Rat tracks at Hobbs Beach spark huge effort to catch invader

From the Dawn Chorus ArchivesEditor: Jim EaglesDate: Dawn Chorus 112 February 2018Photo credit: DOC and Karen O'Shea

Two huge rodent hunts have been carried out on Tiritiri over the past three months (end of 2017 and start of 2018) to preserve the rodent-free status the Island has enjoyed since 1993. Late last year a visitor reported seeing a mouse and sparked off a search which found nothing. Then early this year the discovery of rat tracks near Hobbs Beach led to an even bigger operation.

The rodent saga began on 9 November when a visitor from the US told shop volunteer Chris Eagles he had seen a mouse on Ridge Rd and later repeated the story to relief ranger Dave Jenkins. It was sufficiently convincing for the Department of Conservation to send a rodent dog to check and it smelled something interesting in the area of the sighting. 

As a result, DOC set up a detection zone in a 200m radius around the site and blanketed it with devices 25m apart. These included 300 mouse traps, 500 tracking tunnels with ink pads to record the footprints of anything walking through them and 200 chew cards to record teeth marks. 

In addition, the permanent rodent control network, which has hundreds of traps and tunnels set up at 50m intervals around most of the tracks on the Island, was checked more regularly. Two teams of volunteers were set up – one for Sunday to Wednesday and the other Wednesday to Saturday – to do the monitoring. 

Tony Petricevich, one of the volunteers, said they checked every tunnel/trap every two or three days ‘so between us, we might work on the northern half of the Island one day, the southern half on the next and the central grid on the next and then repeat.’

Left image: delicate footprints left by a skinkRight image: tail and footprints of a tuatara

Left: the footprints of the Norway ratRight: Rodent Hunters (from left) Dennis Green, Yuka Ito, Tony Petricevich, John Kooperberg and Neil Davies (who found the first rat tracks)

Each morning there was a conference call from DOC at 9 am, ‘and then we would be given a map and list of tunnels/traps plus crunchy peanut butter in pottles or small plastic bags. 

‘The tasks were simple: check tracking tunnels and traps, collect and replace cards, reset traps and rebait if required with peanut butter. The weather was very hot and dry and each of us would do up to 40 traps/tunnels, which would take us about 3-4 hours, by which time we were exhausted, hot, sweaty and covered with peanut butter and tracking card ink.’ Nevertheless, Tony said, ‘there was a great team spirit and everyone pulled together to get the job done.’ 

‘We didn’t see any rodent tracks, but what we did see was interesting. After being out for two or three nights many of the tracking cards, especially in damp bushy areas, would be almost black with countless weta prints. Where it was drier and more scrubby some cards would have skink prints and some might have a combination of the two or no prints at all.’ 

Left: the Norway rat on camera approaching a stationRight: the special trap they build to catch the rat

By early December, with no trace of a mouse, in between the monitoring work, the teams started to pack away some of the tunnels/traps from less accessible areas. However, Tony said, ‘We left the more easily accessible tunnels/traps in place for the rangers to collect at their leisure. As it turned out when the search was called off a few days later DOC decided to leave these out and continue monitoring. This was just as well . . .’ 

Only a couple of weeks later the alarm was raised again. Rangers Kata Tamaki and Vonny Sprey could only check the tracking tunnels if they had the time so when Neil Davies, one of the original SoTM mouse volunteers, was on the Island on 7 January he decided to give them a hand. In a tunnel near Hobbs Beach, which had been last checked on 1 January, Neil discovered a rat print and more prints were found in another tunnel. 

The discovery prompted an immediate and massive response. DOC sent two rat dogs and handlers to the Island and scrambled a team of officers to put out more than 50 additional traps and 60 extra tracking tunnels. This time, unfortunately, it was no false alarm. More rat prints were found in the coastal strip from the bottom of the Kawerau Track to the Wharf and further in-land at the top of Cable Rd. A monitoring camera got a photo showing a Norway rat. 

Auckland inner islands operations manager Keith Gell promised: ‘The operation will continue until we can be confident the Island is once again free of predator pests.’ 

When the intruder initially eluded all the efforts DOC staff sought advice from experts and tried different tactics. For instance, knowing that Norway rats are creatures of habit, they put extensions on the tracking tunnels and fitted them with snap traps. 

Brodificum poison was placed in five bait stations around Hobbs Beach and the bottom of the Kawerau Track and monitored daily, with the bait weighed to see how much had been eaten, and checked for teeth marks to see what had been doing the eating. To avoid disturbing the rat, visitors were urged to avoid the coastal strip around Hobbs Beach as much as possible. 

Left: the extensive pest detection network on Tiritiri MatangiRight: rat hunters Finn Buchanan with Pai, and Hannah Johnston with Indie, relax after a day's tracking

The seven takahē were all penned to keep them safe from poison or rat attacks. Studies have shown that tuatara, being cold-blooded, are not affected by brodificum. However, the DOC200 traps being used were raised to keep tuatara out. 

Finally, on 26 January, DOC rangers do- ing an early morning check of traps at Hobbs Beach and found a 280gm female Norway rat in a special device made by cutting a tracking tunnel in half and putting a DOC200 in the middle. Monitoring of tracks continued for a few weeks afterwards, just to be sure, and the takahe stayed in the pens for a while longer because they were due for their health checks and immunisations. But the emergency was over. 

However, the fact that the rat’s footprints were first found near Hobbs Beach, where many boaties come ashore, makes it very likely that it landed from a visiting craft. The incident emphasises just how vulnerable we are and how important it is to maintain strict biosecurity precautions. 

Some useful videos below to show how we can keep invasive pests off Tiritiri Matangi and why it is important to do so