giant australian cuttlefish
Giant Australian cuttlefish are the largest cuttlefish species in the world.
Although they can be found all along the Great Southern Reef, there is a distinct population that gather in large numbers each winter along 8km in South Australia’s Northern Spencer Gulf.
This section of rocky reef that provides an important breeding ground for the Giant Australian Cuttlefish (Sepia apama), and it is within and underneath these rocks where the cuttlefish lay their eggs.
Males outnumber females at around 7:1 but the ratio can be as high as 11:1 early in the mating season. Females reject around half of all matings.
“A lot of people don’t actually know what their sanctuary zones are, and haven’t experienced it. We had to change that and get as many people into the water to see those sanctuary zones as possible.”
Carl Charter, Co-founder and EO, Experiencing Marine Sanctuaries
Making a move
Males pair with, and defend females, rather than egg laying sites. Large males paired with females spend the most time guarding the female from other males with spectacular visual displays. Small lone males spend a considerable amount of time searching for lone females and/or opportunities to mate with paired females.
Fussy cuddlers
When mating does occur, the couple ‘cuddle’ in a head to head embrace. The act involves the male jetting water from his funnel towards the mouth region of the female and the transfer of sperm packages.
Following mating, males may actively try and prevent females from mating with other males (guarding). Males may attempt to displace the sperm of rival males, for example by flushing water through the female during mating, thereby increasing the chances of fertilisation by their own sperm.
Females do not necessarily engage in immediate egg-laying following mating, but can mate multiple times before commencing egg-laying. Eggs are deposited on the underside of rocks or in rock crevices. Females move around in search of egg laying sites and do not deposit all their eggs in the same location. Females may lay 5-39 eggs a day with these eggs being sired by multiple males. The eggs develop for 3-5 months and hatch from mid-September through to early November.
Strength vs. stealth
A number of strategies are used by smaller male cuttlefish to gain more female partners including; open stealth, the smaller mail approaches the guarded female while the consort male is fending off other males; hidden stealth, the smaller male meets potential female partners underneath the cover of rocks and reef; and female mimicry, the smaller males mimic the appearance of a female to prevent larger guarding males from challenging them directly.
“You should definitely come out this way and have a swim.”
Tracking trends
Every year SARDI aquatic sciences sends a team out to Pt. Lowly to assess the population of the Giant Australian Cuttlefish. Divers swim 50m transects and count how many cuttlefish are within a meter on each side of the transect to work out the number of animals per square meter. Within each transect they can count up to several hundred cuttlefish.
Boom and bust
In the late 90s the population estimate was around 180,000 and appeared stable. In 2005 the population dipped a little, but from about 2009 onwards the scientists started to see a concerning decrease in the population, dropping to just 13,000 animals in 2013, which was a decline to less than 10% of earlier estimates.
Many questions were raised about what was causing that decline. Was it pollution, disease, aquaculture, fishing? Because the area is highly utilised by many different people and organisations, there was a lot of speculation. Scientists tried to assess each potential driver, but found it hard to form any form of link or relationship.
Precautionary principle
While studies show no definitive link that fishing pressure caused the cuttlefish population to decline, it was more of a precautionary approach to implement protective bans on fishing in the breeding areas.
From 1998 onwards, the sanctuary zone within the aggregation site has been in place. In March 2013 following record low levels of cuttlefish, the entire northern Spencer Gulf (the line between Wallaroo and Arno Bay) was closed to the taking of the species.
The temperature theory
What studies did find was that the cuttlefish embryos develop at a rate that is parallel to the temperature at which they hatch. Researchers found that during years when the populations increased, the embryos hatched into warmer temperatures. In warmer conditions the growth of the juvenile cuttlefish speeds up and they may become less vulnerable to predation. During years when populations were suppressed, they developed through cooler temperatures, therefore, their growth was slower and perhaps they were more vulnerable to predation.
Rockstars of the sea
Cephalopod populations are known to show considerable fluctuations in abundance, with variations including 10-20 year cycles. When analyzing impact, it is important to recognise their short life span (12-24 months) and the single spawning life history of the giant Australian cuttlefish. Any species with this reproduction strategy is particularly vulnerable since there is no ‘storage effect’ within the population. As a ‘live fast die young’ species, this means they can be highly responsive to environmental stress, causing them to boom and bust dramatically.
Rapid recovery
After a fast comeback in recent years, the aggregation continues to thrive. In 2020, the management removed the temporary ban of fishing cuttlefish from the entire northern Spencer Gulf, and will continue to monitor the aggregation closely.
The local town of Whyalla has really embraced this annual event – after all it is the only place in the world you are able to see tens of thousands of cuttlefish aggregate in such a small area. While this year only South Australians had the luxury of being able to dive with these magnificent animals, we all hope that this is something that will continue to be a huge drawcard for local and international tourists visiting the Great Southern Reef for years to come.
May 2021 update
In the lead up to the 2021 season, in addition to the permanent cephalopod closure in False Bay, from 14 May to 10 August 2021, a temporary cephalopod fishing closure is in place in an adjacent area.
The temporary closure area includes all waters bound by a line commencing from the Point Lowly Lighthouse and following the eastern boundary of the existing closure area to 100 metres offshore from the high water mark and then following the coastline to a point 100 metres south of the boat ramp breakwater near Port Lowly.
Highest level of protection
In May 2023, the State Government has made a ban on cuttlefish fishing in the Upper Spencer Gulf permanent to protect this one-of-a-kind breeding event. Fishers must not target or take any cuttlefish species in the waters north of a line from Northern Spencer Gulf north of the line between Arno Bay and Wallaroo.
This is a much needed win to ensure the highest level of protection for this world-class spectacle. The ban was removed in 2020 by the previous state government but was re-instated temporarily in 2022.
National Heritage Status
To highlight the National significance of this site, the Cuttlefish Coast Sanctuary Zone was given National Heritage status in May 2023. This status will provide further protection of this spectacular breeding event as well as support further research into the species.
Fast Facts:
Species: Sepia apama
Family: Sepiidae (cuttlefish)
Order: Sepiida
Feeding: Small molluscs and crustaceans, especially shrimp and crab
Habitat: Shallow reefs and marine channels
Distribution: Tropical and temperate oceans worldwide
Special Power: Polarised communication
