Fiddler Crab Feeding Frenzy

In some places including Townsville, spring tides are enormous and neap tides are negligible. This creates a boom and bust cycle for animals at the top of the tidal range. If their area has been tidally flooded, then they will come out in a feeding frenzy. This post documents the wildlife I found on a salt pan near Townsville/Cape Cleveland following a big spring tide.

A long salt pan between parallel beach ridges

Beach ridges are long and grassy

The salt pans had million of fiddler crabs (Uca signata). Most were male and they were jousting and leaving the ground littered with ripped off nippers. Even in the distance there were clusters of male crabs. Perhaps they were fighting over a burrow. As the tide goes out, they firstly feed within the colony but when food (diatoms) runs out, they radiate out over the salt pan into areas that have few burrows into which to retreat when the are threatened. Some of the crabs even wanted to fight me! 

Crabs (Uca signata) were present in millions

Most of the crabs were male.

Swarms of crabs covered hundreds of metres. I did not think that a sunbaked salt pan was habitable at all, so it was a major surprise to see so much life. The salt pans belonged to a single species of crab. Other crab species were present within the fringing mangroves and especially within the more luxuriant grow near the creeks. There were large burrows beside the tiny rivulets that drained the salt pans but I could not find out what created those burrows. 

Big burrows near rivulets

Cape Cleveland in distance

Even in the above view, zooming in shows lots of crabs

The mud was sticky and formed ‘moon boots’ on my shoes. It is a change from the sandy mangrove substrates of Mackay or the organic muds of Cairns. The mud seemed to be mostly cracking clay. As the salt pans were long and narrow, one end was a marine environment and the other end was a fresh to brackish environment. The marine end had most of the crab and fish action. Almost no small animals were present on the now dry brackish end, save for some larger birds. These birds probably use the area to rest, as wide open spaces are safe spaces. Dingoes cannot sneak up on them.

Australian Jabiru

Brolga

Big Cyclone, poor mangrove recovery

 Fifteen years ago Category 5 Cyclone (~hurricane or typhoon) Yasi slammed into the coast just north of Cardwell on the Queensland coast in Australia.  The seven metre storm surge caused almost total mangrove death over many square kilometres.  I had not been back to this area in fifteen years.  The last time I saw the area, it was about 3 months after they cyclone, and I walked through a dead standing forest. This site once had a luxuriant Ceriops (yellow-leaved spurred mangrove) swamp.  Remains of the original trees are still standing and a new stand of mangroves is growing up below.  Fifteen years of regeneration have only filled in half the space, regrowth is only about 20% the height of the original trees and dead timber from fallen trees covers the ground.  In contrast, the beach vegetation, mainly littoral rainforest has totally recovered and it luxuriant.

Dead mangroves seen on previous visit fifteen years ago

Beach vegetation that took the seven metre deep surge with six metre waves

Getting into the swamp was a challenge as the landward fringe was filled with bushy mangroves that had retained strong dead twigs on their lower, now shaded stems.  One passed that barrier, I was in a mosaic of open ground with fallen trees and dense bushy mangroves.  The fallen timber was firm and whilst sun bleached, was not weakened by decomposition.  Absent were signs of insect attack or ship worms.  Stepping over the fallen timber would be easy for a young man, and the mud was wet and cracked but not deep or sticky.

Getting in is tough, lots of twigs and mosquitos

In the middle, there are wide spaces and bushy thickets

As far as I wanted to go, it was much the same

Only one type of crab was abundant, a small grapsid or sessarmid (Sesarma messor?) and these crabs ran from me across the surface when crabs usually run down crab holes.  It was strange behaviour that caught my eye.  Just after they cyclone a small dark red crab was everywhere, yesterday I saw just two of them.  Rakali tracks covered the ground and I think that they go down larger crab holes and attack the residents.  I don’t imagine that near complete predation of larger crabs is helping the recovery of the mangrove forest.  Tracks from feral pigs, large and small, indicated that these animals lived in the area and routinely crossed the swamps.  More than crocodiles, I fear encounters with feral pigs and even found a rubbing tree with scars from the pig’s tusks.  Wallaby and bandicoot tracks were also present.

Sesarma messor? Rather small for this species

Rakali scratches

A feral boar rubs against this tree

The one unexpected animal was a moth as large as a small bat.  I could feel the vibration from its wing beats as it flew past.  Fortunately it landed on a nearby tree and I could photograph it.

Despite a number of good years, with above average rainfall and no cyclones, the recovery of the mangroves has been very slow.  The regrowth mangroves are flowering like crazy, aware that half the space is yet to be filled.  Rainforest trees hold back on flowering to concentrate on growth as a larger tree can support many times more flowers and fruits.  It tells me something, that the mangroves are flowering and fruiting so precociously.

Heavy fruit load for such a small tree (Ceriops sp.)

Not much alive on the surface

I did not see any molluscs.  There were a few dead mangrove clams.  I also did not see bird footprints.  It seems that nothing has thrived due to the changed conditions.  The conditions twelve years ago are shown in this post https://queenslandcoast.blogspot.com/2014/06/legacy-of-cyclone-yasi.html.  These photos were taken in Girrimay National Park about 1 km north of the last car park.