The end of radio-trapping operation: troublemakers and haunting questions.

Saturday (16.11.2013) the release and radio-tracking of the European mink in Hiiumaa Island was completed. 15 captive young mink from Tallinn Zoo Conservation Lab were released in the island and radiotracked for more than one and half month. The results of the release were interesting and evidenced remarkably increased release survival. However, the end of the release was a story by itself.
The collars may cause a lot of injuries when animals gain weight in late autumn, so the mink have to be caught and the collars removed. It might seem an easy job – the signal emitted by the transmitters help to locate the very spot of mink. Just set up live-traps in proper way and place along the bank on both sides of the mink location, equip traps with bait and …. voilà – the mink is in trap.
The real process was not so simple – here is the story of retrapping the mink in Hiiumaa Island in 2013.

At the end of the trapping session only 7 mink with collars remained under observations. Our task was to re-trap these 7 mink to remove the collars. The re-trapping was started on 23. of October, when the first collar was removed within one trap-night. On 26th of October the operation was continued and for five mink two traps for each were set up and the remaining one was hoped to get with three traps. The next day was the happy one – four mink in the traps and for the afternoon the collars were removed. Then the luck turned – to get the remaining two mink required notably more traps, nights and efforts. These two mink were simply not going to the traps, despite of all the efforts and long experience in trapping.

The knowledge of exact location of mink helped to set the traps up in the best available positions – real “do not go by” locations; and then the smelly oily smoked sardines as baits – usually very attractive to mink.

These two mink were simply not collaborating. Despite of all the efforts they just passed the traps as if these were non-existent. Even more, they were teasing us…. once it was possible to have a lengthy observation of the swimming mink in near vicinity of the trap.

Other obstacles became evident. In one trapping site close to farm nice black and white domestic cats caught the smell of oily sardines and after that it was impossible to friendly owner to keep the cats away from river bank and from our traps. We have to remove them. Actually, here the mink collaborated and moved upstream, so we followed it with our trapping effort. Then the Norway rats got an idea that the traps were just perfect for nests and they brought leaves and debris into traps till the door of the trap closed. Interestingly, the Norway rats close to water become real semiaquatic mammals. Looking as they jump into water, while escaping from trap, and dive, leaves no doubt – they are very skillful in water.

The time passed and then after some two weeks finally one of the trouble-makers went into trap and were freed from collar. The last one was a real “nuisance” – it just passed the traps – some ten of them in her homerange. We tried salmon instead of sardine as bait with no result. Then we took European mink scats and urine-moistured sand from the zoo to attract the animal – no results again. Finally, we left most of the live-traps empty – without bait. No result.

As a last hope we thought to make use live chick in box inside of livetrap to attract the animal. It was expected that the sound of chick is stimulus for mink strong enough to force her to go into the trap. The take care of the chick we decided to stay in visual distance from the trap, so it would be possible to get to the trap in time.

Then, when we arrived with chicks and equipment, the last troublemaker was in live-trap, without a bait at all in it. It took for us three weeks to get this bugger.

If you look on the graph, it is clear that there are differences in the behavior. Majority behaved like typical nice mink and got to traps as expected, but two .. who were these? Aliens? They were outliers, statistically speaking.. and outliers are important.



A number of speculative questions are rising from this experience:

1. A bit less than third of mink was very complicated to catch despite of their captive origin and ideal situation – we knew exactly the location of the animal. What it means if we use live-trapping as monitoring technique – that is when we even do not know where the animal exactly is? So far we have assumed that every animal have the same likelihood of getting trapped, or, at least, the variation has normal distribution. It is not. So how reliable are our monitoring data if the results depend upon the animals willing or not willing to go to trap? Does this proportion changes with years or with some other variable? We do not know.
2. What it all means in terms of conservation breeding and release operations? Is this around 1/3 ratio typical both for captive population and for wild population? Or it is higher in wild population and the natural selection in captivity actually favors trap-happy mink? Does the ratio  changes with time and duration release operation? What variables are having impact to the ratio? What means trap-happy versus -unhappy type in terms of survival in the wild, are the trap-unhappy type more likely to survive in the wild? Does it mean that if we conduct years-long release operation, we actually provide captive-biased animals for natural selection during release operation to select animals more trap-unhappy? Way more questions than replies.
3. Then, conservation breeding operations starts with founder animals trapped from the wild. Is the ratio of trap-happy and trap-unhappy mink among founder animals is strongly biased towards trap-happy animals? If yes, does it mean that the issue of founder effect in conservation breeding is way more complicated and perhaps overlooked. Again more questions than replies.

Field work with mink is good time after all. You have opportunity to be outside and see nice areas. Have a look at the photos.



The last troublemaker is in this photo. After removal of the collar it swiftly disappeared from opened trap like a shadow. Nice and good animals – good luck to her in streams of Day Island.





In summary, the mink, like all animals, are not simple pre-programmed biorobotes with easily predictable behaviours. They have their personalities, they learn, they change their behavior, and therefore, they are like us, humans, very different from each other. What holds for one person might not be fully correct for other … so lets be careful with our thinking models.


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