20km of barriers, 31 underpasses and 1 bridge: getting ready for the great crab migration.

It’s that time of year, when we await the pitter patter of little feet. About 400 million little feet in fact, as our resident population of 50 million red crabs start their annual migration to the sea to spawn.

There’s a lot of work to do to help them on their way safely, to divert them from traffic, which Parks Australia has already commenced. We spoke to Rob Muller – Chief Ranger, Christmas Island National Park, to find out more.

Typically, when do you start preparations for the crab migration?

Rangers begin preparing for the migration around the beginning of August when the dry season is well under way and there is time to complete the work before the wet season rain comes. We try to have everything in place by mid October. That way we’re ready if the crabs decide to start migrating with the first decent rain, which can be in October, though it is more common that rain comes from November on.

What exactly needs to be done?

The main way we try to protect crabs during the migration is to stop them being squashed by vehicles on the roads. But they still need to be able to cross roads to get to the sea. The best way to do this is to close the roads, but this is impractical for some of the main roads that go through the national park. So on the roads that are kept open we put up special fences to act as a barrier to keep the crabs off the road. But then we still need to let the crabs get across the road so we have installed underpasses in the roads that the crabs use to get to the other side.

The fence barriers on the sides of the roads get damaged by falling trees, (we get a lot of these), and vegetation grows against and over them. The underpasses look like a deep cattle grid and are mostly on dirt roads. They accumulate dirt and gravel that falls off trucks and other vehicles and this washes into the underpasses with the rain during the wet season. If the barriers and underpasses are not maintained then the crabs can climb up the sides of both of these and get onto the road where they don’t fare well against the oncoming traffic.

The preparations that rangers undertake involves two main aspects: cleaning out the underpasses of the dirt, leaves and vegetation that have grown in them since the previous migration; and putting up some barriers, clearing behind them and replacing damaged sections.

To clean out the underpasses, the steel grids on the top have to be unbolted and lifted to the side. A Bobcat loader is then used to remove the built up dirt and clear the entrances to the underpass.  This is followed with a wash down using a water hose, and then the grids lifted back in place and bolted down.

How long has Parks Australia been installing barriers? 

The first time the national park put up and tried the fences, or barriers, was for the migration in 1998-99. That occasion showed that this approach to keeping the crabs off the road worked quite well and over the following years it has been refined and rolled out to greater distances along the roads. 

How many kms of barriers are set up?

There’s just over 20km of barriers in place

How many tunnels are there?

There are 31 underpasses – and one crab bridge, which takes crabs over the road. The bridge is for a location where it is not possible to go under the road.

Are some barriers in place permanently?

Now, there is about 17km of permanent fencing along sections of various roads. That equates to 8.5km of road because there is fencing on both sides of the road. It is made of 4 meter lengths of shaped, colorbond metal panels, held in place by three ‘L’ shaped brackets and nailed, (large nails), to the ground.

The temporary fence is made from 50 metre rolls of vinyl with regularly spaced pockets to fit the same ‘L’ shaped brackets, which are nailed to the ground.

How many people work on the preparations?

There are three rangers who carry out the work preparing for the crab migration, though we usually have an additional person with us when we are working on the road doing the major tasks of cleaning the underpasses and getting the fences in place. Other tasks that need to be done include making sure all road closure gates are functioning, signs regarding road closures and detours are in place, and ensuring community liaison and education takes place.

How are they monitored?

Rangers keep a regular watch on the state of the fences once the migration starts, particularly early morning and later in the afternoon. Fallen branches and trees can flatten parts of the fences, and gaps between the fence and the ground can open up where rain run-off has washed through. The crabs are quick to find these gaps and take any short cut from behind the fence that they can, so we fix these up when we see them.

Are extra people brought to the island to set things up or is the work done by locals?

We’ve found that the set up we currently have is able to be managed by the rangers plus some additional assistance from the other park staff, particularly now that we have moved to the use of permanent barriers where we can.

Locals do play a part though when the crabs are making their way through some areas of town. People look after areas near their house and have their garden rakes handy to move the crabs quickly across the road when cars are coming. Christmas Islanders are quite a considerate lot so most drivers travel slowly during the migration and try their best to avoid unnecessarily hitting crabs.

How effective are the barriers?

The barriers have made a huge difference to the number of crabs killed on the roads and allowed people to use those sections of road without the awful feeling of having run over lots of crabs when they had no choice but to use the road. The perfect solution is to close the roads where crabs are crossing, but where that’s not possible the fences and underpasses are the next best option. It allows business and travel to continue normally and not cause the significant impact on a unique wildlife event that it once did.

Keep an eye on our Facebook page for news of the migration – we’ll let you know when the action starts!