The rats may have come from beech forest areas closer to Westport in the Buller Gorge.
Image: Shellie Evans | ©

Introduction

FAQs about the dead rats found on North Beach near Westport that showed no presence of 1080 after toxicology testing.

How did the rats die if they weren’t poisoned?

Massey University School of Veterinary Science did post-mortem examinations on five dead rats but couldn’t determine the cause of death. Likewise, they couldn’t determine why the weka died.

We don’t know how the rats died. It’s possible they came from beech forest areas in the Buller Gorge affected by flooding.  Rats are in high numbers in beech forests due to the heavy beech mast (seeding) earlier this year.  With the remaining seed now germinating, hunger could be driving rats into new areas or to cross rivers.

Why didn’t DOC send fish or marine life for testing?

Our priority was to collect a sample of rats for testing to determine whether they had been exposed to 1080. 

Secondary poisoning of the fish and other marine life was considered very unlikely and it was not a priority to try and determine the cause of death for these species.

DOC staff were also focussed on the safe disposal of the trailer load of potentially poisoned dead rats.

Why were only 10 rats tested?

Science advice was to test a sample of ten rats, which was considered above the minimum requirement to ensure high statistical certainty of detecting 1080 in the washed-up rats.

Eight rats were able to be tested for 1080 residue (two were too decomposed) and no traces of 1080 were found in any of these rats

Based on this result, statistical analysis indicates that at least 428 rats are ruled out of having detectable 1080, therefore we can logically conclude that 1080 was not the cause of death of the rats on the beach.

Why does Manaaki Whenua - Landcare Research test for fluoroacetate, not fluorocitrate?

Manaaki Whenua - Landcare Research has provided DOC with an explanation of why it does not test for fluorocitrate when testing for the suspected involvement of 1080 in relation to the death of an animal.

The fluoroacetate component of 1080 is metabolised (broken down and transformed) by animal cells into a range of non-toxic substances, plus the toxic chemical fluorocitrate.

The toxic effects of 1080 are a result of this transformation of fluoroacetate into fluorocitrate, which has the effect of interrupting the way that cells generate energy.

There are two main reasons why testing for fluoroacetate is appropriate in this instance:

Firstly, animals poisoned with a lethal dose of 1080 will always die with detectable fluoroacetate in their system, because there will have been enough fluorocitrate to kill the animal before all the fluoroacetate can be metabolised and/or excreted.

Only small amounts of fluoroacetate are converted to fluorocitrate. Savarie (1984) states that the amount converted may be 3%.  Other studies suggest the proportion is around 1- 2.5%.

Secondly, fluorocitrate is more difficult to test for because it remains strongly bound to the aconitase enzyme within the cell’s mitochondria (where it causes the block in the Krebs cycle that kills the organism).

There is, therefore, a much higher chance of detecting sodium fluoroacetate than fluorocitrate in a poisoned carcass. Manaaki Whenua - Landcare Research’s tests for 1080 are sensitive to 0.001ug/g – one part in a billion.

DOC, therefore, considers that if an animal had been killed by 1080 then Manaaki Whenua - Landcare Research would be able to detect 1080 in the carcass by the presence of fluoroacetate alone.

Sources:

How many rats were picked up?

About 680 rats were collected on the weekend of 9 and 10 November 2019.  Another six rats were later collected at Waimangaroa.

There were some reports of rats washed up at Punakaiki but numbers were not quantified. There were no further reports of dead rats washed up after 13 November.

How were the rats disposed of?

DOC sought advice from the West Coast Regional Council on safe disposal of the rats, which were buried in a deep hole well away from any water bodies or habitation.

Why weren’t the rats tested for anything else (e.g. brodifacoum)?

DOC did not test for other toxins such as brodifacoum that could have killed the rats because the Department hadn’t used other toxins in adjacent areas. DOC is responsible for the effects of 1080 operations it undertakes.

We tested for 1080 due to the possibility that the rats could have died from the use of this toxin. With 1080 poisoning ruled out, DOC will not be undertaking further work to investigate the deaths of the rats.

What happened to the crayfish or other sea life that was washed up and why weren’t they tested?

DOC didn’t test any fish or other marine life because 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 did collect a crayfish, but this wasn’t tested, as crayfish are not known to scavenge rat carcasses. The presence of dead marine life on the beach after significant storm events is not unusual.

Back to top