Scientists liberate Gough Island from mouse menace, but how long will it last?

Mousetraps and cheese was never going to cut it. But two-million poison pellets a year have ended in a resounding conservation success story in a showdown between man and mouse on a remote South Atlantic Island.

The Royal Society for the Protection of Birds (RSPB) last week confirmed a huge rebound in the breeding success of the Tristan albatross — up above 75% compared with about half that number for the past 17 years — on Gough Island, which the society says is directly related to their Gough Restoration Programme.

The good news is just rewards for a huge multinational intervention on Gough Island to combat an uncontrollable mouse infestation — to the detriment of many of the island’s seabird species. Mice, which are not endemic to the island, feed on the defenceless seabird chicks. The bird species have not had enough time to evolve a defence against the new menace. 

To combat the mice plague the restoration team use a trusted rodenticide that slowly biodegrades and does not leach into watercourses. They also used helicopters from Johannesburg, the SA Agulhas II research ship, ground crews and a phalanx of top scientists in a collective effort to fight the mouse menace.

“Generally speaking, these excellent breeding success figures show what the albatrosses future could look like on a mouse-free Gough and hardens our resolve to return and remove the mice once and for all,” said RSPB spokesperson Anna Feeney. 

Other species to benefit from the sharp decline in mice numbers include the MacGillivray’s prion (82% chick fledgling rate compared with 0% in 2017), the Atlantic petrel (62.7% compared with less than half that). At a site monitoring the MacGillivray’s prion, for example, no chicks had fledged since 2017. This year, 82% of chicks fledged, and there were no signs of mouse interference on the nests that failed. Atlantic petrel breeding success was 62.7% (more than twice last year's figure), and the grey petrel (75% up from 30%). 

Said Feeney: “There will always be variability in breeding success year on year. However, given not a single wound was found on any chicks this year and given the high breeding success recorded particularly by species badly affected by mice in the past, we believe that this is predominantly a reflection of the significant reduction in mouse numbers.”

Bait distribution began in June 2021 and made an immediate impact on breeding sites closer to the main research base. However, even remote breeding sites on the north of the Island were significantly improved according to this year’s nest survey. “The breeding success figures for 2022 are the first set of figures we have from an environment containing very few mice,” said Feeney. “The greatest gains were from the north of the island where breeding success has typically been very low — this year our team counted 201 chicks at West Point and 177 in Giant Petrel Valley, whereas in previous years the counts have been 30 to 50 and 30 to 90 respectively. In total more than 1,000 chicks fledged this year — for the first time this century (1,186 chicks fledged from 1,570 nests),” she said.

Fledgling success for the Tristan albatross now exceeds the critical threshold required for a sustainable population, though it is unclear to what extent the mice population will rebound. “What we don’t know is how many years of good breeding success we may have before the mice start to predate chicks once again, however, any injection of new birds into the population is a help,” Feeney said.

Gough Island forms part of the British-governed Tristan da Cunha archipelago, considered the most remote inhabited archipelago worldwide. Mice arrived over a century ago by jumping ship when sailing vessels called, and the first evidence of mice eating birds goes back 20 years. Researchers say the birds were particularly vulnerable to mice predation, mostly in winter, when other food sources are scarce.

News of the breeding success surfaced last month on social media, garnering much praise after the restoration programme. “Awesome news. Such a good result from your worthy efforts,” commented Alan Bedford-Shaw on Twitter.  

“I hope you will be able to finally declare Gough a mouse-free island in the near feature,” said Peter Treloar in a follow-up post.  

It’s not the first time scientists have celebrated apparent victory against mice, only to witness a rebound in numbers. The Island was thought to be mouse-free after the initial bait drop last year, only for a mouse to be spotted on a field camera in December. However, the latest seabird breeding statistics suggest victory may be in sight.