Camera trapping a Mink raft - a trial!
Back in August we deployed a single Bushnell HD Max camera trap on a Mink raft at a wetland reserve in central Scotland to try and get an idea of raft usage. This was a first time trial with no expectations, however it provided some interesting and potentially useful results.
Mink rafts have been developed to provide a method of indicating the presence of invasive American Mink along waterways using a wet clay pad situated within a rectangular wooded tunnel mounted on a floating raft. These rafts can subsequently be used for trapping if presence is confirmed.
They are normally tethered to the bank and checked every 1-2 weeks. Encounter rates over a two-week period are said to be approximately 50%.
So is this true?
We used wire to attach the Bushnell facing inwards at one end of the mink raft tunnel (wire looped around two tacks knocked into the side of the raft). The site was already known to have Mink present. In total the camera ran for 123 days recording 6870 images. The camera was programmed to take 3 images per trigger event indicating that it was triggered 2290 times during the period.
The camera recorded at least six species of mammal using the raft:
· American Mink
· Wood Mouse
· Field Vole
· Common Shrew
· Water Shrew
Whilst it may seem a bit of a surprise to have the likes of Wood Mouse and Common Shrew on the list, the raft does often come into contact with the bank, and vegetation can form a bridge to the raft.
As is apparent there were many false triggers, this being as a result of the raft moving in the current and registering a change in the bank side vegetation temperature. False triggers were most prevalent in summer (when temperature differentials are greatest) and almost absent by mid-Autumn. Animal detection also follows a similar pattern with most triggers occurring in summer and early autumn when the temperature differential between a wet animal and the ambient air is greatest. Triggers in late-autumn and early winter only occurred when the animal was exceptionally close to the sensor which then prohibited identification.
Table 1. Mammal triggers by month (time of triggers)
Whilst only an initial trial this method of monitoring mink rafts (or potentially other floating or tubular structures) is most definitely feasible. In summary the key limitations and findings are as follows:
- Many false triggers making analysis timely (Fix: tying and anchor raft movement as much as possible and site in a more temperature-controlled area, for instance under a bridge or overhanging tree).
- Attaching the camera to one open end of the mink raft restricts through access for animals.
- The exposure settings of the camera meant that when triggered bright light surrounding the mink raft chamber became massively overexposed, often completely bleaching out animals on the rim of the raft. Without somehow reducing light levels outside the chamber this may a consistent problem (Fix: Siting under a bridge or overhanging tree may go some way to reducing this issue).
- Mink were recorded frequently (10 times) using the rafts and in all instances (except one) the Mink entered the raft chamber. Interestingly Mink were recorded throughout the 24hr period. The camera images also identified at least two Mink based on size.
- As well as Mink it was interesting to see that the camera detected the presence of other key waterway species such as Otter and Water Shrew (Water Vole isn’t yet known from this site), as well as other more terrestrial small mammals.
- This method could be used to provide some kind of quantitative indication of mink frequentation at rafts. It could also be adapted such that cameras are simple focused onto suitable rafts which may in itself reduce the exposure issues but at the same time increase false triggering due to a wider field of view.
- Further trials adjusting the siting of the camera and raft as well as exposure settings would help to refine the method and the accuracy of the results.
We would be very interested to hear from anyone who has tried similar studies.