EACH YEAR MORE THAN 100,000 bizarre creatures gather in South Australia’s Upper Spencer Gulf Marine Park in what is the only known mass aggregation of its kind in the world.
Nicknamed “the rock stars of the ocean” because they live fast and die young, cuttlefish have a life-span of only 12-18 months. This makes each May-to-August breeding season critical, because the number of surviving eggs in one year directly affects the number of adult cuttlefish that return the next year to breed.
Favourable conditions and a series of law changes, such as fishing bans, have helped the cuttlefish recover from a population of 13,000 in 2013 to 120,000 in 2017. The 2018 count has yet to be finalised but local experts predict the figure to be higher than last year.
About a third of the five-mile strip of rocky reef where the cuttlefish lay their eggs has been a marine-park sanctuary since 2014.
The reef is at Point Lowly in the Spencer Gulf, some 12 miles east of Whyalla and almost 250 miles from the South Australian capital Adelaide.
Organised community dives have been run for the past three seasons, attracting hundreds of visitors from around Australia and overseas.
Natural Resources Eyre Peninsula Marine Parks Co-ordinator Dr Shelley Paull says that recorded cuttlefish numbers peaked in the late 1990s at about 200,000 but declined because of a number of factors, including over-fishing and fluctuating water temperatures.
She says that Upper Spencer Gulf is typically made up of mangroves, sandy bottoms and seagrass beds, making the uncommon habitat of rocky reef around Point Lowly a natural drawcard for the cuttlefish.
“The boulders are ideal for them to place their eggs underneath, and that’s why they head there every year because the habitat, salinity and temperature are perfect for them,” says Dr Paull.
“They get there when the water is really cold, and as the temperature increases as we get towards spring the cuttlefish babies start to form.
“When they hatch after 3-5 months they’ve got to fend for themselves and to try not to encounter predators, and when they’re big enough they will swim out and hang out in northern Spencer Gulf.”
Giant Australian cuttlefish (Sepia apama) are found all along the southern Australian coast in waters off Western Australia, South Australia, Victoria and New South Wales.
However, Dr Paull says that the cuttlefish in South Australia’s Upper Spencer Gulf have slightly different genetics from those found in other areas, so we know that those aggregating in the gulf don’t breed and rarely mix with those further south.
“Cuttlefish elsewhere tend to find a mate and breed at the meeting point,” she says. “They don’t aggregate in these huge numbers, so it’s really a unique thing that happens, and we don’t find these aggregations anywhere else in the world for cuttlefish.”