Week in Review

What killed the Westport rats?

In what’s become the curious incident of the rats on the beach, opinions are divided over the most plausible scenario leading to 680 dead rats.

A flash flood, mass suicide, 1080, substandard swimming skills and rumours of a mysterious farmer applying a different poison have all been floated as reasons 680 rats washed up on a Westport beach.

The Department of Conservation (DoC) had conducted a poisoning operation before the rats were found 140 kilometres away. Heavy rain followed the operation and one theory was the dead rats had washed downstream to the ocean.

This posed a potential issue. While 1080 breaks down quickly in water, it remains in carcasses for some time. If the rats were poisoned they may have posed a risk to other wildlife snacking on their bodies.

On Wednesday, DoC’s test results came back negative for fluoroacetate - 1080. If it wasn’t 1080, what or whodunnit? Pathology results were inconclusive. 

DoC suggested a surge in rat numbers, combined with hunger and a flood may be the cause. 

“Rat numbers have exploded in beech forests due to heavy seeding and now seed is germinating, they are desperate for food, which can drive them into new areas and cause them to cross waterways,” said West Coast operations director Mark Davies.

Not everyone is convinced this is a plausible answer. Speculation over what was responsible for 680 rats, a calf, an octopus, crayfish, and a weka washing up on a beach has been rife. 

Theory one: It was 1080

Before DoC released its negative results for 1080 two groups claimed to have results from independent testing they had conducted. Flora and Fauna of Aotearoa and Clean Green New Zealand Trust said samples collected by volunteers confirmed the substances present indicated the deaths were “almost certainly” the result of 1080.

However, aside from this snippet of information, no other details were disclosed and a request by Newsroom to see the test results went unanswered. Facebook comments suggest the groups plan to release information after more testing is complete.

How could two sets of tests get different results? With one set of tests shrouded in mystery, it’s hard to know. 

The answer could be they were testing for different substances.

Questions have been asked why DoC’s tests looked for fluoroacetate but not fluorocitrate.

A University of Canterbury toxicologist professor Ian Shaw said he is curious at this. The 1080 poison - fluoroacetate - causes a reaction in the body that creates fluorocitrate.

“If you’ve been poisoned with 1080 you would have high levels of fluorocitrate in your body, particularly your blood. That’s extremely well known.”

He said it’s also known 1080 metabolises in the body fairly quickly. 

“If you had some bodies with 1080 in them and they’ve been sitting around on the beach in the warmth, that 1080 is quite likely to be broken down. The fluorocitrate is less likely to be broken down and hangs around for longer, so probably would have been a better way of determining whether these animals have been poisoned by 1080.”

One issue with this is fluorocitrate can be harder to measure. Without being part of the decision-making process, Shaw said it’s hard to know why the tests were only done for 1080, not fluorocitrate.

Asked why this was not tested, DoC said the question should be directed to Maanaki Whenua Landcare Research, which did the testing. DoC’s response came too late in the day for Maanaki Whenua Landcare Research to respond in time for publishing.

If the rat carcasses didn’t contain any 1080, as DoC’s tests indicated, but did contain fluorocitrate, did they pose a risk to other animals that might eat them? Shaw thinks probably not. 

“The amount of fluorocitrate you would find in the rats probably wouldn’t be sufficient to cause toxicity to other animals ... it’s all a matter of dose. 1080 is much more toxic than fluorocitrate but it would be wrong to say fluorocitrate is not toxic.”

With a disclaimer that his knowledge is only based on what he’s heard in the media, Shaw thinks 1080 is the probable killer. He’s heard theories that the rats died of hunger and were washed downstream.

“I don't know much about the ecology of rats. I'm a toxicologist. But it would seem strange that they would all be dying at the same time. It just doesn't hold water to me.”

Theory two: They tried to cross a stream and drowned

University of Auckland’s Dr James Russell has a PhD in biology and statistics. He’s particularly interested in conservation in island ecosystems.

Old records do describe 'waves' of rats. He suspects this was likely to be hundreds of rats scurrying about, rather than rats moving as a colony. 

He said rats normally move in small family groups, not in Pied Piper-style mass stream crossings. If this is the cause it would mean several small groups of rats were attempting to cross the stream around the same time.

Would 680 rats fail in an attempt to swim across a stream as DoC suggested?

“They’re very comfortable in water. We certainly see them swimming up the islands, which is a problem for biosecurity. We know they cross rivers because we've done studies where we monitor them on one side of the river and they’re turning up on the other.”

It’s possible heavy rain may have made swimming more difficult, or could have caused a naturally-formed dam to burst, creating a flash flood.

Theory three: They were washed away in a flash flood

One of the theories put forward is heavy rain caused a naturally-formed dam to burst, causing a flash flood downstream.

While nobody has exact numbers of rats present at the moment, Russell thinks there could be 50 to 100 per hectare after a mega mast. 

“For 600 rats to turn up you would need 10 or 20 hectares to be flooded out. If you’re coming down a catchment from a dam at the top, with a big water event, that is plausible.”

He thinks this theory could also explain the birds and calf found on the beach.

Director of NZ's Biological Heritage National Science Challenge Dr Andrea Byrom also thinks high rat numbers and weather are the likely culprits.

“I would have been very surprised if there had been 1080 in the rats. To have that volume of rats go down a river like that and get washed up on a beach all with 1080 in them, it kind of defies belief. Even if they’re in the bush and eating it, they still have to go in the river, jump in and die. It doesn’t make any sense.”

She thinks it’s a mix of high numbers after a mast year, starvation and flooding. 

With a high density of rats, space would be at a premium and they would have made dens wherever possible. Low-lying dens may have been flooded causing their deaths.

“I can’t think of any other explanation really.”

Rumours and loose ends

One thing Byrom did question was whether any other substances were tested for. One rumour mentioned on social media is a farmer used a brodifacoum-based rat poison. When rats are poisoned with anticoagulant poisons, they often seek water before dying. 

DoC did not test for this: “DoC has not used any other toxins such as brodifacoum in adjacent areas (DoC operations very rarely use this toxin on the mainland), so has not tested for the presence of other toxin.”

Questions have been asked why test results were not available for a crayfish collected by DoC workers at the same time the rat and weka bodies were taken.

DoC did not test the crayfish, saying crayfish are not known to scavenge rats. It also didn’t test any fish or marine life.

“... their secondary poisoning was considered very unlikely and it was not a priority to try and determine the cause of death for these species.”

DoC said all the carcasses had been disposed of and could not be tested further.

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