Given this success, in December 2011 sixteen new Otago skinks were released into the small sanctuary, including some offspring of wild skinks which were hoped to improve the genetics of the population. However, ten days later mice were found inside the fence. Every predator proof fence in New Zealand struggles to exclude mice, and this one was no exception.

 

At first COET was not too worried. Recent research by the Department of Conservation had shown that grand and Otago skink populations recovered when the larger mammalian pests were removed but mice were still present (albeit at very low densities during most years). This research was leading COET to consider changing the design for the planned 14 ha fence to a cheaper design that ‘leaked’ mice. They began to get quotes for this new design. (To read a comparison of the cost-effectiveness of trapping, leaky fences or full exclusion fences see Norbury et al. 2014). 

 

However all this changed when two Dutch students were monitoring the skinks at this time. They witnessed two incidents of mice attacking 25 cm long adult skinks. Both skinks escaped, but the one they re-sighted two hours later had a bloody wound on its head. When they followed the survival of this second release cohort of skinks over the next few months, the results were alarming. Whereas the first release group were showing a survival rate of about 80%, survival of this new group which had been released as the mice entered, plummeted to about a third of this.

 

This was an important discovery. Grant Norbury and the students Michiel van den Munckhof and Sophie Neitzel published the results of their observations in a paper in the New Zealand Journal of Ecology. While painful learning for the Trust, it would be a valuable alert to other groups. It suggested that efforts to exclude mice in lizard translocations were warranted, at least during the first phase of a translocation while the lizards were settling in. Mice are especially damaging in these modified grasslands because introduced grasses and herbs produce abundant seed every year, which maintains very high mouse numbers. 

 

It was thought that the first release group of lizards, who had been in the small fence for over two years before the mice entered, had survived much better because they were familiar with the terrain and had established themselves in well protected retreats. 

 

Needless to say, thoughts of building a 14 ha fence that leaked mice were immediately dropped. A fully pest-proof fence was required.