Post Eradication Monitoring in Glenfern, Sept ‘09 to May ‘10. by Tony Bouzaid

Environmental News Issue 21 Summer-Autumn 2010

Rat incursions have intensified over this dry summer but Glenfern is repelling invaders by effort and ingenuity.

History: Two aerial bait drops were carried out on Kotuku Peninsula on June 24th and August 6th 2009. A track network on a 50m x 50m grid of tracking tunnels had been installed the year between the finishing of the pest exclusion fence and the aerial drops. The grid had been delineated into 16 Pathways – each being 6-8 hours work for a person to place or retrieve index cards in the tracking tunnels. The monitoring process allows us to find and respond to incursions.
Monitoring for rodents commenced monthly in September with peanut butter being placed on ink cards in every one of a thousand tunnels by 8 people over a two day period. Two days later the cards were retrieved to establish the presence of rodents. The buffer zones around each end of the fence were being monitored once in between each monitoring run. Over the first two months only one rat print showed up under a house in Arthur’s Bay (Orama). In October Paul Keeling arrived to ascertain the presence of cats and rabbits and trap for same. Over three weeks on the peninsula he caught one cat and established that there were no rabbits or cats remaining inside the fence.

In November there were incursions around the end of the fence in Arthur’s Bay and across the harbour in Port FitzRoy. In December there were several prints in Arthurs Bay, along the shore line in Port FitzRoy and one rat came ashore in the Kotuku Scenic Reserve and left prints in 5 tunnels across the peninsula. It took five days responding to these prints with traps and bait. In January a rat came ashore at Wingers on the Karaka Bay side and around the end of the fence in Port FitzRoy.

Because of the number of rats that were being caught and detected outside of the fence and along the shorelines on both sides of the peninsula the buffer zones were extended all the way to the Garden Bays on each side. Bait was also placed in the old bait station network along the shoreline around the peninsula. Bait on cradles in tunnels were alternated with traps throughout the buffer zones.

In February multiple incursions were detected along the shorelines of Karaka Bay, Port FitzRoy and Kotuku Reserve. As a result the buffer zone monitoring was stepped up to every 3-4 days. By responding with a trap in the tunnel the same or following day as the prints were found, the rat was generally caught as it seemed to frequent the tunnel that it had found safe. This then became the normal practice and it was decided to decrease the gap between monitoring runs to three weeks. Most of the cards that were still viable were left out or replaced after monitoring this time to provide an idea of what took place during the intervening period.

In March two monitoring runs were conducted. The buffer zones were increased again by extending them inland from the foreshore into the adjacent hinterland to detect those rats missing the shoreline defences. Up until the 5th March there had been 12 rats caught on the Port FitzRoy side and 4 on the Orama side inside the fence. However there were 19 caught outside the fence on the Orama side and 6 on the Pt FitzRoy side.

On the first day of monitoring while the cards were being placed 8 rat prints were picked up in the extended buffer zone in Port FitzRoy, another in Kotuku Bay and one in Karaka Bay that had to be dealt with. On the second day of monitoring a rat was caught in Kotuku Bay, Karaka Bay and one in the Port FitzRoy buffer zone. On the third day of monitoring further prints were detected in the Port FitzRoy and Orama extended buffer zones.

By the 13th, using the instant response method 9 rats were caught in traps where prints were found in Port FitzRoy and Kotuku and another 4 plus a mouse inside and 5 outside at Orama. By the 16th 4 more caught inside at Port FitzRoy and 2 outside with 3 plus a mouse inside and 13 outside at Orama.

The second monitoring run in March showed a similar level of incursions to the earlier one two weeks before. These were dealt with in the same manner as well as the buffer zones being attended to every 2-3 days. This indicated a drop-off in the incidence of incursions in the buffer zones, particularly on the Orama side.

For the April round all the viable cards were left or replaced in the tunnels at the end of the last round and traps were left set in the areas where prints were found.

The team took traps with them this time to place in tunnels where prints were found. These traps are baited with peanut butter and macadamia nuts. When the cards are brought back to base the results are displayed on a large map of the peninsula with coloured pins representing dead rats (red), prints with traps (green), prints without traps (white), traps sprung (black) and bait on cradles (blue). This proved a good way to get an overall feel and plan the response.

In the May round all the cards were replaced as the ink on the cards was not lasting long enough. When the monitoring started 24 rats were found dead in traps placed in the response phase of the April round. Over the next two days another 14 rats were caught from fresh prints and the next 10 days of response monitoring caught a further 12 rats.

Mice prints were detected for the first time this round near the edges of the paddocks, one trapped above the Wingers and one print on the driveway near the Glenfern front gate.

A lot is being learned about rat dynamics in the Great Barrier context where there are no possums, hedgehogs or mustelids.

• When rats trip traps without being caught they become trap shy, so these traps are replaced with cradles, bait and cards.

• A single rat is often responsible for more than one set of prints although traps are placed in all tunnels with prints.

• With the long dry summer the rats that come ashore gravitate up the streams and damp gullies into the middle of the peninsula.

• The shorelines on both sides of the peninsula and Kotuku Bay have the main concentration of prints and rat kills. The next highest concentration occurs on the ridge above the dwellings at Wingers and Arthurs Bay.

• Rats often dwell in barren terrain where there is no ready food supply.

My grateful thanks to the Biodiversity Funds, World Wildlife Fund, Auckland City Heritage Fund, Lotteries, Covertex and the ARC for their assistance with the fence, eradication and monitoring. Also the volunteers who keep coming back and show such commitment to this project.