Rat Eradication

Guidelines for the Eradication of Rats From Islands Within the Falklands Group

These guidelines outline the suggested standard procedures for eradication of rats from islands. They follow tried and tested methods that have repeatedly proven successful in New Zealand and elsewhere around the world. However, careful planning and implementation, and considerable commitment are required to ensure the maximum chance of success of any such operation. One small mistake can easily spell total failure of the operation, and all the effort and expense would be for nothing. It is strongly recommended that any proposals be discussed with an appropriate expert before an eradication attempt is made. Furthermore, the operation leader/supervisor should have had previous experience and success in rat eradication operations.

A most important consideration is the assessment of whether it is even worthwhile attempting eradication of rats from a particular island. If the island is too close to another rat-inhabited site, then natural re-invasion of rats by swimming is very likely to occur some time in the future and therefore ‘eradication’ would only achieve temporary gains. Also, if there are large numbers of visits that create the risk of human-assisted re-introduction then the value of removing rats is questionable - removing rats is only half the equation, keeping them off is just as important, if not more so.

Prior to an eradication attempt, maximising the amount of knowledge of each island’s rat population, in terms of which species is present, the density, seasonal behaviour, and favoured habitats will increase the chances of success. Although most islands are similar in flora and fauna, there may be unusual circumstances on individual islands that could affect the operation. A careful inspection of the island by someone familiar with rodent sign and rodent ecology will be beneficial in terms of developing an appropriate eradication plan.

Eradication of rats from islands must not be considered a part-time effort. Never underestimate the time, budget or effort required. An ‘eradication mentality’ must be developed, whereby to provide the greatest chance of success the field workers must be wholly focussed on the job at hand, and must have the logistical backup to ensure the work proceeds without interruption until its conclusion. Once an operation has begun, it must take priority over all other work, and this should be taken into account during planning. Allocate staff solely to the eradication project – do not have them trying to do other tasks at the same time.

It is important, if not vital, that all those involved in the eradication are totally committed and enthusiastic about the task. A single person lacking motivation or the necessary field skills could spell disaster for the operation if they are not paying sufficient attention to the accuracy and thoroughness of their work.

It is important that sufficient funds are available to finance the project through to its conclusion, rather than to have to pull out due to delays or additional unforeseen costs. It is easier to trim a budget than to add to it.

Logistics are an important consideration, especially if planning to stay on an island for any length of time. Tents and camping equipment, water, food, sanitation, poison bait supplies, transport and communications all need to be thought through. Contingencies need to be considered in all situations, in case the operation does not work entirely to plan.

Don’t forget safety issues, which are critical on the often-isolated islands. Handle the poison baits with care, and wash hands thoroughly after handling baits. Follow the safety instructions on the poison bait containers. Have a reliable method of contact with the support crew off the island e.g. a 2M radio, plus a fallback option (i.e. systems in place if radio contact is lost). Take a First Aid Kit and preferably have at least one person trained in First Aid. Take lifejackets, safety gear and equipment spares whenever on boats. Take extreme care around steep areas and slippery coastal rocks. Ensure you have sufficient and effective wet- or cold-weather clothing.

There are two methods suitable for eradicating rats from islands that are currently applicable to the Falklands. Both methods have proven very successful. If carried out correctly, either technique offers similar prospects of success. The decision as to which technique is used depends on the circumstances for each individual island and in part the preferences of those involved.

The following table provides a summary of the relative advantages and disadvantages of each method.

Helicopter or aerial spreading is not feasible in the Falklands at present due to lack of suitable aircraft, appropriate spreader devices, and experienced pilots. It is strongly urged that only aircraft and machinery purposely set up for aerial bait applications are used, and that only pilots familiar with such operations are employed. Helicopter spreading may become a realistic option in the future. It generally has the same advantages and disadvantages as hand-broadcasting, but can be more accurate due to flying GPS lines, and ability to thoroughly cover very steep ground.

Following sections in this document outline the details of the bait station and hand-broadcasting methods.

To maximise the prospect of success, use only baits that have been proven effective in rodent eradication, not just proven in rodent control. The choices are currently limited to a New Zealand-manufactured pelleted bait ("PestOff 20R") suitable for either bait station or hand-broadcasting, or a waxy block (e.g. "Talon 50WB", also available from NZ) best suited for bait station operations only.

Any other type or brand of bait (including "Ratak") has yet to be effectively tested for eradication work and will add a higher level of risk to any operation. They may be wholly suitable, but as they have not been trialled in an eradication scenario this cannot be absolutely assured. Again, using 'tried and true' methods and materials will reduce the risks and largely eliminate the 'unknown' factor. Use of unproven bait types should only be used in ‘trial’ eradication projects where the outcome is not critical, and generally only if it presents some distinct advantage or potential advantage over the proven baits.

When calculating and ordering bait, always overestimate the quantities of baits required, to allow for various contingencies, e.g. mis-calculation of the treatment area, or accidental loss or damage of the bait (e.g. through spillage, or moisture damage).

Bait stations should be set out on a grid of 50 metres by 50 metres in dense tussac, and in coastal areas, or where seabird concentrations are high. Grazed or open heath-type areas may require lower density of stations, e.g. a 100m x 50m grid pattern (i.e. lines 100m apart, but stations on each line 50m apart), and certainly no larger than 100m x 100m. This equates to a density of stations of between 1 and 4 per hectare. Use a tape measure, or a measured length of rope or string to set out the grid of stations - do not attempt to 'guess' the distances. Place the bait stations on flattish ground, so that the ends of the station are level and water (from rainfall) will not run through. Bait stations do not need to be precisely at the prescribed distance - if possible pick the best-looking place within about 5m radius. Preferably there should be some cover close by (e.g. put the station under the edge of a tussac) as rats tend to stay near overhead cover. Good bait station placement will mean rats will feel more comfortable about investigating and entering an unfamiliar structure.

Bait stations should be placed out by walking accurate compass- or GPS-derived lines. All lines should be parallel, to ensure coverage is even and methodical. All areas of the island, except sheer vertical cliffs need to be covered. If areas are inaccessible for bait stations bait will need to be hand-broadcast over that area - do not ignore the beaches or rock jumbles below cliffs, or steep faces or large caves.

Do some checks after bait stations are placed out, to ensure they are not too far apart (within 5-10m of intended distances should be fine). For ease of relocation, mark all station locations well with waterproof coloured tape or similar if in dense tussac or other tall vegetation. Such markers are easily removed through wind action, or by Johnny rooks, so if possible tie a knot in tussac leaves around the marker to hold them in place.

Bait stations should be put in place at least two weeks prior to the commencement of baiting for rats to familiarise themselves with the stations and to reduce neophobia (avoidance of unfamiliar structures) prior to any baits being placed out.

Bait stations should be of a plastic pipe or similar, of at least 100mm diameter. They should be at least 600mm in length. Ideally they should have a hole in the top with a close-fitting lid, to enable easy checking and replacement of bait. The best stations are corrugated drainage pipes with corrugations on the inside which act to prevent baits sliding out stations, and water entering and wetting the poison. Falkland Conservation has examples of bait stations. Each station should be firmly held down by at least 3 hoops of fencing wire pushed firmly into the ground. These need to be very secure where stock or sealions are present, to prevent disturbance of stations and/or spillage of bait. All stations should be individually numbered so that accurate records of bait take can be kept - write on them in permanent marker and/or use a plastic tag tied to the station or the wire pegs or securely placed nearby. (NB. Take a few spare bait stations to hold on-site during the operation, as some replacements may be necessary)

It is estimated that one reasonably fit person will be able to check 100 bait stations per day, if in dense tussac, and up to 200 stations if in more open areas depending on the spacing and topography. It is strongly advised that these people (and relieving staff) camp on the island for the duration of operations. The continued availability of bait in the stations is critical to the success of the operation, and delays in getting back to the island through bad weather or other factors place the operation at a higher level of risk of failure.

It is recommended that if PestOff 20R bait is used, then at least ten pellets of bait are placed out in each bait station. If using a waxy block bait then 2-3 baits will normally be sufficient. These should be checked daily and replenished as necessary.

Bait take from all stations should be accurately recorded. This provides a record of activity at each station. It also provides an information base for analysis later on, to estimate overall populations and patterns of behaviour, useful and applicable to future operations on similar islands.

Activity at bait stations may take several days and up to a week to begin, before rats overcome their wariness. However, once bait take starts at any station it usually continues uninterrupted for a period of 3-5 days (this is the expected time it takes rats to succumb to the toxin). Norway rats will tend to take all the bait offered, and 'cache' what they cannot eat. Typically, bait take will not occur at all stations at the same time. Rather, it will show a pattern where activity starts at one or more locations and will ‘spread’ from there, slowly sweeping across the island.

The time between start of poisoning and the last bait take (equating hopefully to the death of the last rat) should be between 7 and 21 days. The actual length of the operation relates to various factors such as densities of the rats, how quickly they started taking the bait, and the density of the bait stations. To speed the operation, one or two baits can be placed inside the entrance of rat holes so that rats get a ‘taste’ for the baits and to encourage them to start to take the bait from stations. This can be done at the beginning of the operation or in areas where activity at bait stations is slow to start. If this technique is used, record the details.

Maintain the bait stations in position for at least several weeks after the last evidence of bait take by rats, and periodically check the baits. After this time without any further sign of rats, the stations can be removed. However, ideally and if possible, leave a small quantity of bait (e.g. a single waxy block) in each station over the summer period. This bait has the duel role of acting as an additional monitoring tool and potentially killing any rats that may have survived.

We suggest the maximum size of an island with dense tussac vegetation that can be treated by this method, without expert guidance, is about 30 hectares. This is somewhat dependent on its terrain and shape, as it is easier to ensure full and accurate coverage on a long, thin flat island than a circular-shaped or high one. As experience grows then larger islands can be targeted. Islands with low vegetation will be physically easier to spread bait on, so larger islands in this category may be considered.

Predetermine the sowing rate for the baits. We suggest that on tussac islands a baiting density of 5-6 kilograms per hectare is sufficient, but this may need to be increased to 8-9 kilograms per hectare if an early season eradication is planned or if rat numbers are unusually high. More rats are still alive in the late autumn/early winter compared with in late winter/early spring, meaning more bait is required to ensure toxic doses to all rats.

Always take more bait than you anticipate using, to ensure that in all reasonable eventualities there is enough bait to complete the task. Running out of bait before total coverage of the island is achieved would be a disaster, almost certainly leading to failure of the operation and requiring a repeat attempt in future. A 10-20% contingency of extra bait is the minimum recommended.

If at all possible, avoid times of heavy rain, continual drizzle, or when the ground is unusually wet. Baits will be damaged by excess moisture and will begin to break down, possibly becoming less palatable to rats.

We suggest the baits get spread at intervals of 25m, along parallel lines no more than 50m apart, over the whole island. This equates to 8 spreads per hectare, or 600-750 grams per stop. A 200g measuring scoop means that 3 casts of bait can be made to ensure good spread at each stop point. Bait should be cast as widely as possible to either side, plus trickle a few baits around on the ground closer in.

If possible, marker flags placed out beforehand in a 50m-grid system will provide a visual guide to facilitate accurate coverage. If this is not possible or practical, then mark 'start points' 50m apart along one coast and from these, head across the island on parallel compass lines, using measuring ropes to get accurate spacings for bait spread. Mark the end point of the run on the other coast for later checking, to confirm the lines (and therefore coverage) have been accurate. If possible, it is strongly recommended to have 2 or more teams of two people working across the island parallel to each other, to reduce potential errors of navigation.

It is recommended extra bait gets cast inland (onto vegetated areas) at a similar rate as above (600-750g per 25m) from all walkable parts of the coastline, as it is considered that highest rat densities occur close to the coast.

(Some smaller islands, of less than 5 ha, can be done simply by distributing bait around the coast at 25m intervals, with just one or two 'eye-judged' lines through the middle to cover any inland areas. However, we would urge care with such methods though as it may add potential for errors in coverage, and should only be done on very small islands)

All baits should preferably be spread on a single day, but can if necessary be spread over 2-3 consecutive days. If this is necessary, (e.g. due to the size of the island) work methodically from one end to the other on a ‘rolling front’, and ensure that the edge of the baiting front is very clearly marked at every stoppage. Expert advice should be sought before any hand-broadcasting operation is planned that may take more than 2-3 days to complete.

If in doubt in any area, add more bait. It is recommended someone with experience of rat ecology (i.e. favoured habitats), walks as much of the island as possible after the general broadcast. Their task would be to place extra baits out as deemed necessary in any area with especially dense rat sign or that was apparently not covered well in the broadcasting. Typically, these may be ledges, caves or steep faces on the coast of the island, or isolated rock stacks/islets. The immediate margins of streams, drains and ponds may also require special attention.

If unsure of the effectiveness of the operation, e.g. through doubts about coverage or baits being damaged by heavy rain, a repeat baiting can occur, though it is strongly advocated that strenuous efforts are made to guarantee an initial comprehensive coverage.

In general, rat eradications should avoid times when large concentrations of breeding seabirds are present, due to the fact that such concentrations of birds provide large quantities of alternative food resources, and rats may be less inclined to take the bait, and may occupy much smaller territories. Rat breeding may also be peaking at this time, and there is a danger that breeding animals may be more selective in their choice of foods, and that nearly-weaned young may not have access to the poison bait and may survive an operation.

It is therefore recommended that eradications only occur in the period between late May and early October, but this is dependent on the species of colonial nesting birds present on the respective island. If contemplating eradication early within this period, ensure the birds have departed and obvious food sources (straggling chicks, dead chick carcasses) have all but disappeared.

It is believed rat numbers on islands in the Falklands typically peak in late summer, drop significantly through autumn and winter, and are lowest by Sept/October, just prior to the onset of breeding of both seabirds and of the rats themselves.

Rat eradications may be possible in other months (indeed, some have been achieved successfully in summer months), but they probably have a higher risk factor. Unless there is a compelling reason to undertake an eradication in the 'summer' period, we recommend that this time of year should be avoided.

Both rat eradication techniques present risks to other wildlife, either by direct consumption of baits, or through 'secondary poisoning' via consumption of rats or other wildlife that have themselves consumed toxic baits.

Most rats will die in their burrows, inaccessible to scavengers. However, a small percentage may die above ground, or may be preyed upon while in a weakened state due to the effects of the poison. Scavengers and predators may therefore be at risk of secondary poisoning. These may include turkey vulture, short-eared owl, barn owl, red-backed hawk, caracaras and skua.

Some birds may feed upon the poison baits directly. Birds most likely to do so are the opportunistic or generalist feeders such as seagulls and some ducks. Small songbirds (passerines) may also take bait.

In the operations carried out in the Falkland Islands to date, there has been little or no demonstrable effect on bird species, but do not accept this as a 'given' for any operation. There may be undesired effects and careful planning and implementation is necessary to mitigate these as far as possible. It maybe appropriate to use bait stations rather than hand application in situations where primary poisoning risks to non-target species are known to be high. Where the potential risk is to predators or scavengers through secondary poisoning the choice of technique is not significant. Where there are specific concerns a non-toxic bait trial (using bio-markers) prior to the eradication operation can be used to test for non target bait take and to assess the potential impacts.

However, be prepared in any operation for the potential for some loss of individual birds. Nevertheless, once the rats are removed, an eradication will have long-term benefits for native wildlife. The loss of a few individuals may be an unfortunate but unavoidable consequence in order to make long-term gains for the populations as a whole.

If birds are suspected of being poisoned, assays can be done to determine level of toxins in their system, particularly in the liver. Store the whole bird, or dissect and remove portions of the liver. Freeze the specimens as quickly as possible. (We need to establish how and where to send samples for assay – Falklands Medical Officer of Health, or vets may know)

For either hand-broadcast or bait station operations, attention needs to be paid to the following:

Before you can say you have been successful, you will need to carry out some monitoring to check for any evidence of surviving rats. Standard procedure in New Zealand is that you must allow two summer seasons to pass without any further sign of rats before you can be absolutely sure of success and ‘officially’ declare the island rat-free. This is because evidence of just one or two surviving rats will be extremely difficult to find. However, if any rats do survive they will breed rapidly and sign will become increasingly obvious after one breeding season and the population will be close to normal levels within two.

It is recommended that monitoring occurs in late autumn, when rat numbers (if they have survived) should be at their peak and food is becoming short.

snap-traps (with covers to prevent bird disturbance) baited preferably with fresh meat/fat scraps (e.g. mutton bellyflaps), or fresh fish. Rubber bands or string can hold the bait firmly on the trap plate, reducing the potential for loss of bait without a capture. Other suitable baits include sardines, or a peanut butter and rolled oat mix. Traps should be checked daily, and baits replaced as necessary or if they go ‘off’.

gnaw sticks (dressed un-treated timber sticks of about 200mm length and 15-20mm square, about soaked for at least two days in peanut oil). These are pushed about one-third of the way into the ground, in a vertical position. Rats will find the smell and taste highly attractive and will chew on the stick, leaving distinctive ‘gnawing’ sign. These are quite effective and will be safe to leave on islands for considerable periods between checks.

other 'passive' baits, such as slices of butter (1.5 cm thick), white or milk chocolate, secured to the ground by a nail or wire peg. These should be under covers or out of the sun (e.g. under tussac). Prior to undertaking a check recommend sample baits are placed out in areas with rats so that the monitoring team can familiarise themselves with what rat sign looks like compared with bird or insect feeding. These indicators need to be checked regularly, and replaced and/or sign cleared off at each check. These foods are known to be highly attractive to rats, and they will leave diagnostic gnaw marks on any food remaining.

tracking tunnels (available from New Zealand) – tracking tunnels record the footprints of any animal running through the tunnel on special ink-treated paper on the bottom of the tunnel. The tunnels can have a bait inside as an added attractant. Often rats are less wary of an open tracking tunnel than one with a trap in it.

looking for field sign - such as droppings (particularly under rocky coastal ledges or under sheltered tussac), or footprints in soft mud or peat.

observing rat-vulnerable seabird species (e.g. sooty shearwater or South American terns) - there may be evidence of chicks that have been preyed on by rats.

The increase in numbers of small song-birds, and the possible appearance of such species as tussac birds in good numbers are also possible indicators that rats have been successfully removed.

For all of the options the more of the island that is covered the more confident you can be of the results. Traps, gnaw-sticks or indicator baits should be placed out at regular intervals over as much of the island as practicable, ideally at spacings of no more than 50m x 50m if monitoring just for rats, and 25m x 25m for mice. If time or resources (e.g. number of traps) is limited, focus on the prime rat habitats (e.g. the coastline and dense tussac).

Comprehensive and accurate records will be invaluable to planning future operations. If, for any reason, an eradication fails such records may also provide some evidence of why it may have failed - again of considerable importance for future operations.

If using the bait station technique record the bait take from each individual bait station every time you check the station.

It is urged that a report is completed after each eradication operation and copies stored in relevant locations. This can be a brief summary of little more than a page, or much longer if desired, covering the details as listed above. Such reports will be of considerable value for historical value and for assisting to refine eradication techniques.

It would be extremely beneficial to clearly demonstrate the long-term gains made by eradicating rats from islands. While we can reasonably assume many conservation benefits will accrue, they should if at all possible be proven through collection of accurate data and observations.

Documenting any positive responses of wildlife populations after an eradication will prove to people unfamiliar with eradication (or to any doubters) the value of such work, and will assist in the general support for any future operations.

Therefore, it is recommended as much time as possible is spent recording use of the island by birds and other wildlife prior to the eradication attempt, and similar methods used afterwards to measure any changes

Vegetation may also be affected by rats in subtle ways, so recording plant species and their general abundance and distribution may also be beneficial.

Little information exists on the invertebrate fauna of the Falklands but invertebrates are likely to benefit greatly from rat eradication. Monitoring of pre-eradication invertebrate populations and their post-eradication responses is highly desirable.

Getting rid of rats is only the first step in restoring the natural conditions on the island. Preventing them ever getting back on is a natural follow-on measure. Rats getting back to an island through inadvertent human assistance seems an unlikely possibility but it can and does happen - the consequences of which means you may lose everything you have worked so hard to gain. A few basic precautions and a general awareness of the issues will go a long way to preventing rats (or mice) getting to the islands again.

If you succeed in eradicating rats from an island in the Falklands you will have made a significant and hopefully lasting contribution to the protection of the unique natural heritage of the islands.

The successful eradication of rats from islands is a relatively new concept that in the past two decades has been successfully employed to make some outstanding conservation gains worldwide. However, the learning process is still continuing, and it is urged that accurate records are kept of all operations so that over time a comprehensive picture can be achieved and further improvements made to the techniques.

Eradication work is relatively straightforward, but never underestimate the commitment required to undertake such an operation successfully. Attempting to cut corners or a casual approach to the operation will considerably heighten the risk of failure.

If operations fail, or even worse cause serious damage to native wildlife, it will have negative repercussions on the way eradication projects are viewed, with a possible consequence that funding and general support will wane, to the detriment of long term conservation objectives.

If in doubt, or you have any questions, just ask! Experienced advice is only a phone-call or e-mail away.

Key points for successful eradications are:

Buy-in of all stakeholders is very desirable, if not essential
An ongoing commitment of staff and resources is essential
Know your target animal and ensure the methods put all individuals in the population at risk
Only proven methods should be used
Minimise non-target risks
Result monitoring allows the eradication of rats to be tracked
Monitoring the conservation benefits provides valuable information to sustain support for eradication operations
Ensure re-invasion does not occur, through implementation of stringent quarantine procedures
The right personnel, with good team-work and field skills, and a positive attitude are essential ingredients in effective eradication operations.

Animal Control Products
Private Bag 3018
Wanganui, New Zealand
Ph. 00 64 6 344 5302,
Fax 00 64 6 344 2260
E-mail: pestoff@clear.net.nz

Gerry Williams
Pacific Basin Exports Ltd.
PO Box 612,
Auckland, New Zealand
Ph 0064 9 3734602, Fax 0064 3 3096640
E-mail: pbex@ihug.co.nz

Derek Brown
4 Main Road,
Havelock, New Zealand
Ph 0064 3 5742 422,
Fax 0064 3 5742 544
E-mail: derek.brown@xtra.co.nz

Ian McFadden
Department of Conservation
PO Box 10-420
Wellington, New Zealand
E-mail: imcfadden@doc.govt.nz

Falklands Conservation