Much more problematic for the sooty terns were the predation by the introduced mammals. One of the key drivers for the bird conservation programme on Ascension had been the massive increase in feral cats on the island. Generations of cats had been introduced on to the island since the first sailing ships arrived, some escaped and bred, and somehow managed to survive in the inhospitable environment. The main reason for their success was the prolific abundance of bird eggs and chicks. At one time most of the key bird species on Ascension had all bred on the main land, but, apart from the Wideawakes, the cats in particular had pushed them back to a couple of isolated cliff locations on the south east tip of the island, or the stacks and small islands around. Boatswainbird Island, the largest of these, was where the most flourishing colonies were and now the only safe place for the Ascension Frigate Birds to breed.
There was only one way to deal with this problem and that was to eradicate the feral cats. At first hunters were brought in to shoot or capture the cats. Controversially, cats were often disposed of by throwing them out to sea, where a piranha-like fish called the blackfish would hunt in packs and devour them in seconds. While at first the cats were easy to spot; they were literally everywhere, after the cull had been underway several months, finding out where the remaining cats were hiding out became more of a problem. They were still predating on any birds that tried to nest on land, especially the fruitful pickings at the Wideawake Fairs. But small populations were still to be found around the rest of the island too. Poisoning was used for a while, attracting the cats to bait laced with sodium monofluoroacetate.
Cat eradication programmes are usually conducted on island uninhabited by humans. Here on Ascension they had to be extremely careful not to kill domestic cats. For this reason exclusion zones were set up around the bases and settlements where no bait trapping were conducted. Domestic cats were all registered and tagged with a microchip. In the areas around the villages traps were often used to find the feral cats, and when a cat was caught, it could be checked for a microchip and if a positive reading given could be returned to their owners; if no tag were found the cat would be put down.
As the population of feral cats continued to decline, finding the last few pockets became more and more difficult. If any evidence of cat activity (particularly cat scat or faeces) was found, then an intensive effort to find and eradicate the cats was focused in that neighbourhood. When this evidence dribbled out to nothing, a set of cat tracking stations were set up to make absolutely sure that every cat had indeed gone.