Responses of Stoats to Scent Lures in Tracking Tunnels

1. What they did

Stoats are a serious threat to hole-nesting and flightless birds in New Zealand’s forests, so conservation managers need good ways to measure changes in stoat numbers. The Department of Conservation was developing a standard footprint tracking tunnel method, but stoats often enter tunnels too rarely for the results to be statistically useful. Meat baits help but spoil quickly. The authors tested whether a long-lasting lure carrying a social scent, the stoats’ own anal sac secretions, would draw more stoats into tunnels.

• Making the lures. They removed the anal sacs from 10 female and 17 male stoats that had been kill-trapped and frozen. They extracted the secretions, keeping the sexes separate, and mixed them (about 1% by weight) into small cubes of a slow-release, casein-based material called Albert®.
• Setting the tunnels. They placed 41 tracking tunnels about 200 m apart along an 8 km transect on the shore of Lake Waikaremoana. Each tunnel held an ink pad and tracking paper that recorded the footprints of animals passing through. Every second tunnel got a lure, alternating male and female scent.
• Checking them. They checked the tunnels after a week, then every 3 to 4 weeks, identifying footprints against a reference collection of prints from animals caught locally.
• Testing a blank. From mid March to mid May 1998 they swapped the scented lures for blank Albert® cubes with no stoat scent, to see whether the material itself attracted stoats.

2. What they found

Stoats

• Stoat prints were recorded 30 times, in 19 of the 41 tunnels.
• With the scented lures, stoats left prints 19 times in lured tunnels and only 3 times in unlured ones, a significant difference (0.94 versus 0.15 per 100 tracking nights).
• Female and male scent worked about equally well (11 and 8 records).
• Blank lures did not attract stoats: 5 records with blank lures and 3 without (0.38 and 0.24 per 100 tracking nights).
• Stoat tracking rates fell markedly after about seven weeks, by the end of January.
• One stoat left a scat on top of a lured tunnel.

The lures

• More than 90% of the lures stayed intact for about seven weeks. The rest softened and collapsed after 4 to 6 weeks, or disappeared.
• Dried-out lures could be re-activated by squeezing them, and lures in shady tunnels seemed to keep their scent longer than those in full sun.

Other animals

• Rats left prints 19 times and mice 73 times. The lures had no effect on either: rodents were neither put off by stoat scent nor drawn to the edible casein material.
• While the scented lures were out, all but one of the 15 rat prints came in the first month, while mouse prints appeared in every period except the first.
• Possums disturbed the tunnels 31 times, tipping them over or pulling out the trays, with no preference for lured or unlured tunnels.

Trap trial (unpublished data)

In a separate trial on a heavily trapped stoat population, only 3 of 11 stoats caught were in traps with the scent lures added to egg bait.

3. What they concluded

• Lures made from stoat anal sac secretions increased stoat visits to tracking tunnels and could be useful for indexing stoat populations. This contrasts with earlier synthetic lures, which failed for stoats.
• Equal success of male and female scent makes sense, since both sexes scent mark their territories with anal sac secretions.
• It was the stoat scent, not the casein material, that attracted stoats, although tracking rates were so low that differences between blank lures and no lure would have been hard to detect. The trials should be repeated with more tunnels, or where stoats are more numerous.
• The fall in visits after seven weeks may mean the lures ran out of scent, but stoats may also have lost interest, or stoat numbers may have dropped.
• Tunnels with stoat lures can still be used to index rodent numbers.
• Still needed: comparing lures with meat bait, testing for seasonal changes in how stoats respond, and learning how different stoats (males and females, resident adults and dispersing young) respond.
• Getting enough real stoat secretion is a major limit. Careful testing to find the attractive compounds could lead to a synthetic lure, as it did for ferrets.

4. How tracking was or was not used

The whole study rests on footprints. Ink-pad tracking tunnels recorded the footprints of animals passing through, and the researchers identified stoat, rat and mouse prints by matching them against a reference collection of prints from animals kill-trapped at the lake. Most prints were easy to tell apart by size and number of toes. They also noted signs of disturbance at the tunnels (mostly possums) and one stoat scat. Counts of prints per tracking night (tracking rates) were the measure of how attractive the lures were. No trailing was involved.

5. Tracker or observer certification

Not reported. The paper does not say who identified the prints or how their skill was checked. It does say that most prints were easily told apart by size and toe number, using a local reference collection.

6. Other technologies, skills or innovations

• Footprint tracking tunnels of corrugated plastic on a wooden base, with a central pad soaked in a ferric reagent and tracking paper treated with tannic acid at each end (modified from King and Edgar 1977).
• Extraction of anal sac secretions with diethyl ether, then mixing into Albert®, a bioerodable casein-based material that releases scent slowly. It replaced the plastic rope used in earlier mustelid lure trials.

7. Implications for conservation (from the paper)

• Better tracking rates would give managers a more useful way to measure stoat abundance over time, during control operations, and as stoats re-invade, which matters for protecting vulnerable forest birds.
• Scent lures may also help attract stoats to traps, but this needs more testing.
• Supply of real stoat secretions limits wider use; a synthetic version would be needed.