Saturday, 25 April 2015

Cairns' Disappearing Mudflats

The famous Cairns mudflats are almost no more.  What was the muddiest of places and a magnet for migrating wading birds is becoming a sand flat.  Some people think that the Council’s efforts to make a sandy beach is responsible for the gradual loss of this internationally important shorebird feeding area.  Certainly the amount of sand being placed on the esplanade should be investigated, however other factors might be at play including cyclones and coastal processes. 

Cairns' fine man made beach
Cairns is one of a very few places where you can sit on a boardwalk and view a dozen or more species of usually shy wild birds feeding only metres away.  The birds can’t afford to be shy as there are few good feeding grounds in either direction.  I commissioned a study into one of the major estuaries (Port Musgrave) on Cape York Peninsula and expected it to be ideal wader habitat.  What we found was that when the waders are migrating, there are only high tides so the waders cannot feed there.  It takes a special set of circumstances to create good wader habitat. 

A wading bird less than 10 m from my seat and feeding in the small remaining muddy area near the birdwatchers lookouts.
The rich wader habitat in Cairns may also be partly of human creation.  The shape of the coastline has been changed due to developments like the Pier and this has created a poorly flushed area which is great for accumulating mud deposits.  Long before the Pier arrived, Fogarty Park which juts out into the estuary had been reclaimed.  In the Cairns hinterland, land clearing was at its peak and there was much less concern for soil erosion and the sediment supply coming down the Barron River, Trinity Inlet and even Saltwater Creek near the airport would have been much greater.  Saltwater Creek now only drains urban areas, but when I was young this area was cane fields and would have yielded more sediment.  The local Port Authority pulls out all the mangroves which are attempting to colonise the mudflats just off the esplanade.  This creates an open muddy habitat in an area that would normally be a dense mangrove swamp.  Harbour dredging in times past would have had less consideration for sediment plumes and many think that these are the source of the Cairns mudflats.  Historically, Cairns is said to have had a sandy beach.   However in my imagination the sandy beach would have been above a mudflat as there is in Cardwell today.  This was the case in aerial photos from 1952.

Another wader feeding on the lumpy mud-scape near the wading pool - where the mud builds up today
I have been trying to establish a time series photographic record to make the changes over time visible.  Unfortunately, nobody expects a fact of life such as the Cairns mudflats to disappear, so there is not much in the way of old data.  However it is obvious that at least in some places, the mudflats are becoming sandy as you can now walk out a long way without sinking up to your knees.  The question is where is the sand coming from and/or where is the mud going.  Similar changes are also occurring at Ellie Point which extends from northern end of the Esplanade, If massive changes have been taking place even quite far away from the sandy beach where the council dumps sand, it suggests other factors are involved. 

Dredge working in the shipping channel - note lack of life on exposed tidal flats which occupy the bottom half of the photo .
An almost matching view from 2005.
Telephoto view of the flats - the black object is a beer bottle (2014)
The same area in 2005 was seething with life 
The full story is complex and I am only undertaking informal investigations. It is probable that there is less mud coming out of the rivers.  Even the sea on the Cairns Northern Beaches is more blue than brown these days.  It used to be the colour of milk coffee in rough weather.  Sand supply has also increased due to banning of sand mining in the Barron River and at Ellie Point.  This supply which is estimated to be 23 000 tonnes a year is beginning to pour around the tip of Ellie Point. The renewed supply of river sand could already be making a minor contribution to the sand supply on the esplanade.  Cyclones may have had a bigger impact.  They can wash away vast volumes of surface mud and leave only the heavier sand.  Several big cyclones have passed close to Cairns in recent years.  It is the swells that they generate that do the damage and these swells can change our coastline even when the cyclone is hundreds of kilometres away by creating inshore currents and by lifting sediment into the water column.  Cyclones have always been around but there has been less time between large events recently.

In 2005, seagrass beds were clearly visible from the Esplanade (the dark band).  They appear to have disappeared from most of the Cairns foreshore.
The gradual loss of large areas of mudflat from Cairns probably has multiple causes and it would take a lot of work to apportion blame.   If you are interested in helping to figure out what is happening leave a comment.  All the photos here were taken from near Muddies Playground.  




Saturday, 4 April 2015

Guide to Fiddler Crabs of Queensland

If you are interested in learning about the most visible fauna of the mangrove ecosystem, I have put together a preliminary guide on Fiddler Crabs, which can be downloaded from the link below.

Draft Guide to Fiddler Crabs of Queensland.

It provides a series of colour photos with text about the features that can be used for identifying species.


Friday, 3 April 2015

Same crab a year later


Digital cameras are opening up new avenues for understanding our world.  Today I discovered that I can track individual fiddler crabs using the pattern on their backs.  I first photographed this crab on 18 April 2014 and then yesterday on 4 April 2015.  This is clear evidence that some fiddler crabs live at least a year.  
Adult male Uca triangularis in April 2014

Same crab in same place in April 2015

The ground is a little different between photos too.  In the first photo, the ground had just been scoured by a flood associated with a cyclone.  In the second photo, sand has returned to the substrate and has covered up the exposed roots.

Fiddler Crab Predators

A lot of people have been looking at the “who eats fiddler crabs” post and I was starting to wonder if they had gone on the Paleo Diet.  When I wrote the first post, I was not sure which of our large birds had been dining on fiddler crabs but now I have collected more evidence.  Today on a small mud bank 10 m across, I found 4 detached nippers and bird footprints.  I would say a Beach Curlew (Esacus magnirostris) has been at work.
Uca dussumieri claws
The fiddler crab claws broken off by the bird - collected by me
There are more than a dozen large to very large birds in my area and some smaller ones with strong beaks, so working out who eats crabs is quite hard.  They very rarely eat crabs whilst you are watching and Beach Curlews are notoriously shy.  However I did see one giving something a really hard time one day and went to investigate.  I found the remains of a large sentinel crab.
Sentinel crabs probably put up a struggle and would be a bit bony to swallow, so the bird processes the crabs first and removes most of the hard bits.  It is more common to find the scene of the carnage than see it happening.

Esacus magnirostris, Beach Curlew eating a crab
Beach Curlew working on the shell of a sentinel crab
Sentinel crab remains
The actual crab shell from the Curlew's mouth
Scene of brutality against a crab, see the legs and shell.

Finally, I came across some Beach Curlew poo on a disused road that runs through the mangroves.  The curlews sleep there.  It is clear that crabs are a major dietary item.  When they eat soldier crabs it looks like the road runner in real life as they run the crabs down.  I don’t know how they catch sentinel and fiddler crabs as these crabs are fast and are always near a burrow.  Stealth is the more likely approach and I have seen Beach Curlews on the mud bank but they just stand there like they are waiting for the tide to go out.  They didn’t seem to be in attack mode.

Beach Curlew poo, mostly crab shell
On another web page someone said Whimbrels eat fiddler crabs.  Whilst this is possible and a Whimbrel was feeding near the mud bank, I would want to see more evidence before adding Whimbrels to the list.
Mouth of Richters Creek, Cairns - sort of place where birds catch fiddler and sentinel crabs
Uca dussumieri fiddler crab - the biggest of east Australia's fiddlers
Sentinel crabs in numbers on the sloppy mud.