surface of the water higher than it ordinarily would have been. I don't think you can overlook the potential role of that wave here," Dr Platell said.
Reports of dead and dying fish in the usually healthy waterway first occurred late on Sunday morning. By Monday morning's high tide, thousands of flathead, bream, mullet, eels, whiting, gobies, sole and silver biddies were seen floating in the water.
Department of Primary Industries investigators said on Monday evening that they believed the suspected cause of the fish kill was critically low dissolved oxygen caused by a high volume of organic material entering the waterway.
Dr Platell said the extensive revegetation of a waterway such as Throsby Creek could be a double-edged sword.
"Any sort of model for improving a catchment involves bank stabilisation and the reduction of erosion. Revegetation is the best way to do that but it does produce leaf litter. If there hasn't been a lot of mobilisation that leaf litter can accumulate.
"The dissolved organic carbon that comes from those organic sources will nourish the animals at the bottom of the food web.
"It's checks and balances - we are seeing the natural ecosystem play out."
The fish kill has also thrown a spotlight on the health of the broader 3000 hectare catchment.
A multitude of government and non-government bodies including the Roads and Maritime Services, Port of Newcastle, City of Newcastle, Lake Macquarie Council and Hunter Water, manage the main channel and catchment under the auspices of the Throsby Creek Government Agencies Committee.
The committee produced the Throsby Creek Catchment Agencies Plan 2019-2024, which aims to improve the water quality, health and biodiversity of the ecosystem.
Committee chair and Newcastle MP Tim Crakanthorp echoed the sentiments of local residents who said the fish kill was a shock given the improvement in the creek's health in recent decades.
"My understanding is that the creek has been fairly healthy for a while now. But given we have had this incident, and it appears the catchment is the cause, that certainly needs to be looked at and how it can be better managed," he said.
Mr Crakanthorp said he was keen for as much information as possible about the incident to be shared.
"We will definitely be looking to get as much information as we can from the experts and feedback from the community to ensure this doesn't happen again," he said.
A Hunter Water spokesman said there had been no adverse pollution incidents in the upper catchment recently or operational issues within its network that would have impacted the health of the creek.
The community is encouraged to report any fish kills to the Fishers Watch Phoneline on 1800 043 536. More information on fish kills can be found at https://www.dpi.nsw.gov.au/fishing/habitat/threats/fish-kills
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