Oysters in the Mangroves

Did oysters disappear from Queensland mangroves or were they never there to start with. There have been some big environmental changes but they occur slowly and go unnoticed. This is the shifting baseline phenomena, where powerful changes occur too slowly for people to see from personal experience but which totally change places from generation to generation.

A document from the NSW government included a photo of an aboriginal man harvesting oysters from the mangroves. Even similar swamps in southern Queensland are oyster free. Did the oysters disappear from Queensland swamps before my time? If so why?

Aboriginal man harvesting oysters (photo: NSW museum)

Oysters thrive in the tropics and are present on the rocky or concrete breakwaters, often as thick crusts. In smaller mangrove creeks where stilt roots descend from high branches and dangle into deep water, clumps of oysters form at the club ends where the roots stop growing. Obviously, it is not a water quality issue that prevents oysters from growing in the mangroves.

Oysters on thriving on rocks inside Cairns Marina

I began to notice broken shells on the mud below mangrove trees and even patches of bark crushed off the place where an oyster was attached. This implicates something that can nip or bite. As the oysters on dangling roots are fine, we can rule out fish. This leaves crabs. Famously, Queensland has mud crabs (Scylla serrata) and these very powerful crabs can crush shellfish. The lumps in the gape of their nippers are even called molars and these lumps serve to crush hard shells. Another crab is the thunder crab, which is so named as you need thunder to get them off if they latch on to you.

Remains of an oyster

Crunch marks on mangrove stilt roots

Thunder crabs (Myomenippe fornasinii) are probably specialised oyster predators. These heavily built crabs are very slow moving but powerful. They are relatively hard to find but live in mangrove estuaries and passages. They are probably the ones that crush oysters from mangrove roots.

Thunder crab in action!

Thunder and mud crabs are probably not entirely responsible for depleting oysters in the mangroves, as oysters are abundant in places that these crabs cannot reach or in more southerly areas where cold water temperatures limit crab activity.  Historical photos suggest that at least some mangrove areas were oyster encrusted and is possible that crabs were eaten or controlled by indigenous people to allow this to happen. At river mouths and lake entrances in NSW, there are carpets of oysters such as can be seen in the top photo in this post.

Oysters on mangroves at Kurnell, Botany Bay, 2019

Oysters carpet the mangrove pneumatophores of the seaward zone

More information on oyster reefs in Far North Queensland can be found in this post

Mangrove Recovery 10 Years after a Category 5 Cyclone

During the early hours of the morning on 6 February 2011, the most powerful cyclone in Queensland’s recorded history struck the coast.  Cyclone Yasi crossed the coast as a Category 5 storm between Cardwell and Tully Heads.   Where it crossed the coast, the storm surge was said to have been 7 m high.  Fortunately, the cyclone crossed the coast in an unpopulated area of swampy Girrimay National Park.  Several kilometres to the south saltwater inundated the Cardwell esplanade and adjacent town up to the ceilings of low set houses.   

Immediately after the cyclone, it is hard to inspect the impacted area.  Low clouds and rain prevent aviation and aerial imagery.  On the ground, power lines are down and often areas are closed to the public to prevent looting.  It was not until the middle of April that I could go in and record the damage.  My role was to assess the level of damage to trees and coastal reserves.  Some of the records that I collected are presented in previous posts including Environmental Damage from Cyclone Yasi  and Legacy of Cyclone Yasi.

By April, the mangroves were completely dead.  Wherever there were deep green mangrove forests, there were now broad grey fringes of dead forest.  Something similar happened in Darwin after Cyclone Tracey.  However, neither the process of mangrove death or the gradual recovery of the mangrove ecosystems were recorded.  Exactly why the mangroves die is not known to me.  I suspect that the mangroves are shaken so hard by waves that the soil releases its poison gasses into the roots of the mangroves and that the trees are poisoned.  Coastal trees growing on thin fingers of sand within the mangroves survived and had partially regenerated their crowns within only a few months of the cyclone.  Whatever kills the mangroves is mangrove specific and extremely lethal.  

Dead mangroves on Cardwell foreshore, April 2011

The best area to observe the long-term impact of the mangroves is near the boat ramp at Meunga Creek, which is about 2 km north of Cardwell town.  In the ten years since Cyclone Yasi, mangrove forests that were approximately 20 m high have recovered to about 5 m tall.  Log piles formed from toppled adn washed-up mangroves there lined the seaward fringe are now gone.  Gaps in seaward stands where trees once stood remain open as there is very little regeneration within the remnants of these once dense stands.  Sonneratia mangrove seedlings formed a band in front of the tall stilt mangroves but ten years on, they remain shrubby and wind burned.

Seaward mangroves in 2014

Seaward mangroves in 2018

Seaward mangroves in 2021

On the sandy levee which runs through the mangroves, there is now a 5 m tall stand of a type of Orange Mangroves - Brugiera parviflora.  These trees can grow taller than 25 m, so they are still young.  After the cyclone, it was difficult to walk here as fallen mangroves were so densely packed that they almost formed a deck that could be walked on.  The maelstrom of crashing waves and grinding logs would have cleared a patch of ground on which these mangroves could regenerate.  Mangrove propagules would have been washed in and could start growing.  

Orange mangrove stand in 2014

Same stand in 2018

Inside stand in 2021

Looking up inside stand in 2021

In the dense stand behind the levee the mangroves were not toppled but died off in droves for reasons unknown.  There was so much fallen timber that very few mangrove propagules would have been washed in.  Also as very few mangrove trees survived, local production of propagules was almost non-existent. Regeneration in these back swamps has a slow start.

The back swamp was mostly dead in 2014

By 2018 some new mangroves had established

In 2021, the mangroves are starting to close the gaps

It has taken 10 years for mangrove regeneration to fill most of the gaps.  Based on gaps created by Cyclone Winifred in 1986, which were visible from the now closed mangrove boardwalk at Cairns Airport, it will take approximately 30 years before the regenerating mangroves start to blend in with the trees that survived.  It might take twice as long for the forest to recover its original stature.

Southernmost Mangroves in the World (almost)

In Australia, mangroves reach as far south as Corner Inlet at the very bottom of Victoria and are almost the southernmost mangroves in the world. The mangroves described in this post, were only 40 km further north at Western Port Bay which is an internationally important wetland. 

I wanted to know what mangroves are like at the extreme end of their distribution and how they compare to tropical mangroves? Which mangrove fauna and flora species are missing and which remain at the geographical limits of existence? 

I found the mangrove ecosystem in Western Port Bay to be as simple as a mangrove ecosystem can get. However, the saltmarsh behind the mangroves is sensational. There is a riot of colour and visual texture.

The saltmarsh stretches to the horizon

Jellybeans (Disphyma crassifolium)

Selligera

Triglochin

Samolus repens

The saltmarsh in early summer is a vibrant carpet of green as it is really a seasonal freshwater wetland that is watered by groundwater. In winter the rainfall is slightly more than summer and the evaporation rate is low, so groundwater rises to the surface. At least twenty species of succulents and grasses contest every patch of space. In the saltmarsh, there is even an understorey of miniature species growing below the canopy of small shrubs and herbs. Between the inner saltmarsh and the mangroves is a wide band of tall succulent bushes belonging to the saltbush family.

Metre high thicket of Tecticornia arbuscula

Close up of succulent stems of Tecticornia arbuscula

Tiny flowering daisies grow between triglochin plants

Other tiny daisies are almost all flowerhead

The best place to see mangroves and saltmarsh in Victoria is at Hastings on the Mornington Peninsula. There is a saltmarsh and mangrove boardwalk which is a few kilometres long and there is also a carefully maintained coastal grassland which also has tracks. Having boardwalks is fortunate as the mangroves are shrubby and as thick as a hedge.

Native coastal grassland

Warringine Park Boardwalk

As this part of the world has cool wet winters, there is a season excess of groundwater that flows through the sandy soil and emerges within the saltmarsh. In summer, the groundwater levels fall and the regular 3 m tides can flood the area with seawater. Hence the plants that are present all need to be tolerant of salinity. There were no salt pans, however, which makes this ecosystem markedly different from saltmarshes or samphire flats in Queensland.

With 3 metre tides, the creeks are quite deep yet narrow

In shallow lagoons, black swans would feed on aquatic plants. Wrens and honeyeaters would flitter around in the succulents. Birdlife reaches its peak on the mudflats and seagrass bed beyond the mangroves and thousands of ducks feed in the shallows.

Black swans feeding on aquatic vegetation (Lepilaena sp.)

White ibis

White-eared honeyeater

Crabs species are very few. There appears to be only 4 species of crab, of which only one species is abundant. This main species is the semiphore crab and it occupies all of the niches from sand flats beyond the seaward fringe to the upper intertidal. It only needs regular seawater inundation to thrive. It is a surface feeder like fiddler crabs. Fiddler crabs are conspicuous by the absence and the Queensland mud crab is also. The other mangrove crabs are opportunistic feeders and would feed on algae, dead insects and possibly mangrove propagules. In the adjacent forest, I found fox poo full of crab shells, so the crabs need to be fast at escaping predators including both native ones like ibis and introduced like foxes.

Adult semephore crab  (Heloecius cordiformis)

Red-fingered marsh crab (Parasesarma erythrodactylum)

Fox poo with crab shells in it

Common fish within the mangroves seem to be limited to yellow-eye mullet and smooth toad fish. Tiny gobies are present in the creeks but as herons are abundant, they zoom of rapidly when they see you. Molluscs were also limited to two species of pea-sized snail that feed on the mud surface.

Smooth toadfish

Salinator snails

Ophicardelus ornatus snails

In the seaward mangroves, the lower trunks have barnacles and support a species of gastropod, which feeds on the algal turf growing at the base of the trunk. The mangroves look healthy although some places had sooty mold infestations on the leaves and scale insects. That is a possible sign of environmental stress.

At the seaward fringe, the Avicennia marina mangroves reach about 3-5 m

Barnacles on a seaward mangrove

Sooty mold on mangrove leaves, white dots are scale insects

Beyond the mangroves are yabby flats and sea grass beds. Wading birds were not present when I was there in November. Wading birds actually take time to move down the Queensland coast so appear in Victoria much later than they do in Queensland. Instead, ducks, herons, cormorants and sea gulls and black swans were present in numbers.

Seagrass beds are exposed at low tide

Ducks grazing in seagrass meadows

The sand flats are soft and you quickly find yourself knee deep in sediment. Dead bivalve shells are abundant within the ground and over the surface. The seagrass beds trap a layer of warm water at low tide and I wonder if this helps the biota escape from the cold for time. Staying upright whilst moving through the soft ground of the seagrass beds was difficult and in the few metres I could survey, I did not find any fauna.

The marine yabbies are so abundant, they made the ground soft

Stingrays bring old shells to the surface when the make hollows

Underwater photos of the seagrass bed

Deeper in Western Port Bay, there must be substantial populations of fish as near the boat ramps, were dozens of large fish skeletons. Giant seagulls pick away at them.

Juvenile Pacific Gull - these things are seriously over-engineered