The term Residual Trap Catch (RTC) is commonly used in relation to pest management operations. It is an area that can confuse the general public and this confusion arises from the wording of the term. It is the word “residual” that causes the confusion.
In the minds of the average person, the term “residual” means “what is left”. However, in the case of RTC, “residual” actually means “randomised”. I have long argued that it should be called Randomised Trap Catch as that is what it actually is.
It is not a precise measurement of what number of possums may be left in an area. However, I can understand how people are led to think it is because of the term “residual”.
It is a method of laying out traps in straight lines of 10. The number of lines and the placement is decided by computer or manually in accordance with THE PROTOCCOL or a combination of both. If you really want to punish yourself for starting to read this stuff and want to read the full document, send me an email by clicking on this link simon.friar@xtra.co.nz and I will send it to you. It is 36 pages of scientific reading which might enthrall you.
Here is a sample:
The theory behind this RTC method is that it removes human intervention from the statistical process. It means that sometimes the trap lines may be laid in a place where possums are likely to be. It also means the converse – the trap lines may be in an area where there is little likelihood of encountering possums. It is a statistical method which allows one area to be compared equally with another area, despite possible differences in area etc.
The lines are monitored over three nights and the results put through the equation process. If the figure is less than a given figure, then that is regarded well. When it is over then more consideration is given to it.
In relation to possums, the EW RPMS provides that the level should not exceed 5 Residual Trap Catch (RTC). The RPMS stipulates that if the RTC level is between 5-7%, then it is a staff call as to whether it should be included into a programme. This is variable. It might be included if it was adjacent to another block that was going to be subject to another operation. Otherise, it might wait until a later time.
Over 7% means that it is automatically considered for a project. However, it must take its place in the priority queue. Finance is constrained. There are lots of things which would be good to do but we must have regard to the rate-payers ability to pay, especially these days.
According to Landcare Research, a 5% RTC is roughly equivalent to 5 possums per 10 hectares of area. So in a 100 hectare property, the possum population should be around 50. If the RTC was 6.5%, then the population would be around 65 possums. Some consider that the target level of 5% is too low. That is a matter of debate. I have to concede that I find the trigger point of 1-2% RTC over the target as being pretty fine. But others hold a different view.
The Animal Health Board through its TBFree propgramme are dealing with possums as a carrier of TB. I do not think anyone will argue that we should not be vigilant in eliminating TB. TBFree has a target of 2%RTC. If an area has no TB or has been cleared of TB, then the pest management operations are for biodiversity reasons. As far as possums are concerned, this involves forests and plants. There have been some cases of possums attacking bird’s nests but rats, stoats and ferrets are the main offenders there.
As far as plants are concerned, it depends on the plant itself as to what level of possum population it can sustain. Some species are impacted at levels of 2% RTC. Other plants can sustain levels up to 20% RTC. This again, is an area of debate as to what level is appropriate.
