A review of potential techniques for identifying individual stoats (Mustela erminea) visiting control or monitoring stations

1. What they did

Stoats are a major threat to some endemic birds in New Zealand, and controlling them is long and costly. To place traps, bait stations and tracking tunnels just densely enough that every stoat meets one, managers need to know how often individual stoats visit a station. This review grew out of a study on that question. The authors reviewed ways to identify individual stoats, and similar small carnivores, from their visits to bait stations or monitoring devices, focusing on “remote” methods rather than direct observation such as ear tags.

They set out seven criteria for a good marking method: it must tell individuals apart, not harm behaviour or fitness, cause minimal pain and stress, last for the study, be easy to apply in the field, be affordable, and be easy to read. They then compared four groups of techniques:

• Footprint tracking in tracking tunnels and on sooted plates.
• Genetic markers from hair, skin or faeces.
• Electronic methods: passive integrated transponder (PIT) tags and radio-tracking data loggers.
• Chemical and dye markers placed in bait or applied to the fur.

2. What they found

Footprint tracking

• Tracking devices are the cheapest and most practical way to show that a species is present, but little research had been done on recognising individuals from their prints.
• Natural tracks in snow, fine soil or sand are unlikely to show enough fine detail to tell individual small carnivores apart.
• Ink-print tracking tunnels are the method of choice in New Zealand. Their prints are solid, with no fine detail inside the pads, mainly because coarse brown paper is used. Glossy white paper would give better prints but may put some animals off. Prints also smudge in wet weather.
• Individual fishers have been identified from pad scar patterns on sooted plates, and from digital images of tracking-paper prints; individual black-footed ferrets have been identified from print characteristics. The authors say chance identifications like these are not enough: a study needs a deliberate way of marking each animal’s pads.
• Toe-clipping has been used on rodents and stoats. A study that toe-clipped feral ferrets found it neither reliable nor worthwhile: it took great effort, stressed the animals, and the prints could not be reliably identified, partly because partial prints from clipped animals could not be told from poor prints of unclipped ones.
• The authors’ own pilot study of small markers glued to the pads failed: the markers were lost on the first night.
• Often there are too few good prints to identify animals. Other species can also cover stoat prints, take the bait first, or leave scent that may deter stoats.

Genetic markers

• Microsatellite DNA can identify individuals without capture, and enough markers were known for stoats.
• Hair caught by brushes, wire springs, sticky traps or an adhesive rubber band (used on stoats in New Zealand beech forest) is the most practical sample. At least 3 to 5 hairs with roots are needed.
• Collecting stoat faeces would need live-trapping, and carnivore faeces may be contaminated by prey DNA.
• The main drawback is cost, plus contamination and degradation of samples.

Electronic methods

• PIT tags (about 2 mm by 10 to 11 mm) are reliable and permanent, with few adverse reactions and low tag loss in most studies. But readers must be within about 10 cm of the tag, and costs are prohibitive for large-scale monitoring. Loop antennae around tunnel entrances can record visits automatically.
• Data loggers can record radio-collared stoats within about 30 m, but would also be very expensive, as stoats range so widely that each station would need its own equipment.

Chemical and dye markers

• Marked bait shows which animals in a population have eaten bait from a station; animals are trapped or observed later to check for the marker. In Table 1 these markers detect bait uptake but do not identify individuals.
• Iophenoxic acid raises blood iodine for several weeks; it was found in wild stoats up to 27 days after baiting. A new detection method needs far smaller blood samples, but the equipment is expensive.
• Rhodamine B shows up as fluorescent bands in whiskers. It was detected in all 11 captive stoats dosed; repeated doses gave separate bands, and wild stoats showed two to eight bands, suggesting several bait visits.
• Radioactive markers are tightly restricted and can damage tissue; tetracycline is an antibiotic controlled by law in many countries.
• External dyes and paints can last days to over a month on fur, but an applicator for tunnels would still need to be developed.

3. What they concluded

• Few of the techniques were ready to use without further testing.
• PIT tags are probably the most reliable way to identify individuals but by far the most expensive. With enough funding, coil readers for tunnels and automatic recorders should be a research priority.
• DNA is also likely to be too expensive for many large programmes, but hair-capture devices could be placed in tunnels routinely, with samples stored for later analysis.
• If funds are limited, priority should go to studying the quality and fine detail of prints from sooted plates and ink-print devices. Tunnels should be lengthened to collect more prints, and entrances made to keep out larger animals.
• Bait markers may be the most suitable way to tell which animals used a particular bait station. Rhodamine B’s reliability, simple sampling and lower cost probably make it the preferred option.
• A practical and humane way to recognise individual stoats would help managers monitor and control them more effectively.

4. How tracking was or was not used

This is a review, not a field study. Footprint tracking is one of the four groups of techniques it assesses: natural tracks in snow, soil or sand; ink-print tracking tunnels and sooted plates; print measurement keys for identifying New Zealand small mammals to species; and identification of individuals from pad scars, print characteristics or deliberate pad marking (toe-clipping, pad markers). It also mentions a study that followed toe-clipped stoats by snow-tracking.

6. Other technologies, skills or innovations

• Passive integrated transponder (PIT) tags with loop antennae and automatic data recording.
• Radio receivers and data loggers recording radio-collared animals.
• Microsatellite DNA analysis using polymerase chain reaction (PCR), with hair snares and sticky traps.
• Bait markers detected by laboratory analysis (iophenoxic acid, using high performance liquid chromatography) or fluorescence microscopy (rhodamine B in whiskers).

7. Implications for conservation (from the paper)

• Stoats are widespread, have keen smell and hearing, climb and swim, and are hard to control: they can be hard to trap in some seasons and places, disperse quickly over long distances, and increase rapidly when food is plentiful.
• Knowing how often individual stoats visit stations would allow traps, bait stations and tracking tunnels to be spaced efficiently, protecting the largest area with the funds available. Too many stations wastes money; too few could put a control operation at risk.