It’s incredibly sad to be sitting down and writing this – the last thing we need in this Country is more fish deaths, especially when this one isn’t related to drought or mismanagement.
In comparison to other lakes in the country which are struggling from lack of water and drought, Blowering Dam is considered healthy. Sitting at 48%, we can rule out drought and lack of water for this fish kill.
During the week a video went viral in the freshwater fishing community about Murray Cod deaths in Blowering Dam. The video title ‘Murray Cod Massacre of Blowering Dam? Big Cod Nightmares’ was produced by the Big Bass Dreams Australia crew who found numerous dead fish in concentrated pockets during their night session on the lake.
This initial video had me worried but with no idea as to what had caused the fish kill. I was initially thinking it could have been poor handling from a group of anglers who had been camped in one spot for a few days.
As sad as this is, I was hoping this was the cause and that it wasn’t something much worse…
First Sightings of Dying Cod
I spoke to Cal from Big Bass Dreams Australia in an interview which is included in the podcast on this topic and he said, “We started in the evening fishing and we smelt the fish before we saw them.”
Initially Cal thought it was strange to find a dead fish as he hadn’t ever seen many dead fish over the last 10 years fishing the lake. “We have only ever seen one or two here and there”, Cal mentioned.
“50 metres up the bank there was another one and then 100 metres further there was another one. Then we stopped fishing to have a look and then it really snowballed from there.”
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Cal explained that to him it looked like there were two distinct times the fish were killed. Like batch one and batch two. The first lot were higher up the bank and were more decayed than the second batch closer to the water.
After speaking with Cal, I knew this was a much bigger issue than just poor handling. This had me extremely worried that this incredible fishery could fall apart.
Blowering Fish Kill – Bigger Issue than First Predicted
On Thursday morning, Social Fishing team members Tallis and Jack were up fishing the lake. They sent me through a video of a large dead cod floating up near the dam wall of about 90cm in length.
Later they found another dead fish over a metre in length sitting on a bank in Log Creek.
After contact with numerous anglers and Fisheries it’s estimated that there is at least 30 large Murray Cod which have died over the past two weeks in Blowering Dam. This number could be much higher as more fish are continuing to pop up.
I spoke with Cameron Lay from DPI and he said, “The first reports came in early on Monday morning on the 25th of November.”
“Fisheries officers who attended the site believe that they’ve been at least two separate mortality events and some of the fish could have been floating there for up to two weeks,” said Cameron Lay, DPI Fisheries.
Interestingly, the fish seem to have been reported in high concentrations in two main locations of Yellowin and Browns Bay. These bays are located a long-distance from each other which leads to the thought of an environmental impact on the species.
“These kind of localised fish kills are not uncommon in other impoundments, but we don’t really have much of a history of this happening in Blowering.”
Fisheries have known about these fish kills for a few weeks and have been sampling the water and the fish. They are doing everything they can to figure out what has caused the deaths. Sampling has been sent away and it will take some time before results come back. From there we believe Fisheries will release a statement explaining the cause.
The CAUSE of the Fish Deaths
There are a range of reasons as to what could have caused the fish kill. I’m going to share the facts and everything we know about this event.
From the evidence above we can rule out improper handling of fish. If this was the case the deaths would be on a much smaller scale and not in concentrated pockets. We do see fish deaths in Blowering but usually only in the middle of summer when the water is at its warmest and the larger fish struggle.
The falling water levels of the lake are also not to blame as water is released from Blowering in massive volumes every single year. Looking over records this year has had minimal releases of water compared to previous years. It has remained relatively steady over the past two months and has only fallen 4% in the past 3 weeks.
From our research we’ve found there are two different scenarios that could have possibly caused the deaths:
Scenario 1: Water ‘Roll Over’
This is the process that happens naturally in large and deep lakes every year coming into winter. What happens is the water on the surface is warmer than the water below. As winter rolls in the surface temperature cools quickly and become dense and sinks to the bottom causing the water from below to move to the surface. Click here for more info.
It currently isn’t winter… but three to four weeks ago the system experienced a dramatic change in temperature. From 40 degree days to a sudden 17 degree day and cooler nights, this shift in temperature could have caused the surface water to cool and cause this roll over effect.
What affect does it have on the fish?
When the water flips this quick it can cause the sediment to become dislodged, increases turbidity and the less oxygenated water from beneath quickly replaces the water in the shallows. Therefore, these two large shallow bays would have copped the full force.
In most cases a ‘roll over’ event goes unnoticed and wouldn’t usually cause this many deaths. In saying that with the cod in a ‘spawning like’ mode they may have stayed with their nests instead of retreating for deeper water and avoiding the poor oxygen levels in shallower reaches of the lake.
This roll over event along with the stress post spawn and the falling lake levels, could all have contributed to some of the larger and weaker fish not being able to survive.
Since this sudden change in temperature the fishing slowed right down, with many anglers struggling to catch fish and this is when the dead fish started showing up.
I spoke to Clint Hansell, who is a local angler who spends countless hours out on Blowering every week about what he has noticed. He believes it could quite possibly be the ‘roll over’ of water.
Over the past 3-6 weeks Clint has noticed the massive change in fish activity and he said,
“For me I was averaging a decent fish every 5-6 hours of fishing up there. In the last 6-8 weeks personally I’ve done in excess of 100 hours and only 1 cod on the cast.”
This could well be the cause of the deaths but there is another alternative… which is much worse.
Scenario 2: Contaminated Water
As we started to research the fish deaths, we found that there is a possibility of water contamination from a spray that is used to kill weeds and blackberries throughout the Pine plantations.
Unconfirmed sources have told us that one of the sprays commonly used is called ‘Brush-Off’.
An article published on June 12 2019 in the Illawarra Mercury states, “Brush-Off, with the active ingredient Metsulfuron methyl, has a material safety data sheet (MSDS) which warns it is very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment”.
This quote has also been taken from the MSDS which is available to read here.
In contradiction to this a Chemicals Information Sheet published by DPI Tasmania states the opposite about the chemical Metsulfuron-methyl. This information sheet can be found here.
Which one should we believe? I’m not too sure and it’s not confirmed that this is the exact chemical they use. Regardless this is the evidence we have found.
The fish started to slowly show up after a heavy rainfall event over 4 weeks ago and have continued to pop up in several locations across the lake. These large rains could have quite possibly washed the spray in along the edges of the Pine plantations.
I spoke to a local who use to grow apples in an orchard many years ago and he mentioned that majority of the sprays they use warned that they are toxic to aquatic organisms.
We have also since spoken to other locals who have farm dams located through the Batlow/Tumbarumba region, and they too say many of there stocked fish in farm dams have died over recent years. They believe it to be a mix of the chemicals that run off the land.
The major fish deaths have been concentrated to Yellowin and Browns Bay which are both major shallow inflows along the State Forest side of the lake.
Big cod have also been sighted along the banks in Jounama Pondage which sits directly upstream of Blowering Dam. It too boarders areas with pine plantation.
Update: 30 November 2019
After publishing this article, I visited the lake to do some research and to have a look for myself. We searched the flat banks on the western side of the lake and south of Browns Bay. We found 4 large dead Murray Cod along a 2 km stretch.
Finding these fish meant the fish kill wasn’t isolated to just Browns and Yellowin Bay as initially thought. It was affecting most of the dam, but the deaths were isolated.
We only found 4 dead fish between 95-110cm along a bank that we know would hold possibly 60-100 large Murray Cod. Based on this we can tell it isn’t affecting all the fish, only a select few.
While on the water I received a message from Aaron on Instagram and he explained he found 8 dead cod between 80-100cm along the bank nears the Pines Access, which is on the eastern side of the lake.
We boated over to have a look and we too found deceased fish. Based on this, it almost rules out the probability of scenario #2 being the sole cause of the fish kills. I’m almost certain now that the water quality quickly diminished in the lake, most likely from roll-over and the males guarding the nests in shallow water have decided to sacrifice themselves, rather than retreat.
These fish stayed to protect the eggs even with the poor water quality. If it was any other time of the year these fish would have moved down into deeper water. We also found that these dead fish were all very close to structure which has just become exposed due to dropping water levels. This means the nests were in shallow water (less than 3m) and where the oxygen levels would’ve been poor.
We didn’t find dead cod in any of the steeper areas which makes sense because their nests would be down in better quality water.
This is also why we aren’t finding any fish in the 120-140cm caliber because the males aren’t commonly as big and most of these larger fish are female.
We are only sharing the facts, and we still aren’t sure what has caused the deaths. Fisheries are looking into the deaths and after water tests, they will be able to confirm what has caused this devastation. From reports only Murray Cod have been found deceased with no confirmed sightings of other species.
I wouldn’t recommend eating any fish from the waterway at this stage until further tests have been conducted.
Please send through any information you have, any photos or any comments you have on the situation.
For more information and for the full interviews check out the latest podcast episode on the Blowering Dam Fish Kill. Also available on all the good podcast apps.