Extract from ABC News
Murray cod (Maccullochella peelii) is the apex predator of the Murray-Darling Basin river system. (Flickr: Murray Cod/Guo Chai Lim/CC BY-NC-ND 2.0)
At up to 1.8 metres in length and weighing more than 50 kilograms, the Murray cod is the apex predator of Australia's inland waterways.
An adult "pondi" (its Ngarrindjeri name) cruises the country's rivers and creeks — from the Campaspe in central Victoria to the Condamine in Queensland — undaunted by any native rival.
Even European carp make themselves itself scarce when a big cod is on the prowl.
Murray cod will feed on juvenile carp. (supplied: Victorian Fisheries Authority)
But during the 1970s they all but disappeared from much of their range across the Murray-Darling Basin.
Now the big fish of the basin is on the way back.
The decline
Murray cod (Maccullochella peelii) and related species such as trout cod (Maccullochella macquariensis), eastern freshwater cod (Maccullochella ikei) and Mary River cod (Maccullochella mariensis) once dominated the rivers of the Murray-Darling Basin.
However, overfishing (especially commercial fishing from the late 19th century into the mid-20th century), the building of vast dams like Lake Hume which altered water flows, and habitat degradation resulted in a drastic decline.
Commercial fishing took its toll on Murray cod numbers up until the mid-20th century. (Supplied: Stuart Rowland)
"In the 1970s and 1980s, even into the 1990s, so many of our recreational fishers told us Murray cod were very hard to come by," Victorian Fisheries Authority (VFA) recreational fisheries manager Taylor Hunt said.
Murray cod is still classified as vulnerable according to the Australian Environment Protection Biodiversity Conservation Act, but the trend is positive.
Taylor Hunt with an Ovens River Murray cod. (supplied: Victorian Fisheries Authority)
The recovery
An electrofishing survey of the Ovens River in Victoria found more than 110 Murray cod per kilometre, with flourishing populations in the Goulburn and Loddon rivers as well.
Meanwhile, the Lindsay River and Mullaroo Creek cod, devastated by blackwater fish kills in 2022, have recovered to pre-2021 levels.
Craig Eury and daughter Charlie with a large Murray cod they caught in Lake Eildon. (supplied: Craig Eury)
Dr Hunt said the VFA's scientific research and angler feedback were telling him a similar story.
"We've seen an amazing resurgence in Murray cod numbers and now they are back and in good condition," he said.
"We see this in our scientific surveys, and we've just completed a big study of recreational fishers where we asked about their own experiences.
"People are telling us they see cod everywhere."
"They are seeing lots of big ones, lots of them within the legal limit, and lots of little ones coming through as well."
The VFA survey had more than 3,500 responses.
More than 90 per cent supported Murray cod conservation efforts, including catch-and-release and adopting ways to handle caught fish in a manner that improves their chances of survival once let go.
The legal catch limit for Murray cod in NSW and Victoria is 55cm to 75cm, with strict bag limits (one per person in Victorian rivers, and two in NSW and in specified Victorian lakes). Smaller and larger fish must be released.
Angler Kate Norman with a massive cod. Murray cod can grow to more than 1.8m. (supplied: Kate Norman)
"In many rivers in Victoria, we have seen more than two-to-five-fold increases in the numbers of Murray cod since the 1990s," Dr Hunt said.
"This includes the Goulburn River, Ovens, Campaspe, Broken Creek, Broken River, and Gunbower Creek.
"It's a similar story in our lakes. In Lake Eildon, for example, there has been an 81 per cent increase in Murray cod numbers in just five years between 2014 and 2019."
Dr Hunt said the increases across Victoria were thanks to a combination of fish production and stocking, habitat restoration, fishing regulations and size limits, and improved recreational fishing practices, such as increased voluntary catch-and-release.
There was good news across many tributaries in New South Wales as well, including the Murrumbidgee River and Edward River.
Not all good news
The Darling River, however, represented an ecological disaster, according to retired research scientist with NSW Fisheries Stuart Rowland.
While there was a concerted effort to restore cod populations in other streams, the Darling has been neglected.
And according to Dr Rowland, known as The Codfather for his lifetime study of the species, where cod is absent, carp numbers explode.
Dr Stuart Rowland says native fish are the victims of river mismanagement. (Supplied: Stuart Rowland)
"The Darling River ecosystem, where the top predator has largely disappeared, it has damaged the whole food chain," Dr Rowland said.
"The aquatic fauna has so changed (due to carp), it has altered the aquatic flora drastically. That's why we have things like toxic blue-green algae blooms.
"All species are important in an ecosystem but perhaps the most important is the top predator. It shapes the entire ecosystem."
Stunning surveys
Electrofishing is a survey method used by researchers to physically count fish within a specific area. The data is then used to extrapolate a broader population.
It involves running up to 1,000 volts of direct current through the water which stuns any fish within a 5m radius without harming them. Researchers then catch and count each fish and, in the case of native species, release them back into the wild.
Electrofishing allows researchers to stun fish within a small area, count them, and return them unharmed to the water. (supplied: Victorian Fisheries Authority)
Recent electrofishing surveys across Victoria paint a positive picture for Murray cod but also illustrate the threat to other inland aquatic species.
The VFA estimates European carp populations to be 30 times higher than record levels before Victoria's 2022 floods.
A European carp. When cod disappears from a waterway, carp populations often explode at the expense of native flora and fauna. (Supplied: UNSW)
"Murray cod is ahead of a lot of our native species because they grow so big. They can grow much bigger than carp," Dr Hunt said.
"For a lot of our other native fish the overabundance of carp is still pushing down their numbers.
"We'd love to see more control programs for carp like we see for rabbits, foxes and other pest species.
"All the evidence is there to show we can have these wins so we can also bring back trout, Macquarie perch and yellowbelly (golden perch), and more of our small-bodied native fish."
State of play
Murray cod populations vary across south-eastern Australian states, according to Dr Hunt and Dr Rowland.
Lake Eildon (Vic) — Australia's best big Murray cod fishery with trophy sized fish (over a metre long and up 30kg) being caught by anglers daily. Since 1990 the VFA has stocked Lake Eildon with more than 6.3 million Murray cod. Between 2014 and 2019 there was an 81 per cent increase in overall numbers.
Campaspe River (Vic)– Huge increases in Murray cod numbers, particularly since 2019. Stocking of 100,000 Murray cod per year, combined with habitat restoration are likely contributors.
Goulburn River (Vic) — One of Victoria's most popular rivers for Murray cod fishers with a flourishing population. An electrofishing survey saw 60 Murray cod caught and released. There is strong Murray cod breeding downstream of Goulburn Weir as well.
Loddon River (Vic) — Murray cod have gradually increased since 2014 thanks to stocking of up to 160,000 fish per year, combined with habitat restoration and improving river health.
Lindsay River-Mullaroo Creek (Vic) — Reputedly home of some of Australia's biggest Murray cod. Numbers have resurged since the 2022 blackwater event which devastated the population.
Ovens River (Vic) — Considered Australia's healthiest river for Murray cod. An electrofishing survey in the Ovens netted 185 Murray cod, ranging in size from 50mm to close to a metre. The Ovens also supports populations of other threatened native fish including trout cod and Macquarie perch.
Voluntary catch-and-release by anglers has contributed to the Murray cod's recovery. (Supplied: James Norney)
Murrumbidgee River (NSW) — Described as a very healthy population with high angler catch rates.
Darling River (NSW) — Once a stronghold for Murray cod when the fish was under threat elsewhere. Now a river of greatest concern, with a dramatic decline in Murray cod numbers.
Lower Murray River (SA) — There remains concern about the amount of Murray cod in South Australia because breeding has been very limited.
No comments:
Post a Comment