Granite Island’s little penguin population has just 22 birds – and 11 active burrows – according to the latest census conducted on the island.
It’s a similar number to recordings over the last decade, and remains a far cry from the thousands of seabirds counted on the island in the early 2000s, when they were greatly beneficial for local tourism on the Fleurieu Peninsula, south of Adelaide.
Granite Island is joined to the foreshore of the adjacent Victor Harbor township by a long causeway and is accessible to mainland residents.
But a combination of factors has conspired to reduce the numbers of little penguins on the island. This includes climate change impacts, predation by feral, domesticated and native animals, and access to food sources.
South Australia’s decade-long ‘Millennium Drought’ which broke in 2009 is believed to have critically impacted the colony.
Flinders University behavioural ecologist Dr Diane Colombelli-Négrel led a study that found a relationship between river outflows and the mass of potential penguin prey, such as local southern garfish and sardines.
The research, published in Frontiers in Marine Science, shows reductions in river flows from the Murray Mouth – about 20km east of Granite Island – was a key contributor to the collapse in population.
Decade-long drought cuts penguins and their prey
Penguins forage near the Murray Mouth. Colombelli-Négrel and her colleagues found that reductions in nearby garfish populations was a strong predictor of lower penguin numbers in annual census activities.
“When the millennium drought happened, the [river] flow to the ocean went to zero, and that’s when the little penguin population on Granite Island started declining,” Colombelli-Négrel told Cosmos as part of a feature to be published in Cosmos Weekly in December.
“In 2010, when the flow started going again into the ocean, what’s when the population that was going down started stabilising. We found there’s an interaction between the rainfall, the river outflow, the primary connectivity, the fish – which are obviously directly impacted as well – and then the penguins. It’s all linked together.”
Protecting against predators
While numbers have held in the 20s for several years, a critical population ‘floor’ for the colony may have been reached some time ago.
Climate change is expected to increase the duration and severity of drought events. Warming oceans may also force penguin prey into more cooler waters, further off-shore.
One of the few things that locals and researchers alike can control is on-land predation.
Feral animals are known to cross the 500-metre-long causeway, and in 2020, a fox attack reduced numbers to a mere dozen after being as high as 44 the year prior.
Do you care about the oceans? Are you interested in scientific developments that affect them? Then our new email newsletter Ultramarine, launching soon, is for you. Click here to become an inaugural subscriber.
Baits, gates and specialist shooters have all formed part of the protective arsenal against feral predators making the trip to the island.
Human interactions are also taking their toll: from enthusiastic visitors using disruptive torch light which can blind penguins, to walking dogs that can harass or attack birds.
Dog scents also leave a marker that attracts other predators – including foxes – to penguin burrows.
Colombelli-Négrel and colleague Andrew Katsis say Granite Island penguins were markedly more aggressive than those on other islands in the southern ocean due to these disturbances.
It’s led to the local council approving measures to dissuade locals and tourists from walking dogs on the island, including permanent causeway signage.
“[The signage] is more educational,’” explains Stephen Hedges, who previously operated penguin tours on the island and now coordinates a local volunteer group that surveys penguin burrows.
“I’ve found that sign has turned alot of people back of their own accord. Even locals are braver because they have a reason to talk to people calmly.
“I think the dogs have improved, in the last few months, six months.”
While the highly-publicised little penguin colony at Phillip Island numbers in the tens of thousands, other mainland communities are less abundant.
Just as Granite Island in South Australia’s most prominent colony near populated areas, Manly Beach in suburban Sydney has also seen feral animals diminish its penguin numbers, with a stable population of 40 birds after devastating fox attack killed 27 birds. Colonies in Western Australia have also seen similar declines.