Melbourne’s Spider Crab Migration

Photo credit: Matt Testoni, MT Underwater Media (Spider Crab Alliance)

 

When we think of animal migrations our minds will usually wander to large African mammals crossing a continent. However, just in the shallow waters of the Mornington Peninsula, a migration happens every year that is well worth checking out.

As the waters of Port Phillip Bay drop to a chilly 15 degrees at the start of each winter, 10s of thousands of native spider crabs Leptomithrax gaimardii ascend from the deep for a couple of weeks. This allows the spider crabs to moult their old shells and harden their new under shell all with the protection of the group. This creates a feeding frenzy for many ray species and Port Jackson sharks as they hover along the bottom sucking in all the newly moulted soft crabs.

Spider crabs have a body of around 15cm and when you add their spidery legs, they can reach over 30 cm across. So, watching them pile up to eight deep and the feeding frenzy that goes along with this is an amazing event for all to watch.

The spider crabs often congregate near one particular pier each year, so people can watch from on the pier, snorkel or scuba dive this wonderful event.

Photo credit: Matt Testoni, MT Underwater Media (Spider Crab Alliance)

 

The Problem.

The spider crab migration has been a well-known secret to Melbournians and avid scuba divers for many years. In recent years they gained further fame through the David Attenborough film Blue Planet Two and a host of social media traction alerting people to this amazing event. This caused an uproar of epic proportions as some people decided they wanted to eat them. For those of us who have followed this migration for many years, we have always known them to taste largely of sediment and seawater as they use this method to expand their bodies as they moult their old shell and grow a new one.

The Victorian Fisheries catch limit regulations states 30 crabs a day, however, this is for catch for the non-native, invasive green shore crab Carcinus maenas or larger crabs caught in the deep seas off a boat. This would seem reasonable is you were running all day to catch this many.

This past winter (2019) we saw people on mass coming to the one place spider crabs could be seen by all (a pier) and taking 30 a day per person.

We watched in amazement as people carried bucket loads of them back to their cars. Within about 10 days they were gone. It is hard to know the exact figures on how many were taken for human consumption and how many went back to sea.

They are caught in crab pots with a chicken carcass as bait. It isn’t hard to get them as the piles are often directly under the pier. My last dive with them had me in tears as I literally pushed discarded chicken carcasses away from my path. I watched in horror as the crab nets were lowered and quickly ripped upwards to catch a spider crab again and again. The crab net was often pulled across pier pylons and walls ripping sponge, coral and ascidian growth from its home. This could create large ecosystem changes within the area.

 

Photo credit: Matt Testoni, MT Underwater Media (Spider Crab Alliance)

 

Science and the Solution.

Like many smaller marine animals, there is little scientific work done. Research has just been funded to find out more about these wonderful creatures and the reasons for their annual migration from the depths of the ocean. The current knowledge comes from many scuba divers with a diverse range of backgrounds from ecologists, photographers and people who have been diving and watching this migration for over 30 years.

Help is needed for this wonderful event. A petition has been set up from the Spider Crab Alliance (you can find them on Facebook). This page also highlights community outcomes from all that enjoy the migration and gives the latest news. This includes meetings with the Victorian Fisheries Authority, local council and other groups to find out the social and ecological value of this wonderful migration.

David Attenborough was contacted, and a kind handwritten reply was received about the troubling situation. The lucky person who received that!

It is hoped that this wonderful migration can be preserved as an environmental event. So that all people from all walks of life can head to a pier near Melbourne to see the spider crab migration for a few weeks every year for many years to come.