With Cayman on the brink of embarking on a nationwide green iguana cull, the population of the invasive species continues to creep upward.

There are now between 1.1 and 1.6 million green iguanas in Grand Cayman, according to the latest population survey.

Department of Environment researchers spent two weeks in the field counting iguanas at 164 different survey points for the annual population survey. Using a technique called distance sampling, they are able to estimate the animal density at each of the sites and extrapolate those results to get a total population estimate.

Fred Burton, head of the DoE’s terrestrial resources unit, said it was an internationally recognized technique that enabled researchers to complete an otherwise impossible task.

He said this year’s population estimate was 1.3 million, but could potentially be higher, given that hatching season was not finished at the time of the survey in August. While the given figure is an estimate, he said the DoE could say with a high degree of certainty that the population was between 1.1 and 1.6 million.

He said the figures for this year are lower than expected, suggesting a slight slowdown in the growth of the invasive population.

Since 2014, the green iguana population has been approximately doubling each year, and hit the one million mark last year.

“They may be approaching carrying capacity in some of the more densely populated areas,” he said.

While this is a good sign, he said, there was no reason for complacency.

The Department of Environment is about to embark on a massive nationwide iguana cull, aiming to kill one million of the invasive lizards in the next year. Mr. Burton said it was unclear how the iguanas would respond to such an intense cull and warned that reproduction rates could initially increase as the population reduced. He said the cull managers would have to adapt and revise their targets based on the results over the first few months.

“It is a moving target,” he said. “When you embark on something like this that nobody has ever done before on this scale, you have to expect the unexpected.”

He said the cull was funded for at least the first three months and the DoE and government would be monitoring the results carefully.

Jane Haakonsson, a research officer at the DoE, said the green iguana population survey takes place during the first two weeks of August every year at the same locations. She said the research team used a mix of survey points alongside roads, off roads and in reserves.

Researchers work in teams of three, with one observer recording the data and the other two measuring detection distances from the observation point to the iguanas.

Ms. Haakonsson said the survey showed that green iguanas were more abundant in urban areas than in undisturbed forest environments.