Will improving the kill rate of existing traps make much of difference in our ability to eliminate predators?

Today we tackle the question of the kill rate of existing traps.  The arsenal of traps available to trappers includes some well-designed, field-tested, and hardy workhorses.  And yet we know that even the most skillful deployment of these in the field only delivers a level of suppression, not the total elimination we strive for.  Today we discuss why that might be.

Improved kill rates

Some people have noticed that predators either escape from or don’t do enough to trigger existing traps (https://zip.org.nz/findings/2019/7/the-zip200-project-to-develop-an-improved-rat-and-stoat-trap-comes-to-a-surprising-end). Increasing the kill rate up to 100% (from say 80%) seems to be a sensible sort of strategy to help get to total elimination. Using the same model as before we can easily show that improving the kill rate doesn’t make as much of a difference as you might think.

As usual, just to make sure we're all talking about the same thing, a couple of definitions:

  • Kill Rate: The elimination success rate of each device.  Defined as the percentage of animals that are eliminated when they trigger the trap.

  • Trap Interaction Rate:  The number of times a predator interacts enough to trigger a trap in a given time and area.  

Referring to the graph below:

  • the blue line shows the population trend for a low kill rate device
  • the red line shows the population trend for a high kill rate device


See the previous blog post for full explanation of the simple model. The reason improved kill rate by itself doesn’t make much difference is that most predators just walk on by the current devices. We don’t say that lightly. We have observed this over years of using the highest sensitivity tracking camera and the high level data from the 60 best monitored projects in New Zealand support this low interaction rate.   Obviously if you have a high interaction rate trap then having a high kill rate is important but it is the interaction rate that is the key to improved trapping.

In our next blog post we discuss a number of ways this can be improved. 

As always, we welcome your feedback so don't hesitate to get in touch - leave a comment below or email us at blog@cacophony.org.nz.


Publication Date: 
Saturday, 22 August 2020