7 November 2011: Ulva Island rat eradication update

Ulva Island Update - So far, no rats. Early signs are encouraging that the rat eradication operation has been a success. Results from monitoring are starting to arrive, so I'll update you on what we have in this update.

Rodent dog check underway

Our first check of Ulva Island, since the bait drop, with a rodent detecting dog is underway. As there are no trained dogs in the South Island, we had to fly Mirium and Oki down from Northland. Mirium has a connection with Stewart Island, having had a spell on the Darwin's barberry eradication team before becoming a professional dog handler.

Mirium and Oki arrived last Friday and started searching the island on Saturday. So far, they haven't detected any rat sign. The two of them will do another full day on the island Tuesday and then fly home on Wednesday. They will come back down for another check in about six months.

Two months of trapping

While I write this update, staff are out checking the biosecurity traps again. This check brings up two months of trapping with the full network of traps. No rats have been caught in this time.

The network has been upgraded over the last two months, with all existing traps being replaced with new trap boxes and stainless steel traps. We have also increased the amount of marking, so the network of traps is a bit more obvious. This is a purposeful shift in approach. In the past, we have tried to keep the trap network in the background with relatively discrete marking. But, now it is important that anyone who checks these traps can easily find them and we think that they are an important part of the Ulva Island story. The traps are part of what is keeping Ulva rat free and we should be reminding people about this effort at every opportunity.

Biosecurity plan

The revised biosecurity plan is still being rewritten. The key addition that we have been working on lately is a "1st response plan". This sets out the actions that should be taken immediately and for the first month, should a rat be detected on Ulva Island.

Thank you to everyone who provided comments and ideas on the draft biosecurity plan. I'm aiming to have the final biosecurity plan available by Christmas. If you are wondering what to do in the meantime, just continue to check over your gear and boats for rats, mice and weed seeds before departing for Ulva and make sure that your clients or visitors do the same. If we all take responsibility for raising awareness about biosecurity, then I am sure that we can continue to enjoy a rat free Ulva Island.

Bait monitoring and signs

If you have visited Ulva Island lately, you will have noticed that all of the poison warning signs are still in place. These signs are required by law to remain in place until no bait remains on the island. To determine when the bait has completely broken down, we have put some bait under cages in a variety of habitats. This enables the bait to weather, but stops any interference from animals. When the bait under the cages has completely gone, then we will take the signs down. I expect that this will be in about another month.

Bird monitoring

The University of Otago bird research team conducted bird monitoring before and after the rat eradication to measure the impact of the rats and the bait drop on Ulva's birdlife. The results of this monitoring should be available next week.

I've been out to Ulva several times since the eradication operation and there appears to have been negligible impact on the birdlife. I have been pleased to see plenty of robins, saddleback, mohua, brown creeper and riflemen. Weka have reduced in number, but I have seen and heard them across all of Ulva Island, so the actual reduction seems to be well less than our "worst case scenario" expectations.

Testing of seafood

Ten days after the bait drop, a raft of samples were taken from the Mataitai reserve immediately around Ulva Island to test for residues of the rat toxin. We wanted to ensure that there was no rat poison present before we stopped warning people not to fish in this area.

As the samples take a while to analyse, we prioritised blue cod and paua to be done first as key food species. We are still waiting on results from testing of trumpeter, banded wrasse, spotties, pipi, kina, limpets and mussels. The lab has said that we should have those results sometime this week.

The paua samples have all come back clear. Of the six Blue Cod samples, two came back with trace amounts of brodifacoum present (0.026 and 0.092 micrograms per gram). Blue Cod was sampled in three locations, with ten fish per site being caught. As brodifacoum is concentrated in the livers, and livers will have the highest concentrations of any part of the fish, the livers were what we initially got tested. Landcare have started testing the flesh of the cod from the two affected samples. These results should be available at the end of this week.

These amounts are trace levels. A fatal dose of brodifacoum for a human adult is estimated at 15mg. This is equivalent to 750 grams of bait containing 20ppm brodifacoum, or approximately 370 cereal pellet baits weighing around 2 grams each. While there are no firm estimates on the minimum oral exposures of brodifacoum that produce any effect on blood coagulation in humans, it appears that 1-2mg would cause measurable increases in coagulation times. Using the highest concentration of brodifacoum measured in the blue cod (0.096 micrograms/gram), over 10.8kg of cod livers would need to be ingested for a 1mg exposure.

This was an unexpected result as the deflector bucket proved very effective at keeping bait from being thrown out into the sea and all other testing nationally (where deflector buckets haven't been used) has resulted in very few positive results (only 2 pipi in the Bay of Islands have tested positive before now).

We will continue to warn people not to fish in the area until testing reveals that there is no brodifacoum present at all. Therefore, we collected more blue cod samples on Friday and will be sending these away for analysis this week.

DNA testing

A project to gather DNA samples from likely rat departure points has been underway for the last few months. This will hopefully allow us to determine where this recent invasion came from, but also allows a DNA library of Ulva and mainland populations to be built up for comparison with any future invasions.

Of the rats caught on Ulva over the last year, 60 have been DNA profiled. Results from this show that the rats are all very closely related, probably resulting from one female.

Samples from mainland sites have started to be collected and we have sent 15 away for profiling. We are expecting a report on this within the next two weeks. As Norway rats only make up a small proportion of the rats around the coast of Paterson Inlet (most are Ship rats), it is taking a while to collect the required number of samples from each site. Nevertheless, we will keep sending samples away as we catch them and will keep you informed as results become available.

Brent Beaven
Biodiversity Programme Manager
Kaiwhakahaere - Koiora Rereketanga

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