One of Australia’s most unique underwater events is facing a new challenge. The Australian giant cuttlefish mating aggregation—the world’s only large-scale cuttlefish gathering—has seen the first major decline in numbers since the event was protected from commercial fishing nearly 14 years ago.
Each winter the sleepy industrial town of Whyalla becomes a bustling regional center. Divers, scientists, documentary filmmakers and tourists armed with snorkel gear flock to the dusty red South Australian mining town to see one of the world’s most unique marine spectacles. Each winter thousands of Australian giant cuttlefish (Sepia apama) migrate from the reaches of the Upper Spencer Gulf to the shallow rocky reef between Fitzgerald Bay and False Bay to participate in the penultimate event of their short lives. The annual gathering, starting in May and ending in August, is their one chance to mate and pass on their genes to the next generation.
It’s a three-month marathon of competition and complex courtship, which sees these normally solitary and shy animals gather in concentrations as high as one cuttlefish per square metre. The cuttlefish throw caution to the wind in their attempts to mate, completely ignoring divers and snorkelers.
In just a few metres of water, smaller males can be observed outsmarting their stronger rivals by cross dressing as females. It’s a sneaky but effective strategy, thanks to the amazingly diverse array of colors, patterns and textures made possible by specialized skin cells called chromatophores.
Whyalla Diving Services owner, Tony Bramley, knows the cuttlefish better than anyone. He has been nicknamed, the Godfather of Cuttlefish, because of his experience diving with the mating aggregation since 1979.
Bramley estimates only 20 percent of the cuttlefish from the 2010 season were seen in 2011. In the early 1990’s there was a small amount of fishing during the mating season, mostly for bait, but when demand for cuttlefish in Asia ramped up in the mid ‘90s the aggregation was specifically targeted by commercial fishing operations.
Bramley and other divers successfully campaigned for protection for the cuttlefish after the aggregation was nearly fished out in 1998. The number of cuttlefish at the annual mating aggregation has been steadily increasing ever since.
The 2005, 2006 and 2007 events were bumper seasons, still not as abundant as when Bramley first started diving with the cuttlefish, but a healthy increase on the first official government counts of 2002. There was a slight dip in 2008, and the 2009 and 2010 seasons appeared to plateau but showed no signs of decrease or widespread ill health in the population.
This past year’s season was the first time cuttlefish numbers had declined significantly since intensive commercial fishing in 1998.Bramley believes warmer than average water temperatures are the most likely culprit.
“There was no difference to any activity that I know of in the gulf last year,” he said. “It’s not like there was some development or dredging or new industry coming online. As far as I know, other than the temperature difference, it was just another season. Right up into the middle of June the water temperature was up around 17°C. Normally, it gets below that by May.”
The giant cuttlefish in the Upper Spencer Gulf are particularly vulnerable to fishing and other pressures, like increased water temperatures, because of their dense concentrations during the mating season and their short life cycle.
After hatching, juvenile cuttlefish move out into the gulf where they lead solitary lives, taking shelter under ledges from predators such as dolphins. Less than a year later, they return to the shallow rocky reef of their birth to mate and then die shortly afterwards. Thus, each year’s population is directly dependent on the success of the previous year’s aggregation. The fewer adults that are able to mate, or the less eggs that survive to hatching, the fewer adults there will be to mate the following year.
“Those two years where we had full commercial fishing, in 1997 and 1998, the flow-on effect was really noticeable right up through 2002,” Bramley said. “There was no recruitment in those two years.”
There is hope that since the cuttlefish successfully recovered from those fishing pressures, they should also be able to bounce back from last year’s unseasonably warm temperatures. But that’s only if temperatures return to the normal 12-15°C range, and while fishing efforts can be regulated, it’s much harder to do anything about water temperature.
“It’s very worrying, but we’ll know a lot more at the beginning of this season,” Bramley said.