Rat tracking - bait at the ends or the middle?

I was interested to hear at the 2011 Sanctuaries Workshop at Nelson that some folks were still confused about whether they should bait tracking tunnels for rodents (mainly ship rats) at the ends or the middle.  It seems to me that there is no substantial real issue here, and it may help to explain why.

The key matter to resolve is: why are you undertaking the tracking in the first place?

•  IF you want to use best practice SOP (Standard Operating Procedure) so that your results can be compared reliably with other folks’ published work, then you must follow the protocol.  Simple as that.  That is, if for example you wish to verify that you have <5% tracking to help kokako, then you should use the protocol verbatim because that’s how the correlation with kokako nesting success was derived and published.  The protocol clearly says “This [bait] is smeared on the vertical face of the wooden base at each end of the tunnel’.  These days, the base may not be wooden, but you should still put bait on the vertical face at each end of the tunnel.  The key point of an index is not that it is as big as possible.  The key thing is that it is IDENTICAL wherever you use it.  Repeat:  IDENTICAL.

• IF you want to just compare pre- with post- results in your own trial, but not necessarily compare with anyone else’s results, then you should use the same technique at all your sites, regardless of what it is.  So if you want to bait in the middle rather than the ends, that will work fine, but the results strictly cannot be compared with the SOP results.  That is, a 5% index derived from baiting in the middle of the tunnel cannot be expected to deliver the benefits calculated for kokako by using tunnels baited at the ends.  There may be better results, or worse.

• IF you are not deriving an index at all, but are for example, trying to detect reinvaded rats in a pest-free sanctuary, then again you need not follow the SOP but should use the technique that has the highest probability of detecting an animal.  The problem at the moment is that nobody has actually done the research required to show whether baiting at ends or middle affects the detection rate at all.  Until the work is done, then practitioners have to make their own educated guesses about which technique is best.

The main conclusion is clear:  if you want to ‘plug in’ your rat outcomes to published accounts of benefits, such as kokako nesting success, then you should use the current DOC protocol verbatim, ie bait at the ends.

I should note though, that the widespread use of the 5% target for rats and possums for ecosystem recovery is just a somewhat wild stab in the dark as a target.  That is, the benefit of this target cf any other is as yet unpublished, as far as I know.  It’s as good as any other smallish number, and it is substantially smaller than what you will get if you do no population control, but it has no published relationship with ecosystem health at present.  So violating the SOP tracking measure in ecosystem restoration is just adding a bit more uncertainty to an already uncertain relationship between index and outcome.

John Innes

Landcare Research, Hamilton

6 September 2011.