PARK WATCH June 2020 |

AJ Morton, scuba diving business owner of Dive2U, Reveals how one of Victoria’s greatest underwater spectacles could be under threat.

Every year, through March to July, our southern coastal waters are invaded by bizarre, alien-like crabs, aggregating in their thousands.

Covered in seaweed and sponges, a mass of legs crawling this way and that, they pile over the top of one another in a hectic battle for position. Scuba divers, nature lovers, educators, and coastal residents eagerly anticipate this annual natural phenomenon. People travel to Melbourne from all over the planet to experience this fascinating event.

The annual Spider Crab Aggregation holds many mysteries to us all, but one question many people ask that we do know the answer to, is: “why do the Spider Crabs risk travelling from the depths of the Bass Strait to the shoreline shallows?”

The answer is to moult.

Moulting is an extremely vulnerable phase of the Spider Crab’s life. The process of shedding a shell and waiting for a new shell to harden takes time, leaving the crab exposed, and easy pickings for predators such as the large Smooth Stingrays which cruise these waters. Moulting alongside thousands of other crabs increases their chance of survival.

Made globally famous by David Attenborough’s BBC documentary Blue Planet II, the Spider Crab aggregation has become a tourism drawcard. The crabs can often be seen from piers when the visibility is clear, and those keen to brave the 10-12-degree waters often jump in for an up-close scuba or snorkel.

In what would usually be an off-peak season for the Mornington Peninsula, crowds flock to see this amazing sight, unlike any other in the world. Unfortunately, with the fame has come some unexpected attention.

During the 2019 aggregation event, there was an influx of fishing of the crabs. An event which drew passive observers, such as photographers, videographers, educators and visitors alike, was now subject to fishing pressure that has not been observed at this scale before.

Much about the Spider Crab’s life remains unknown, i.e. their population size, how far they travel to moult, where they go after moulting, and whether they return to the same location each year, or are even if we are seeing the same individuals year on year.

What we do know, however, is that in just two weekends, large numbers of crabs were removed from piers on the Mornington Peninsula.

For the tourists and locals who had come down to admire the migrating crabs, the sight of them being hauled up in nets and carted down the pier was shocking and stressful for many. With so many basic questions around the life cycles of the crabs unknown, it is difficult to estimate the impact fishing may have on their population.

Until 2019 people that came to witness the aggregation only took photos, and left bubbles. These same people are concerned that if this fishing pressure continues to escalate in these easy to access locations, this unique event may no longer be witnessed in the future.

Representatives from the dive industry, conservation and education sector sought a meeting with the Victorian Fisheries Authority (VFA) to express their concerns and see what could be done to address the issue.   

Presented at the meeting on behalf of the ‘Spider Crab Alliance’ (an alliance with the above sectors, with the addition of concerned community members) were views from nearly 20,000 petition signees; 50+ statements of position from tourism operators, education providers and businesses; over 1000+ personal statements from marine scientists, educators, photographers and community members; and a handwritten letter from Sir David Attenborough himself. They all called for a halt on fishing Spider Crabs during their peak aggregation months of March to July – at least until we know more about their population numbers and the impact fishing has on this valuable tourist attraction.

The VFA’s concerns do not appear to be echoed at the same level, with their efforts going towards education, instead of any regulatory change or ban on fishing Spider Crabs in their peak moulting season. It would be prudent to take a precautionary approach to protect the aggregation and the benefits brought by the visitation to the local economy. The VFA’s automatic assumption of abundance and the statement that “lots of crabs get to moult”, as quoted in a recent article in The Age, is disappointing to many.

Fears for the future of the Spider Crab Aggregation partly stems from events encountered by our South Australian neighbours. Over there, during the Giant Cuttlefish mating migration on the Spencer Gulf, populations were believed to have dropped by more than 90 per cent, which overlapped with a history of intensified fishing pressure. In response to community concern and action, fishing closures were implemented, reversing this decline. The Giant Cuttlefish population is now enjoying a strong recovery.

Due to the huge benefits the Spider Crab spectacle brings to tourism, local economies, and simple public enjoyment, balanced with the imminent and real threat it appears that fishing has on this prized crab – more consideration should be given to its management efforts.

Introducing an interim halt on fishing Spider Crabs during their critical aggregation period of March – July is a good start, at least until more research is undertaken to gain an understanding of the species population numbers.

We hope the VFA will see the huge value these Spider Crabs bring, and what is at risk.


Did you like reading this article? Want to be kept up to date about this and other nature issues in Victoria? Subscribe to our email updates.

You can also receive our print magazine Park Watch four times a year by becoming a member. Find out more here.