The island has a terrestrial area of 4,405 km² and is 145 km long (E-W) × 54 km wide at its widest point with a population of > 4000 people. In 2019/2020 > one-third of the island was severely burnt by bushfires, reducing the feral pig population by approximately 90%. This provided an opportunity to eradicate the low-density population while vegetation recovery enabled high detectability. After the 2019/2020 fires, the feral pig population was estimated to be < 450 individuals (Page et al. 2021). As part of the Island’s post-bushfire feral pig eradication program there has been extensive ground shooting (one intensive three-month period that removed 165 pigs), trapping, and baiting for pigs across the island. Ad-hoc aerial culling occurred in March 2020 while a helicopter was on the Island for aerial baiting. This approach resulted in three 2 h shooting sessions and removed 7 pigs. The remaining individuals are increasingly difficult to bait or trap, with animals restricted to the western end of the Island, and typically located in inaccessible parts of farms, former plantation forests, as well as parks and reserves. The TAAC program was undertaken over creek lines of Flinders Chase National Park, The Ravine des Casoars Wilderness Protection Area (WPA), Cape Torrens WPA, Cape Bouguer WPA, Kelly Hill CP, Western River WPA and Vivonne Bay Conservation Park.

The Limestone Coast is a low-lying region of south-eastern South Australia that consists of ancient, low sand dunes and incorporates important Ramsar listed wetland areas such as the Coorong and Bool Lagoon. Feral deer occur throughout the region with fallow deer the most common deer species. The TAAC program took place across patches of bushland on seven private holdings totalling 173.7 km². Properties included a mix of grazing and cropping, with areas of remnant tea-tree (Melaleuca alternifolia) and eucalypt woodland. Visual shoots have been done in this area 1-2 times year^-1 for several years. Landholders also engage contract, volunteer and/or recreational shooters to assist with deer management and a commercial harvester also worked across most of the seven holdings regularly between 2018–2020. A visual aerial cull was done over the area 10 days prior (13–17 September) to the TAAC program. The visual shoot crew flew 89.9 h, using 2 helicopters over 5 days, and shot 603 deer across 128,103 ha², with the aim of removing as many deer as possible as quickly as possible. While vegetated areas were included, targeting occurred mainly on open pasture.

We followed the guidelines in Iacona et al. (2018) for collecting and reporting intervention-level costs in 2021 Australian dollars (AUD). The following costs are indicative rates only and might vary between providers and/or countries. They are also only for operational hours and do not include mobilisation costs such as ferry of the helicopter to site, positioning of a fuel truck or travel costs for the crew. Where day rates were given, we calculated the hourly rate as day rate × number of days ÷ number of hours. We calculated the hourly cost of control as: hourly cost = helicopter hourly wet rate + shooter hourly rate + thermal operator hourly rate + imager hire hourly rate + (cost round^-1 × animals shot hour^-1 × shots animal^-1). We used the following rates to calculate program costs: helicopter wet rate for a AS350 B3 Squirrel = $2500 hour^-1, shooter rate = $1200 day^-1, thermal operator rate = $650 day^-1 and imager hire = $900 day^-1. We averaged ammunition costs at $2.25 round^-1. The cost for the pig program (36 hours flown over 13 days) was $3515.93 hour^-1. There have since been two additional thermal-assisted aerial cullings on Kangaroo Island, with a total of 394 pigs removed over the three programs. The cost for the deer program (15.5 hours flown over 3 days) was $3092.25 hour^-1. The deer trial was stopped after 15.5 hours of shooting because the land manager felt the trial was successful and did not require further evidence of its effectiveness.

Types of firearms and ammunition used in aerial culling are determined by each jurisdiction. In the Australian programs a Springfield M1A 308 calibre self-loading rifle was used with 135 grain hollow-point ammunition for pigs, and 150 grain soft-point ammunition for deer. In New Zealand a variety of firearms are used. The common calibres are 223 with 55 grain soft or hollow point for goats or 64 grain for deer, 308 using 130 grain soft or hollow point for goats and 150 grain for deer, and a 12 gauge shotgun using 9 or 12 ball “00” lead Buckshot for goats and deer. A full discussion of firearm development for TAAC can be found in Supplementary Information 2.

The reported number of deer lost during the Limestone Coast case study is potentially overestimated. When density is high, it is difficult to track all individuals in a group. During a shoot, individuals would often leave the group or additional individuals would be acquired. Without being able to identify animals individually, it is difficult to know if lost animals are in fact lost, or whether they are reacquired as part of another group. For this reason, ‘lost animal’ data from the deer shoot, and likely the pig shoot, are probably inaccurate. The deer data in this case study are for fallow deer only. Therefore, we cannot make reliable inferences about effectiveness for other deer species. Similarly, for pigs, the number of lost animals is likely an overestimation. While individuals were lost, quite often revisiting that area the next day would result in the destruction of the same number of animals that were lost in the area the day before. It is highly likely that these were the same individuals.

In New Zealand several aerial culling configurations, both with and without thermal, have been trialled. The configuration that has been tried and tested for over 50 years for visual shooting has the shooter in the front seat opposite the pilot. This configuration gives the shooter the best of everything: widest field of view, ease of access, balance of the aircraft, a good place to communicate with the pilot from, and the shooter can support the pilot with tail rotor clearance information on that side. This is important in the steep terrain of New Zealand where programs often occur in areas of ≥ 45° slope. The shooter also acts as an observer looking for obstacles along with the pilot.

For the addition of thermal, an operator was trialled in both back seat positions – either behind the pilot or behind the shooter. Placing the thermal operator in the back behind the shooter was the logical option. The operator provides essential support to the shooter to find new and escaping targets, to confirm identification, and to confirm death. Having the thermal operator behind the pilot gave little benefit to the shooter other than identifying additional targets. In the type of terrain and vegetation that the programs occur this just adds additional time to acquiring the animal due to the repositioning that is required. Having the thermal operator on the same side behind the shooter makes orientating the shooter easy and logical.

Recently (June 2021) a trial was undertaken which included a second visual shooter behind the pilot during a TAAC program. This second shooter was a local project manager and had knowledge of which parts of the area the effort should be concentrated. This was particularly important in the vast landscape of the South Island which couldn’t be covered effectively within the projects flight time constraints. The configuration maximised our target acquisition and used the flight hours to greatest effect.

Feedback from pilots who have flown both visual and TAAC aerial culling programs was that TAAC programs were safer than visual culling programs due to the height, speed and nature of the required flying. One pilot described it as “boring flying” compared to visual aerial culling (K. Jacobi 2021, 13 November). Animals are detected from farther away using the thermal imager, thereby allowing the crew to find the optimal angle of approach, and approach from a higher altitude to set up the shot which reduces disturbance to the animal. The average height of detection was > 60m AGL for both species. The average height of a distribution line power pole is < 9m AGL (AS/NZS 4065:2010 and AS/NZS4676:2000) in Australia and New Zealand. While there were instances where parts of the shoot were below average power pole/line height, these occurred in later stages of the shoot after the terrain and obstacles had already been assessed prior to pursuit. TAAC programs requires a much lower flight speed because it avoids the low-altitude, rapid flying necessary in visual programs to flush animals, (particularly pigs), from dense vegetation (e.g. tangled lignum, Muehlenbeckia florulenta).

Feedback from the shooter on the Kangaroo Island program was that welfare outcomes were improved. There appeared to be lower stress from less panic and fleeing compared to visual shoots where animals move quickly because the helicopter is low and close. The slow and high approach reduced the chances of animals scattering and targets being lost. Having the shooter and operator on the same side of the aircraft meant there were always two observers, and two sets of thermal equipment (imager and thermal scope or binoculars) to track an animal. Additionally, if there was more than one target individual, the thermal operator continued to monitor other individuals and the environment while the shooter focused on the target. This reduced the number of lost animals and decreased the time taken to re-acquire. The shooter felt that injured animals could not easily be lost in the TAAC configuration, and that animal loss was a consistent worry associated with visual-only culling. Both the shooter and the thermal operator verbally confirmed death before moving to the next target.

· Don’t come in too hot on animals. You can identify animals on the fly when the two observers have thermal and that speeds up the process. Come in high and slow at 300ft 30-40 knots, then when an animal is identified the pilot makes an approach while the shooter gets ready (picks up firearm and loads). If you come in too low the animals pick up speed, but if you stay higher, they move slower and it provides greater opportunity to get a good fast and accurate shot.

Summary

The improved performance of the latest thermal imagers has the potential to offer a simple, quick, cost effective control/ eradication tool to detect and dispatch deer (and other ungulates) through dense canopy. In August 2017 a trial was conducted to: (a) help assess the effectiveness of thermal imaging to detect and dispatch red deer on Five Finger Peninsula, Resolution Island, and (b) develop and optimise the way this technology is deployed. One hundred and fifty-five deer were located with the thermal imager and 138 were successfully dispatched over the trial.

We determined the biggest advances to effectively using thermal for targeting deer through the canopy were in CRM and our ability to work as a team. That just took time and the will to be better. We felt that the change in resolution from the Jenoptik to the Vayu imager gave a 40% increase in detection rates and the increased FOV made a significant difference to the thermal operator’s ability to track animals through the canopy. The addition of both the 75mm and the 50mm Theon scope to our suite of thermal tools enabled us to target deer that before would have been impossible to do. Even though the FLIR Breach only had 320x280 resolution, the small size and long battery life, together with a 19mm exit pupil having an extremely wide FOV, enabled the shooter to look in places and detect deer outside the FOV of the thermal operator.

The importance of ergonomics, reliability, and keeping things simple and intuitive, all stood out over this trial period. Simply put, if a piece of equipment we used or tested didn’t meet those criteria no matter how good you thought the idea was, it simply didn’t get used. Some of the less significant things that are mentioned in this report are not big milestones but are included to demonstrate that attention to detail has a cumulative effect on the outcome of any encounter.

Background

At 208 km² Resolution Island is the largest island in Fiordland, it is the country's fifth largest island and it is New Zealand’s largest rat free island with mice being the only rodent species present. Resolution Island is separated from the mainland of the South Island by Dusky Sound and Breaksea Sound. The island is roughly rectangular, with the exception of a long narrow peninsula on the west coast known as Five Fingers Peninsula (3000ha).

In 1891 Resolution Island (20,860 ha) was gazetted as the world’s first “Reserve” for the protection of native flora and fauna. Richard Henry, as curator of Resolution Island, saw the value of the then stoat-free island and over several years translocated 572 birds, mostly kiwi and kakapo, to the island sanctuary. This early restoration programme was in response to the extirpation of native birds on mainland New Zealand as recently introduced rodents, mustelids and cats spread across the country.

International Union for Conservation of Nature. guidelines define eradication as the complete removal of an alien invasive species (IUCN Guidelines for the Prevention of Biodiversity Loss caused by Alien Invasive Species, May 2000). The operational plans for Secretary and Resolution Islands define the term “eradication” as the complete removal of the resident stoat and deer population and the establishment of long-term control programmes to manage re-invasion. Neither Secretary and Resolution Islands technically meet the requirements for eradication as the risk of re-invasion is modest for Secretary and high for Resolution Island.

Both Secretary and Resolution Islands lie close enough to either the mainland or other islands that they act as steppingstones for pest re-invasion. For this reason, pest management is also carried out on islands adjacent to them. Operational objectives for these eradication programmes are outlined in separate plans (see Golding et. al. 2006; Brown et. al. 2006; Crouchley et. al. 2007; McMurtrie et. al. 2007; Crouchley and Edge, 2009).

The eradication strategy for deer

The deer eradication project for Resolution Island started in mid-2009 and ran until 2013 removing 1058 over that period. As a result of ground hunting introduced as the primary eradication tool at the start of the project and using the “one hunter per one block approach” it was apparent that the remnant deer population was becoming increasingly wary. Comments from hunters indicated an increasingly higher percentage of deer encountered were not able to be dispatched. The remnant deer population on Resolution at that point can still be measured in the multiple hundreds. Despite this, the population overall was reduced to a level where the understorey coastal faces responded to the reduction of browsing. In 2013 it became evident that unless there was a radical change in the eradication strategy, and significant increase in funding and resources, there was an overwhelming likelihood that the eradication would have failed, beaten by the pool of ground hunting resistant deer and the regenerating understorey. At that point the decision was made to put the eradication on hold and the funding was transferred to Secretary Island to ramp up the removal of the last few deer on the Island (by mid-2015 the last deer on Secretary had been despatched). Since then, ongoing deer control on Resolution has consisted of a limited aerial shooting program primarily aimed at limiting browse damage in alpine areas.

Aerial shooting is a very cost-effective tool but is generally most effective in open country, clearings, slips and riverbeds where deer are visible. Open areas in New Zealand’s offshore islands generally cover only a small part of the land mass which means that only the deer that have home ranges in these more open areas can be put at risk. While it is possible to get a very quick population reduction in these areas with aerial shooting, there are often large areas where aerial shooting is not effective. The only other option to reduce the deer population in the areas with dense canopy cover has been to use ground hunting dogs early in the eradication. However, using hunters with indicator dogs when the deer population is still high has led to a pool of “hunter resistant” animals that are extremely wary and difficult to kill (as found during both the Secretary Island and Resolution Island eradications).

The loss of funding that put the eradication of deer from Resolution Island on hold gave opportunity to investigate innovative ways to reduce the base population to the point hunters with indicator/ bailing dogs (which have the highest detection probability) are most effective. This is when the deer population is at very low densities (i.e less than 5% of the population remaining). It is also evident that to successfully reduce deer to zero detectable density on an island the size of Resolution quickly, it is essential to keep the remnant population as naive as possible throughout the eradication. To accomplish this, it is essential to have tools in our eradication toolbox that are effective at each population density (high, medium, low, zero detectable density) and which are applied in the correct sequence as animal numbers decline.

Before we embark on largescale deer eradications such as those for Resolution Island we need to be able to do the eradication; (i) quickly, (ii) cost effectively, and (iii) without making the remnant population wary to ground hunting. The only two options that could be considered to achieve this were (i) cutting the dense vegetation and poisoning with 1080 Gel [1], or (ii) undertaking aerial shooting with thermal detection. The use of poison was the least palatable option for recreational hunters, other park users and the general public, so utilising thermal imaging technology was investigated.

The development of thermal-assisted aerial culling (TAAC)

Aerial thermal hunting is in its infancy in New Zealand for wild animal management. At this point only a few people have invested in using this technology in New Zealand, fewer still have the skills to use it to its full potential. During this development work we learned that to consistently detect ungulates through the canopy is no easy feat, to convert detection into a lethal result is even more difficult.

Grant Halverson from Airborne Technologies (Whangarei), has been operating thermal imagers for 20+ years all around the world on fires, powerline, wildlife and plant surveys. He also has an extensive professional hunting background. In August 2017 we started the process of learning how to detect and despatch deer under the canopy. Our starting point was based on the work that Airborne technologies had previously done with David Paine (Environment Bay of Plenty) looking for goats in the Motu River (North Island). From14^th -28^th September 2017 an initial trial to test the efficacy of thermal assisted aerial culling through dense forest canopy was conducted. The trial involved nine days of aerial hunting. A total 571kms, flown in transects, effectively covered approximately 50% of the Five Fingers Peninsula at approximately 300m intervals. Seventy deer[2] were detected of which thirty-five were successfully dispatched (See Appendix 1 & 2). At the start of 2018 we received additional funding which enabled us to continue to develop thermal to detect and dispatch deer through a closed canopy. The use of thermal equipment to target deer through the canopy is still in the development phase, but it is showing that it could be the most significant leap forward in ungulate control in the last 40 years.

Although thermal imagers have been around for some time now, it has taken a while for technology that was developed primarily for military applications to be made available to the civilian market. ITAR (International Traffic in Arms Regulations) imposed by the US Government highly regulate the export of this evolution of thermal equipment. Unless you are the military or the police of an approved country, it is almost impossible to get this equipment out of the US legally. Airborne Technologies received ITAR approval to export a suitable imager to Australia, and then applied for and received approval to export it for use in New Zealand. The decision to buy this thermal imager was based on a combination of specification, opportunistic availability, support and cost.

Equipment

The thermal imager that was used during the first trial was a Jenoptik 1024- HD (Figure 1, Table 1). The thermal detector and chipset that is incorporated into the Jenoptik is a generation behind the latest technology and had been around for at least five years. Nevertheless, the imager was able to detect deer consistently through reasonably dense canopy. The imager was integrated into a handheld frame that was suspended by an ‘Easyrig’ that was attached to the thermal operator. A powerful laser pointer (bore sighted to the centre of the imagers field of view) and a 4K video camera were also integrated into the frame.

Prior to the start of the second phase of the project, Airborne Technologies took delivery of a new thermal imager (Vayu HD) (Figure 1, Table 1) from the US and there were several significant improvements to the role equipment used on this project which made an estimated 40% increase in our ability to detect and dispatch deer. Since this was the first Vayu off the production line, there were a few user interface and design issues that needed to be resolved. The Vayu was designed to operate via a laptop in a stationary environment so getting it to function in a dynamic environment (passenger compartment of a H369D) with constantly changing environmental conditions proved to be a challenge. This required the use of a hand controller and significant development of the operating software in collaboration with the manufacturer in the US. Because the imager was so sensitive the movements in adjusters had to be calibrated precisely so that the they reacted quickly enough to the changing environmental conditions but were not so aggressive so that the caused a massive over or undershoot of the optimum setting. To aid in effectively operating the imager without the need for a laptop, we also worked with the manufacturer to develop a hand controller. The layout of the hand controller was designed here in NZ and built into a standard commercially available controller housing in the US. While this worked, the size of the housing did compromise its positioning in the handheld frame and as a result the ergonomics of using the controller were also compromised. This was identified early on and subsequently improved significantly but was still wasn’t integrated into the imager system as well it should be. We also worked through other issues such as the angle of cable fittings coming out of the back of the imager housing that were problematic for our application (they needed to be right angle fittings instead of straight), also cable connectors on the back of the Vayu were not “Milspec[3]” and caused a few reliability problems.