Driving Down the Costs of Predator Fences

Background

Constructing and maintaining a predator fence is not cheap, as it involves costs of earthworks, materials, fabrication/engineering, and ongoing maintenance.  Currently, a traditional 1.8 m high fence seldom comes in cheaper than $250/m, and this cost can reach as high as $500/m on steep, tricky to access terrain.  

Over the past five years, we’ve been undertaking trials to attempt to reduce the cost of a predator fence. This update summarises the results of the trials.

Our first phase of trials, which started in 2017, explored minimum fence height. We discovered that a 1,100mm high fence, fitted with a standard rolled steel cap, was sufficient to contain 95% of stoats and possums, and 100% of ship rats. This design is cheaper to construct than traditional 1,800mm fences, because its construction requires less mesh, staples, and fewer posts.

Since 2018, we have continued testing alternative fence designs, with a focus on trying to reduce the cost of the fence by using either (i) a cheaper material for the cap, or (ii) an alternative fence design that does not require a cap.  The results are outlined below; we have yet to undertake a whole-of-life assessment of these designs.

A Cheaper Fence Cap

Multi-species predator fences in New Zealand are traditionally capped with a rolled or folded steel hood. Steel caps can be effective against a range of species but may be excessive in some contexts. The steel cap is a significant portion of the overall cost of a fence, and in some situations, a cheaper, species-specific alternative may be a more feasible option.

A ship rat attempts to climb over a low-height fence capped with a length of 90mm wide HDPE pipe during trials at Lincoln

High-density polyethylene (HDPE) pipe is a common, durable and non-brittle material, often used for water pipes. The smooth and hard surface is largely resistant to scratching, giving rats and stoats less purchase or grip and limiting climbing behaviour. One potential advantage of this material is that could be feasible to heat-bend around corner sections on-site (unlike steel caps where the corner sections must be precisely prefabricated off-site).

During testing at the animal behaviour facility (at Lincoln University), we found that 110 mm diameter HDPE pipe, fitted onto a 1,100 mm high, 12 mm x 12 mm mesh fence, was able to contain 94.7% of ship rats (18 out of 19 rats tested) and 95.2% of stoats (20 out of 21 stoats).This diameter pipe was not able to contain possums

You can find out more about this trial in the report we prepared for Predator Free Wellington (who asked us to run the trial).

Please note: we are not aware of anyone, as yet, using this method to cap a predator fence. And there are aspects of the research and development that we have not worked on, such as how to efficiently attach the mesh to this capping type, or how to join sections of piping.

An Alternative to Fence Caps - Electric Hotwires

Fences combining wire mesh and electric hotwires, have been used around New Zealand as a means of possum control for many decades. Working alongside Otago Peninsula Biodiversity Trust, we tested a design consisting of a 900 mm wire fence retrofitted with mesh and also with three single hotwires on 10 cm outriggers (at 400, 800, and 1,000 mm height).

 Possum deterrent fence consisting of mesh and three electric hotwires

This fence only contained 62.5% of possums (10 out of 16 animals rested). Despite the high rate of escapes, the ability to retrofit this type of design, at very low cost, onto existing fences may be a useful tool for helping to slow the migration of possums (especially when used in combination with other control techniques, such as bait stations).

For further details please see Technical Report #11: Testing a modified electric farm fence as a possum-specific barrier.

Alternatives to Fence Caps – Sheet Metal Panels

Our current fencing trial work centres on finding a low-cost option for rat-specific barriers. In particular, we are interested in whether aluminium sheet metal can be used to fence rats.

Aluminium sheet metal collars are already widely used on trees and power-poles to prevent possums from climbing them.  And is a cheaper, and lighter, alternative material than steel.

We are currently trialling whether a 300 mm strip of 0.45 mm aluminium sheet metal, secured to the top of an 800 mm mesh fence, is an effective barrier to ship rats.  To date, 82% of ship rats have been contained by this fence design (18 out of 22 rats tested).

Mock-up of an aluminium sheet metal fence used for ship rat trials

We are undertaking further tests to determine the efficacy of this design on Norway rats and stoats.

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General Advice about Predator Fencing

We regularly get asked for advice about fencing. Based on trials at our research facility, we think:

• A standard capped predator fence at a height of 1.1 m, with a reduced skirt, generally does a good job at keeping possums, stoats and rats out of fenced areas.  Note though, 1.8 m is the industry standard to keep cats out of sanctuaries.

• Mice can be excluded with predator fence at a height of 1.1 m, provided "mouse proof" mesh is used. Standard 12 mm mesh won't protect against mice, but it will against rats.

• 110 mm diameter HDPE pipe shows promise as a potential replacement for standard pressed steel capping. Please note that this does not deter possums.

• A hotwired fence used in combination with bait stations and/or kill traps may be a more cost-effective solution than a full-scale predator fence to deter possum reinvasions.

• A good seal around the outside of any gates is a must, particularly if you are trying to exclude rats

This information is not intended to replace seeking out a solution that is suitable for your project – a predator fence may not actually be the most effective option. You may need to get someone to do an ecological assessment to determine the best ways for your project to become and remain predator free. 

Before committing to predator fencing, it is also important to understand the whole cost, as well as the cost of potential alternatives. You will also need to consider any added social cost of having high fences in people’s backyards. 

Testing a modified electric farm fence as a possum-specific barrier

Background

ZIP has been providing technical support and advice to our friends at Predator Free Dunedin and Otago Peninsula Biodiversity Group (OPBG), who are working to eradicate possums from the 9,500 ha Otago Peninsula.

In 2019, we visited the project site, and were shown a modified electric farm fence developed by a Trustee of OPBG to slow possum migration across rural farmland. Based on anecdotal success, we decided to carry out further testing of the fence as a possum barrier, at our predator behaviour facility in Lincoln.

What we did

We adapted the original fence design, a 900 mm high wire-mesh fence fitted with three single hotwires, into an octagonal testing pen inside the 2 ha enclosure at our predator behaviour facility.

We tested the fence against a total of 16 wild-caught possums—8 female and 8 male. Each possum was released into the pen overnight and their interactions with the fence were monitored by continually recording video cameras.

Results

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Overall, the pen contained 62.5% of individuals (10/16). Male possums were more likely to escape the pen (62.5% escape, 5/8 individuals) than their female counterparts (12.5% escape, 1/8 individuals). The most common mode of escape was termed ‘jump and climb’. A possum, having previously encountered the lower hotwire, would jump past the first hotwire onto the mesh, then climb over or under the mid and upper hotwires. A disparity in initial jump height between male and female possums may help to explain the sex bias observed in this trial.

Conclusion & discussion

Despite the high overall escape rate in these trials, this low-cost fence may offer benefit to control operations as a means of slowing possum migration. The higher containment rate of females is promising as it suggests male-skewed invasion, which would limit the breeding potential of the population. Improvements to the fence design should aim at targeting these male individuals. Alterations that could be considered include raising the height of the mesh, or installing a fourth hotwire between the mid and upper hotwires.

It is also worth noting that placing an individual possum in a pen generates a very high motivation to escape, and therefore actual rates of migration across an electric fence may be lower than those we saw during this trial. Wild animals are likely to interact with a barrier fence over longer timescales, but may face lower motivation to cross when they do. In a landscape scale application, an exploratory shock may result in avoidance or edge following behaviour, which are not feasible for individuals within the confines of the trial pen. This may help to explain why some field applications of possum-specific electric fences report higher rates of efficacy than that observed in this trial (Cowan & Rhodes 1992).

Want to learn more? Check out Technical Report #11.

Using lured trail cameras to detect predators at low density

We have prepared an accompanying field guide, outlining how to install a lured trail camera in the field, for the purpose of detecting predators.

Background

The use of trail cameras has increased across the conservation industry in recent years, as they have become more affordable and accessible, and as it has become increasingly evident that they are a highly sensitive tool for survey, monitoring and detection relative to other available methods (Smith & Weston 2017). Cameras also provide a wealth of data, allowing researchers and practitioners alike to make inferences wider than simple presence/absence of a species (Dilks, Sjoberg, & Murphy, 2020) — such as animal behaviour (Caravaggi et al., 2017) and activity patterns (Kays et al., 2009).

ZIP has learned that a network of trail cameras paired with our ‘MotoLure’ automated lure dispensers is an effective method for detecting possums, rats and stoats, particularly for projects where it’s critical to detect relatively few animals in a large landscape.

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This method is now the basis of a detection network we have established in the Perth River valley to provide the first indication that one (or more) of these predators is present within the field site.

Cameras also have the added benefit of detecting other species. The Perth River network has also detected native birds such as kea, kakaruai (South Island robin) and ruru (morepork), along with game animals such as tahr, deer and chamois.

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Purpose

The primary purpose of this Finding is to describe ZIP’s current approach to possum, rat and stoat detection using lured trail cameras. This approach will continue to evolve on the basis of what we and others learn about this technique, and as new tools become available.

ZIP’s current approach to the use of trail cameras

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This camera, though covered by snow and ice, was still functioning well in the depths of winter in the Perth River valley (5th August 2019)

Preferred Camera

Our preferred trail camera is the Dark Ops range by Browning. We have generally found these cameras to be more reliable, more waterproof, and easier to use than other brands we have trialled; although the harsh wet conditions of the Perth River valley can occasionally result in moisture damage.

Over the last three years we have used four slightly different models as Browning have evolved their product range. The Dark Ops models we have used in the Perth River valley are the Extreme (model BTC-6HDX), 940 HD (BTC-6HD-940), the Elite HD (BTC-HDE), and most recently, the 2020 Dark Ops Extreme Max HD (model BTC-6HD-MAX).

This model has a maximum image size of 18 MP, and a minimum delay trigger setting of 1 second. This is very useful, because valuable imagery can be missed with longer trigger delays. It is also slightly more compact than the other models (10.6 x 7.50 x 6.25 cm).

All of these cameras have similar specifications, including a maximum video size of 1280 x 720p (low compression), infrared LED night-time illumination, infrared motion detectors, and adjustable trigger speeds and detection range.

Browning Dark Ops cameras use standard SD cards, and run on six AA batteries.

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Limitations of the PIR Browning camera sensor zone

Our preferred practice is to place cameras at a distance of 1.2-2.5 metres from the lure.

On the Browning camera, the PIR sensor zone is bow-tie shaped, and detects animals more readily when they approach from the left or right of the camera’s field of vision. We have found that animals approaching from the top, bottom, or corners are unlikely to trigger the PIR until they near the centre of the camera’s field of vision (ZIP, unpublished data).

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The diagram to the left illustrates the estimated extent of the PIR sensor zone, at a distance of 2.1 metres. At this distance, an animal must be within the area outlined in red to trigger the PIR sensor.

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SD cards              

We use Sandisk 16GB Ultra 48 MB/s or 80 MB/s SD cards. We have found that these cards offer good value for money, without compromising on storage size. A 16GB card can hold up to 30,000 still images, which is sufficient to last at least two months in the field with our preferred settings. Sandisk cards are also waterproof, drop-proof and magnet-proof, which makes them very reliable in the field. It is good practice to format SD cards after removing footage, to ensure there are no residual files that could affect future recording.

Batteries

We currently use Panasonic Eneloop PRO 2450 mAh Ni-MH rechargeable AA batteries. We have found these to be a reliable option in the field. The batteries retain a high charge for significantly longer than other rechargeable batteries we have used, and work well in the cold conditions of the Perth River valley.

In general, batteries will last approximately 2-3 months in the field depending on the number of trigger events; some have been known to last over 6 months.

To ensure our cameras are continually functional and ready to detect invading possums, rats or stoats, our current practice is to check the batteries every 4-6 weeks, and change batteries when the charge drops below two bars.

We have also found that it is a good practice to fully discharge the batteries, using the discharge function on our chargers, before charging them again. We use Tenergy TN160 chargers, which have capacity to charge up to 12 batteries at a time. A full discharge and recharge cycle takes up to 12 hours, while a normal charge straight from the field takes 4-5 hours, depending on the remaining charge in the batteries.

Lure

The ZIP MotoLure

In order to maximize the chance of detecting possums, rats and stoats in a low-predator or predator-free environment, we pair trail cameras with the ZIP MotoLure, which dispenses a pre-set amount of fresh mayonnaise each night to attract possums, rats and stoats into a camera’s field of view. Dispensing mayonnaise lure each night encourages repeat visits by predators.

We believe that deploying trail cameras paired with lure dispensers before a predator removal operation may increase predator visitation rates; which in turn may increase confidence in the network as a method for detecting any remaining individuals after the operation is carried out.

Corflute ‘chew cards’

For possums and rats, peanut butter-lured chew cards are an affordable alternative to the MotoLure, although these will need to be replaced more frequently as the peanut butter is eaten or degrades. This should be roughly every 3-4 weeks in summer and can be extended during the winter depending on environmental conditions. To lure trail cameras using this method, we recommend purchasing unfilled 10 cm x 10 cm chew cards, and filling them to a depth of 4-6 cm on one side with a high-oleic smooth salted peanut butter (e.g. Pic’s), mixed with enough icing sugar to thicken it to a paste consistency that clings to the back of a spoon without running off – a ratio of approximately nine parts peanut butter to one part icing sugar.

Network design

In order for the Remove and Protect approach to be economically viable across large landscapes, it is important to minimise costs associated with both the network of trail cameras and labour required to service them.

In the Perth River valley, we are currently trialling a network of lured trail cameras at a density of one camera every 35 hectares, across all predator habitat within the field site – including tussock and alpine scrub, exposed ridges, open forest and wet valleys.

We are very confident this density is sufficient to detect individual possums and stoats, so that our team can initiate a response to remove these individuals (using traps, toxins and hunting with a predator dog). We are still determining whether or not this density is sufficient to quickly alert us to the presence of an emergent population of rats, while this population is still spatially restricted and can be removed using a targeted approach.

Footage review

We have developed a software tool that:

• makes it easier, more efficient and more accurate to manually review trail camera footage to record detections of target species;

• can be used offline, as our team often need to review footage in remote backcountry, with limited internet service, in order to enable a timely response to a detection; and

• is able to process both image and video files.

The software we have developed meets all of our requirements. When in use, the software: (i) enables a reviewer to record detections using ‘quick keys’, each of which is associated with one of up 26 target species or other items of interest; (ii) automatically extracts metadata associated with each detection (e.g. time and date); and (iii) is capable of exporting results into a spreadsheet (in CSV format) and folders for specific targets (for easy reference at a later date). Somewhat unimaginatively, we currently refer to this tool as “The Classifier”, and we are continuing to refine it.

Similar software tools already exist (e.g. Snoopy, ViXen and FastPhotoTagger). However, none of these tools met enough of our specific requirements for us to easily apply them in our context.

A project to improve the efficiency of the use of cameras to detect predators

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ZIP’s prototype detection device, currently undergoing field testing in the Perth River valley

While trail cameras are increasingly recognised as the most sensitive tool available for detecting the presence of predators, it can be very expensive to maintain a network of trail cameras – particularly in remote locations. This is largely due to the high labour cost associated with changing batteries and SD cards every 3 to 6 weeks, and with processing footage from the cameras. Manually servicing trail cameras in remote areas does not necessarily provide timely notification of the detection of individual predators, given their mobility and/or reproductive capability.

Consequently, for the past several months, the ZIP team, with the help of some talented contractors, has been developing a new predator detection device. The work has been funded by DOC, NEXT Foundation, Predator Free 2050 Limited, and the Biological Heritage National Science Challenge. The team also acknowledges the pioneering work of Grant Ryan, from The Cacophony Project, to develop a similar device particularly suited to more accessible areas.

The ZIP device will comprise a highly sensitive thermal camera, on-board artificial intelligence (A.I.) video analysis software, and remote reporting capability. The device will be need to be rugged, and use little power in order to run without servicing for as long as possible; ZIP’s target is for the battery to last more than 6 months, but that will depend upon the number of triggers. It will need to be able to reliably identify possums, rats and stoats to a very high level of accuracy, and also be able to remotely report target detections to conservation managers.

We’ll provide an update about progress with developing this device in the coming weeks. For now though, some more information is available in this short video.

Reference

Caravaggi A, Banks P, Cole Burton A, Finlay CMV, Haswell PM, Hayward MW, Rowcliffe MJ, Wood MD 2017. A review of camera trapping for conservation behaviour research. Remote Sensing in Ecology and Conservation 3(3): 109-122.

Dilks P, Sjoberg T, & Murphy E 2020. Effectiveness of aerial 1080 for control of mammal pests in the Blue Mountains, New Zealand. New Zealand Journal of Ecology 44(2): 1-7.

Kays R, Tilak S, Kranstauber B, Jansen P, Carbone C, Rowcliffe M, Fountain T, Eggert J, He Z 2011. Camera Traps as Sensor Networks for Monitoring Animal Communities. International Journal of Research and Reviews in Wireless Sensor Networks 1(2): 19-29

Smith DH, Weston KA 2017. Capturing the cryptic: a comparison of detection methods for stoats (Mustela erminea) in alpine habitats. Wildlife Research 44(5): 418-426.

Assessing the use of the ZIP PosStop trap, with daytime lockout, in the Perth River valley

Background

Since early 2018, ZIP (with the support of DOC, NEXT Foundation and Predator Free 2050 Limited) has been carrying out a programme of work at a field site in the Perth River valley, South Westland. This project aims to completely remove possums, and potentially rats, and stoats, from 12,000 hectares of rugged back-country, and to then permanently prevent these predators from re-establishing.

One of the tools ZIP has developed to help protect the site against possums is the ZIP PosStop, a live-capture possum trap based on the PCR No. 1 leghold trap, presented in a raised platform to minimise risks to ground-dwelling birds. Each trap is attached to the platform with a chain, which is in turn fitted at the trap end with a short length of bungee cord to reduce possum escapes. The platforms are mounted on a tree or post, and lured with a white powder-coated aluminium visual lure.

Previous work by ZIP and others has shown lone possums can roam large areas (in the order of 50−100 ha) in search of other possums, so a relatively lean network of devices is believed to be sufficient to intercept them when they enter an otherwise possum-free area. In earlier trials by ZIP, a network of PosStop traps at a density of 1 per 50 hectare successfully prevented possums from re-establishing on the 400 hectare Bottle Rock peninsula in the Queen Charlotte Sound.

In the Perth River valley, we initially intended to deploy a network of PosStop traps, paired with the ZIP OutPost automated reporting system, at a density of 1 per 21 ha, within all forested areas of the project site (a total of 143 traps). However, the Perth River valley is home to an estimated population of 75-120 kea. While we anticipated that the removal of possums from the site would ultimately be beneficial to kea, they are curious and intelligent birds, with a reputation for thoroughly investigating novel objects in their environment. Therefore, before deploying a live network of ZIP PosStops in the Perth River valley, we needed to:

1. Identify and mitigate any impacts that curious kea may have on the performance of these devices at catching invading possums; and

2. mitigate any risks to kea from these devices

The ‘SafeLock’ function for daytime lockout

Early in the project, we identified a need to prevent kea from fiddling with components of the trap set up, which could reduce the effectiveness of the trap and possibly cause harm to kea.

To that end, we developed a version of the PosStop trap that included a ‘SafeLock’ mechanism to enable the traps to be locked during daylight hours, when kea are most active – while still enabling the trap to operate during darkness, when possums are most active. In the video below, our engineer John demonstrates how this mechanism works, using an early 3D-printed prototype.

Trial in the Perth River valley

Between December 2018 and July 2019, we ran a trial in the Perth River valley to assess the timing, frequency and nature of kea activity at ZIP SafeLock traps. Ten SafeLock traps were installed in the field site: five along an exposed alpine ridge, and five below the bush line. Each trap was monitored with trail cameras programmed to take 30 second videos upon triggering, with a 5 second delay between each video. To ensure the safety of kea during this trial, all 10 traps were permanently locked and unable to be sprung.

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ABOVE: The exposed ridge where five SafeLock traps were deployed - three trap set ups are visible (circled) (L) and one of the five traps installed on the ridge, monitored with two trail cameras (R)

Over 213 days (equating to 1,838 trap nights), the trail cameras recorded 278 kea interactions with the SafeLock trap set ups. All of these occurred at the five ridge traps, with the exception of a single daytime interaction with one of the traps below the bush line.

From December through to the end of April, kea activity at the SafeLock traps was infrequent (34 records), and always occurred between the hours of sunrise and sunset – the time during which the traps would be locked in operational use. We observed a sharp increase in kea activity in late April 2019, which continued for the remainder of the trial. Kea activity increased at the trap sites as kea began to discover, and play with, the chain and bungee cord components of the SafeLock traps. The timing of increased behaviour coincides with the end of the fledging season, a time when kea chicks from the most recent breeding season reach independence and start to explore the world around them.

Our observations suggest that kea learning to play with the trap components may have provided an ‘entertainment’ reward which resulted in an unusually high level of kea activity at, and repeat visitation to, the SafeLock traps, and the occurrence of interactions outside of daylight hours.

In a ‘real world’ scenario, in which these traps were not locked during the night, 12 of the 278 interactions we recorded may have triggered the leghold trap, and been unsafe for kea. While it was difficult to distinguish individual kea in the trial footage, repeat visitation by kea is known to be common at sites where a reward is provided, so we believe these interactions are likely to represent a small number of repeat visitors. All nocturnal activity occurred after the traps had been in place for 6 months, and at a substantially lower rate (0. 01 per trap night) than daytime activity (0. 2 per trap night).

All five of the ridgeline traps were disarmed or had their bungees damaged as a result of kea activity during this period. Left unattended, these traps would not have been able to catch invading possums – and, in fact, may have led in some cases to trap-shyness, by partially catching or simply frightening any possums that triggered them. 

Attempts to remove the entertainment ‘reward’ for kea from the trap

Based on these results, we initiated work on a solution to protect the chain and bungee from kea access, and reduce the incentive for kea to interact or play with the traps.

We designed a stainless-steel prototype ‘protection plate’ to fit between the set SafeLock trap and the chain bucket, and tested the effectiveness of the plate in the Willowbank Wildlife Reserve kea enclosure.

To our disappointment, the steel plate was found to have little effect on restricting kea access to the chain and bungee. Kea rapidly learned to hook their beaks around the steel plate, pull the bungee out, and chew on the bungee, within hours of installation (see below). As we observed in the field trial, they also flipped one trap jaw up, which in a field situation would render the trap ineffective at catching possums. A second design, with smaller corner gaps, also failed to prevent kea interfering with the chain and bungee.

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ABOVE: A SafeLock trap with jaw flipped up and chain pulled out due to kea interaction

We determined that further prototypes of this type were unlikely to be effective, unless the trap was seriously modified so that the front jaw could not be flipped up, or the whole platform was redesigned with a deeper chain bucket and in-built protection plate. This was not a feasible option for the Perth River valley field site, as retooling, testing and production would likely take months to complete, when we needed a solution much sooner.

Conclusion

We can draw several conclusions from the data gathered from the trials in the Perth River valley and at Willowbank Wildlife Reserve:

1. The ZIP SafeLock system works to prevent unwanted trigger events. Despite 216 total ‘virtual trigger’ events in the field (and multiple at Willowbank), while traps were locked, none were able to be triggered by kea.

2. Kea in the Perth River valley are significantly more likely to interact with traps in exposed alpine locations, than with traps below the bush line. Of the five bush traps in this trial, only a single kea interaction was recorded over 895 trap nights in this environment.

3. Over time, kea can learn to interfere with the bungee and chain components of the traps in a way that regularly renders them ineffective for catching possums.

4. Sub-adult or juvenile kea are more likely to interact with traps and render them ineffective.

5. While there were night interactions on the exposed SafeLock traps on the ridge, these occurred at an extremely low rate, and did not occur until after the traps had been in place for 6 months, when kea activity increased (most likely as a result of fledging kea learning to play with the bungee and chain components of the traps).

6. Overall, risks to kea from the SafeLock trap are very low, and can be further reduced by minimizing kea exposure to the trap.

Where to from here?

Based on the conclusions described above, we plan to minimise opportunities for kea to learn to play with the bungee and chain components of the SafeLock trap, by minimising kea exposure to traps in the Perth River valley field site.

Instead of deploying an extensive, permanent network of automated reporting SafeLock traps as the primary tool to detect and respond to invading possums (as we originally intended), we will use lured trail cameras to detect invading possums.  SafeLock traps will then be used to respond to detections in a temporary, targeted network, along with other devices and hunting by our predator dog Pepper and her handler Mike. These traps will then be removed once the possum is caught. Furthermore, SafeLock traps will only be deployed below the bush line, where the probability of kea interacting with traps is substantially lower.

The network of lured trail cameras used in the Perth River valley has proven to be a highly sensitive method for detecting possums, rats and stoats. In addition, the use of trail cameras as the primary detection method for possums is expected to significantly reduce the infrastructure costs associated with protecting the site long term, relative to the cost of a permanent network of automated reporting traps.

A single interaction may be the only opportunity we have to capture an invading possum in the Perth River valley. These measures will ensure that the opportunity is not compromised, while ensuring that kea exposure to the ZIP SafeLock trap is carefully managed.

The damage possums do to native forests is well documented: they are the major cause of the decline of many tree species, including rātā, kāmahi, kōtukutuku and pōhutukawa. But they are also opportunistic predators, and have been recorded preying on the eggs and chicks of native birds, including kea. We are already beginning to see the benefits of removing possums and other invasive predators from the Perth River valley, and our observations suggest that kea are thriving in the field site as we work to maintain predator freedom.

Technical report pending.

Assessing survival of Himalayan tahr during the Perth River predator removal operation

Since early 2018, ZIP (with the support of DOC and Predator Free 2050 Limited) has been carrying out a trial at a field site in the Perth River valley, South Westland, which aims to completely remove possums, rats, and potentially stoats from 12,000 hectares of rugged back-country, and to then permanently prevent these predators from re-establishing. In autumn/winter 2019, ZIP carried out an initial predator removal operation using a modified technique for applying aerial 1080. This operation was carried out in two phases, each consisting of two applications of non-toxic prefeed, followed by a single application of toxic bait.

The Perth River field site contains Himalayan tahr, which are highly valued by hunters. While there is little evidence to suggest that ‘standard’ aerial 1080 operations have an impact on tahr populations, ZIP recognised that some aspects of the modified technique used in the predator removal could elevate the risk to tahr. In order to better understand this risk, ZIP invited the Game Animal Council (GAC) to collaborate on a research programme to assess tahr survivorship through the Perth River predator removal operation.

Before the predator removal operation, a sample of 21 tahr were fitted with collars containing radio transmitters. The sample was made up of nannies and juveniles of both sexes, as these were deemed to be more likely to be at risk of 1080 poisoning than adult males.

These tahr were monitored throughout the operation using Sky Ranger, an automated system designed to monitor wildlife from an aircraft. Sky Ranger flights were carried out as soon as practically possible before and after each application of prefeed or toxic bait, to provide information about each individual tahr’s potential exposure to 1080.

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Three tahr are fitted with collars containing radio transmitters. During this procedure, the tahr wear blindfolds and horn protectors to ensure their safety and welfare.

The high mobility of tahr, along with the unavoidable time lapses between 1080 applications and Sky Ranger monitoring, meant that we could not confirm with certainty the exposure of some individual tahr to 1080 baits. However, statistical analysis based on the locations of radio-collared tahr during each Sky Ranger flight indicates that 11-15 of the radio-collared sample were exposed to 1080 during the first phase of the operation, and 8-14 were exposed during the second phase. All of these tahr survived the operation.

The full report, authored by Geoff Kerr, Professor of Environmental Management at Lincoln University and former Game Animal Councillor, is available here.

The results of this trial suggest that tahr survival during the Perth River valley predator removal operation was extremely high, and there was no evidence that any tahr died as a result of the operation. Together, these results support the general view that tahr populations are unlikely to be at risk during aerial 1080 operations.

ZIP appreciated the opportunity to work closely with the Game Animal Council on this project, which also benefitted from the advice and feedback of the Department of Conservation, the New Zealand chapter of Safari Club International, the New Zealand Deerstalkers’ Association, and the New Zealand Professional Hunting Guides Association.

“The Game Animal Council was pleased to contribute to this project. It is critical that pest control operations and the use of toxins are based on good science, which is why our collective work on the Perth Valley operation that showed aerial 1080 to have little to no impact on tahr is so important. We are excited about the animal management and conservation opportunities that will result from this ZIP-led work.”

— Tim Gale, Game Animal Council General Manager