North Australian FISH FINDER forums

Golden snapper ("fingermark" to the rest of Australia) have been in the crosshairs of NT fisheries managers for a while because numbers in the waters around Darwin are down. Fisheries says they are at about 14% of historic levels.

If you don't already know, snapper are a slow-growing reef fish that taste too good for their own good. Juveniles grow in estuaries before heading out to reefs around the 20m to 50m zone where spawning takes place. They are found across northern Australia.

From what I can find online, regional stocks are believed to be mostly genetically separate, but with some mixing, which is good news if there is localised depletion, but clearly fisheries managers are still worried about the Darwin stock.

Reef Fish Protection Areas closed to fishing were introduced across the Top End in 2015, and there have been tightened bag limits, but it appears to not have rejuvenated golden snapper stocks off Darwin.

In its plans to rebuild stocks, the NT Government has released a survey giving respondents various management scenarios to choose from, including a nuclear option of closing a huge area - from Anson Bay almost to Point Stuart, for four years. And that's for all reef fishing!

It's a perplexing proposal, as much of this area would rarely see a sinker.

The nuclear option might seem unlikely but fishos are aware of what happened in Western Australia, with a ban on demersal fishing in the West Coast bioregion until 2027, covering a huge area from Augusta to Kalbarri.

The NT is not WA however. The Top End has far fewer people and the habitat is different.

Online I see arguments among WA and NT fishos about the true status of stocks. Charter operators say they are still catching fish. I assume that if the fish were almost gone charter operations would be gone too - no customers.

Another point of online discussion is how these blanket bans are happening consecutively, as if a ball has begun rolling. South Australia kicked it off in 2019 with a pink snapper ban over most of the state's waters, which continues to this day, then WA with its demersal ban.

There have been other bans or near-bans before these - Shark Bay's genetically distinct pink snapper were subject to very strict management after the stock plummeted. The fish made a comeback. Also Perth's garfish. Queensland had a brief ban with jewfish, and there is no-take on some fish, for example barramundi cod in Queensland and blue groper down south.

Everyone is blaming each other for the NT's snapper problem. Some things can not be easily proved. How much illegal fishing is there? Consider also the new wildcards - climate change and habitat degradation.

Unfortunately the problem may get worse for various reasons, and I suspect this is on the minds of fisheries managers. The NT Government survey asks respondents if they think climate change and habitat degradation are part of the problem. Regardless of what some people think, the waters are getting warmer, marine heatwaves are already killing fish, and there have been pollution incidents, including various reasonably minor spills in Darwin Harbour, that would have had negative effects. The shallows around mangroves are where many juvenile reef fish start their lives, and these areas are vulnerable.

Looking at the survey options, an annual reef closure in the build-up months of Oct/Nov and late wet season to May might be a good choice, as there are fewer tourists about at these times, and these are the barramundi fishing "seasons" when fishos are occupied elsewhere.

A reef protection area off Dundee might help.

IMO snapper should simply be a no-take fish in the Darwin region until recovery is seen, and jewfish should be off limits to everyone during the spawning season. Not a perfect solution, but none are.

Not mentioned is a minimum size rule. A minimum size would help because small snapper are caught in shallow water where barotrauma is less likely, so these fish can be released. A lot of small snapper are taken in Darwin Harbour, they never get a chance to reach spawning age.

The nuclear option of a four-year closure over such a large area is a potential disaster. People live in Darwin for the fishing lifestyle. Many businesses rely on fishing-related revenue. If a blanket bluewater demersal ban were introduced boats would be sold, charters would close, some people would leave town.

In other states permanent area closures have become the norm courtesy marine parks. Consider Queensland's Great Barrier Reef Marine Park and its network of zones. However the parks generally allow for fishing around regional centres.

There is good news. The giant 2026 Top End wet season should be great for marine fish (hopefully not too much of a good thing). It might be just what is needed to bump up the numbers.

Lastly, if fuel prices stay sky high for an extended period, which looks likely courtesy Trump, fishing pressure will be reduced because bluewater fishing is fuel-intensive.

Meanwhile, the survey is here ... https://haveyoursay.nt.gov.au/golden-sn ... win-region

Minimum size rule seems applicable for this species, as above.

Yeah the charter guys say they still catch good snapper but the proof to me is how far they now travel to find them. Gone are the half-day charters catching snapper and jewies

Gone are the half-day charters catching snapper and jewies

Still operators advertising half days online??

Yeah there's a market but they're not catching what they used to

So glad to see such a well-informed take on this, cheers Matt!

Any closures should not include areas that are less that 10 m deep, then have strict size limits. This encourages people to still fish, but can release the small ones.

This would change behaviours of fishos long term.

I also believe in creating artificial reefs in shallow areas where there is currently no reef. Then closing these areas for a few fish generations to allow them to build up. The small bio artificial reef that fro is part of are looking promising. This way we are not loosing areas we currently fish. When it is opened to fishing in the future, size limits and catch and release is still possible. Could even have max size limits if the big ones are the breeding stock.

Changing fishos behaviours to reef fishing shallow will be key to have a long term benefit to goldies.

Good suggestions.

I wrote years ago in the Sunday Territorian newspaper about the easy access to large quantities of insectides in hardware stores.

Pesticides and herbicides are very harmful to fish, amphibians and aquatic invertebrates even at low concentrations.

The impact of this stuff washed into estuary nurseries, creeks and rivers might be 1000X more destructive than fishing pressure. I'm not sure anyone knows the answer, but the potential is obviously there. It gets into the water from normal use but the fact the public can buy litres of the stuff and some of it probably gets tipped down drains as leftovers is kinda stark.

Hard to get a handle on the problem because monitoring might not pick up single spill events. Also, there might not be enough in the water to kill visible larger fish, but there might be enough to kill the micro stuff that tiny fish/larvae eat.

Add on top rising water temperatures, general pollution (including all our pharma drugs going out through the septic system), and you can see that fisheries management controls might just be a blunt instrument.

Or not, coz no one knows.

I asked AI to summarise the pesticide/aquatic damage subject, with some NT focus ... apologies for the AI, but I don't have time to dig up all the data and two-finger type it in.
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Here are the main groups and notable examples:

1. Organophosphate insecticides (very high acute toxicity). Chlorpyrifos, diazinon, malathion.

Impact: Extremely toxic to fish and aquatic insects. Can cause rapid die-offs after runoff events. Also harmful to birds and mammals.

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2. Pyrethroid insecticides (extremely toxic to fish and invertebrates)

Synthetic versions of natural pyrethrins. Permethrin (commonly sold as ant and wasp powder), cypermethrin, deltamethrin.

Impact: Among the most toxic pesticides to fish. Bind to sediments → long-lasting exposure. Highly toxic to crustaceans (e.g., shrimp, yabbies)

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3. Neonicotinoid insecticides (chronic ecosystem effects).

Systemic insecticides that affect nervous systems. Imidacloprid, clothianidin, thiamethoxam.

Impact: Lower acute toxicity to fish than pyrethroids, but very harmful to aquatic insects → disrupts food chains, persistent in water.

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4. Organochlorines (legacy but extremely persistent)

Many are banned but still present in sediments. DDT, dieldrin.

Impact: Bioaccumulate in fish and predators. Long-term ecological damage. Still detected decades later.

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5. Herbicides with aquatic toxicity concerns

Generally less acutely toxic than insecticides, but still impactful. Atrazine, diuron, paraquat.

Impact: Damage aquatic plants and algae → disrupt oxygen and food webs. Some (like diuron) inhibit photosynthesis in corals and seagrass. Atrazine linked to endocrine disruption in amphibians.

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6. Glyphosate (context-dependent toxicity)

Non-selective plant killer.

Impact: Pure glyphosate: relatively low toxicity to fish. Formulations (with surfactants) can be much more toxic.

Can affect amphibians and algae.

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What makes a chemical especially dangerous to aquatic life?

High water solubility → spreads easily into waterways.

Persistence → doesn’t break down quickly.

Bioaccumulation → builds up in organisms.

Sediment binding → long-term exposure source.

Run-off potential after rain.

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Most acutely toxic: pyrethroids, organophosphates.

Most ecologically disruptive long-term: neonicotinoids, organochlorines.

Herbicide risk: more indirect (ecosystem-level effects).

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In Australia, the pesticides that get the most regulatory attention tend to fall into two overlapping groups:

1. Frequently detected in waterways.

2. Known to exceed (or approach) aquatic safety guideline values.

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Most commonly detected pesticides in Australian waterways.

1. Herbicides dominate monitoring results, especially in agricultural catchments: diuron, atrazine, 2,4-D, hexazinone, simazine.

Why they show up so often: Widely used in cropping, also in domestic gardens. Mobile in soil → easily washed into rivers. Persistent enough to remain detectable.

Evidence: Repeated detections of atrazine, diuron and 2,4-D across Queensland catchments, for example.

Frequent detection of diuron in the Murray–Darling Basin and other surface waters.

Large monitoring studies list diuron, atrazine, simazine, hexazinone among the most common contaminants.

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2. Neonicotinoid insecticides (increasing concern)

Imidacloprid. Why it stands out: Extremely toxic to aquatic invertebrates, highly water-soluble → travels easily into streams. Often detected at concerning levels

Evidence: Found in 5/6 sampling sites in an Australian coastal catchment. Recorded at very high concentrations (up to 294 µg/L)

Key point: One of the most concerning modern pesticides for waterways due to both frequency and toxicity.

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3. Organophosphate insecticides (less frequent but high impact)

Dimethoate, chlorpyrifos, diazinon.

Why they matter: Highly toxic to fish and aquatic insects. Even low detections can be ecologically significant.

Evidence: Dimethoate frequently detected alongside imidacloprid in waterways.

Chlorpyrifos and diazinon flagged as problematic in monitoring datasets.

Key point: Less persistent than herbicides, but more acutely toxic when present.

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4. Other commonly detected agricultural chemicals

From large multi-pesticide screening studies: Carbendazim, metalaxyl, iprodione, MCPA.

These are: Frequently detected but less publicly discussed. Often not routinely monitored despite presence.

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Regulators like APVMA, NSW EPA, reef programs focus heavily on high-priority substances:

Diuron → coral/seagrass toxicity, long persistence.
Atrazine → endocrine + ecosystem effects.
Imidacloprid → invertebrate collapse risk.
Hexazinone → frequent detections in reef catchments.

Also flagged in monitoring / enforcement - Chlorpyrifos, diazinon, 2,4-D.

These are often targeted because they pose risk to fisheries and reef ecosystems.

A nuance often missed - it’s rarely just one chemical. Australian scientists increasingly emphasise “mixture effects”—a cocktail of low-level pesticides that together can harm aquatic life even when each individual chemical is below guideline limits (this has come up in fish-kill investigations and monitoring debates).

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In the Northern Territory (NT), pesticide patterns are a bit different from the southern states because of the tropical climate, wet–dry seasons, and different agriculture (mangoes, melons, horticulture, cotton, urban mosquito control).

Here’s what’s most commonly detected and/or regulated in NT waterways and nearby northern catchments:

1. Herbicides dominate monitoring after wet-season runoff.

2,4-D, atrazine, diuron, hexazinone.

NT-specific context: Washed into rivers during intense monsoonal rains. Detected in Darwin Harbour catchments and northern rivers. Similar profile to Queensland’s wet tropics.

Insecticides used in horticulture (higher toxicity) used in mangoes, vegetables and cotton.

Imidacloprid,chlorpyrifos, dimethoate.

NT-specific context: Detected less consistently than herbicides, but much higher ecological impact per detection, particularly risky to aquatic insects and small fish.

Imidacloprid is a standout concern due to high water solubility, strong toxicity to aquatic invertebrates, increasing detections across northern Australia.

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Mosquito-control chemicals (public health use) are important in the NT due to disease control programs.

1. Temephos

2. Bacillus thuringiensis israelensis (BTI)

Used in wetlands and urban areas, generally targeted application, but still monitored. BTI is considered relatively low-impact compared to chemical insecticides

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Big picture for the Northern Territory.

Most commonly detected: → Herbicides (2,4-D, atrazine, diuron)

Most ecologically concerning currently: → Imidacloprid (chronic food-chain effects)

Most dangerous when present (acute toxicity): → Organophosphates (chlorpyrifos, dimethoate)

Unique NT factor: → Mosquito-control pesticides in wetlands

Separately, Australian governments seem very slow to control dangerous substances. An example is chlorpyrifos. This was used in pet flea collars sold in Australia until just a few years ago even though it was known the chemical was linked to lowered IQ in babies exposed in the womb. How many pregnant Aussie mums were patting pets that wore those flea collars?

Chlorpyrifos ... https://usrtk.org/healthwire/chlorpyrif ... c-disease/

It has been studied as a possible cause of autism - autism and ADHD seem to be at epidemic levels.