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Port Davey Marine Reserve


Unusual Water Features
Bathurst NarrowsThe tea-coloured waters of Bathurst Narrows
are the result of tannin staining
(Photo copyright Matthew Newton)


Dark, layered water
When freshwater meets saltwater, layering occurs because freshwater is lighter in weight than dense saltwater. Within the marine reserve, this causes the dark band of tannin-rich freshwater to sit on top of the clear saltwater. This phenomenon is known as stratification. In Bathurst Harbour and Bathurst Channel, the freshwater layer is at its deepest – around 4 m – in winter, when rainfall is highest. The narrow zone where freshwater meets saltwater is called the halocline, or salt gradient. Mixing of the halocline disturbs marine life, as some species have adapted to live in either fresh or saltwater.

Reducing boat speed helps avoid mixing the fresh and saltwater layers.

Low nutrient levels  

It is highly unusual for an estuary to have low nutrient levels, as the waters that drain into them usually contain sediments rich in nutrients. In the southwest however, the waters have drained through the nutrient-poor, shallow soils of eroded quartzites. 

The low nutrient levels affect the entire food chain, with less plankton, less fish and less filter-feeding invertebrates, other than at locations with rapidly flowing currents. 

The marine invertebrates in Bathurst Channel and Bathurst Harbour exist in – and rely on – this low nutrient environment. Any increase in nutrients can poison the animals and encourage invasive species. 

Discharging boat sewage and sullage increases nutrient levels. Boat owners are encouraged to discharge outside the marine reserve. 


Port Davey halocline

Port Davey
is exposed to the Southern Ocean’s currents and waves. Its shoreline includes exposed cliffs, islands, sheltered inlets and sandy beaches. These habitats support a variety of seaweeds, fish and invertebrates, with occasional visits from marine mammals. Further inside the port, in Payne Bay and east of the Breaksea Islands, the marine waters are mixed with tannin freshwater flowing out from Bathurst Channel and the Davey River.

Bathurst Channel halocline

Bathurst Channel
connects Bathurst Harbour with Port Davey. The centre of the 12 km long narrow channel varies in depth from 15 to 40 metres. Tannin freshwater enters from Bathurst Harbour, the Spring River and numerous creeks and rivulets, as well as tidal saltwater from Port Davey. The dark waters restrict growth of seaweeds to the top few metres. Below this, a magnificent display of colourful invertebrates attach themselves to the rocky channel walls and seafloor reefs.

Bathurst Harbour halocline

Bathurst Harbour’s shallow waters (5-7 m deep) are strongly influenced by tannins in the freshwater draining
from the Old and North rivers, and myriad creeks and rivulets. Heavier saltwater lays beneath, carried in tidally from Port Davey. In the inky dark harbour waters, phytoplankton and marine plants struggle to survive. Few invertebrate species live on the muddy harbour floor, although heart urchins, sea cucumbers and polychaete worms are common. Dogfish and skates move about the seabed.

Sea pens

Sea pens within Bathurst Narrows

Marine invertebrates

More than 500 species of marine invertebrates (animals without backbones) have been recorded, with many still to be described. Some, such as anemones and zoanthids, bear more resemblance to flowers than animals. Most do not move, anchoring themselves firmly to the seafloor or channel walls, where they feed on other marine animals, or filter-feed plankton and nutrients from the swiftly flowing passing current. 

There are single animals, like sea urchins and seastars, and colonial animals, which look like one animal, but are in fact hundreds of tiny animals living together. There is safety and efficiency in living as a colony. Each animal within a colony has a particular function – some gather food, some strengthen the colony and others even clean the colony! 

Many of the marine invertebrates found in Bathurst Channel are usually found in much deeper and often inaccessible ocean waters.
White-spotted dogfish - Graham Edgar

White-spotted dogfish
(Photo by Graham Edgar)


Fish, sharks, rays and eels

In outer Port Davey an unusual mix of fish species occurs. This is due to the incredibly exposed reefs and some influence from the tannins draining from the Davey River and Bathurst Channel. Fish species usually found around the Tasmanian coast, such as leatherjackets and barber perch, are absent. Instead, some species that do occur here are uncommon elsewhere. 

In Bathurst Harbour and Bathurst Channel, the dark, poorly oxygenated, low nutrient waters attract few fish – with the most common being eels, sharks and skates. One species of cusk eel is unique to Port Davey. The endangered Maugean skate (also known as the Port Davey skate) occurs only in Bathurst Harbour and Macquarie Harbour. Both the Maugean skate and an unusual relative of the ice fish that also occurs here are related to species found in the fjords of New Zealand and South America – land masses to which Tasmania was once connected. Another species is the white-spotted dogfish, a species common in the region but listed on the IUCN Red List as threatened because of its globally declining population.
Red algae

Of the three basic forms of seaweed
(brown, green and red), red seaweeds
are able to survive with the least sunlight,
at the deepest levels.
(Photo by Graham Edgar)


Seaweeds and seagrasses

In the clearer marine waters of outer Port Davey, the typical variety of brown, green and red seaweeds thrive to depths of 10 m or more. However in the darker waters of Bathurst Harbour and Bathurst Channel, where tannins block out light, marine plants are restricted to the top few metres. 

In quiet bays and coves, seagrasses form underwater meadows, providing shelter for breeding fish and invertebrates, and feeding areas for black swans and other waterbirds.

The Landscape

Many people mistake the Port Davey landscape, with its steep-sided valleys and convoluted shoreline, for a glacial fjord. It is in fact a drowned valley or ria. At its centre is Bathurst Harbour, which was once a large plain that flooded as the sea level rose about 7000 years ago with late melting of the last ice age. 

The harbour has since been gathering a deepening layer of ooze, some from the surrounding plains, but mostly from tiny sea animals, like bryozoans. Bathurst Channel, connecting Bathurst Harbour with Port Davey, is a drowned river. Swiftly flowing tidal currents continually sweep the channel floor, exposing resistant rock. 

As you walk across this magnificent landscape, you’ll often notice crunchy white gravels underfoot. These began as sand and mud laid down in shallow seas up to a billion years ago – long before Tasmania looked anything remotely like it does today. Over time, the sand and mud was gradually buried, metamorphosed by heat and pressure into quartzites and schists, then uplifted, folded and partially eroded away. Younger rocks (only half a billion years old!) called conglomerates are made up of pebbly fragments of those earlier quartzites. 

As well as the thin white lines of walking tracks, quartzites and conglomerates appear as snowlike patches on the surrounding mountains and white cliffs along parts of the shoreline.
The soils that blanket much of south-west Tasmania are organosols. Loosely referred to as peat, the thin organosols form from undecomposed plant material that slowly accumulates under extremely wet, humid and cool conditions. 

The organosols support the buttongrass moorland  and heathlands, which are so typical of the south-west. The moorlands have developed over thousands of years, assisted by regular burning by the Aboriginal people. 

The shores around the marine reserve are often lined with tea-tree, banksia and eucalypts. In more sheltered areas where mud slowly accumulates, marshes and marsupial lawns occur – the latter evolved from constant grazing. In the steeper, darker, wetter gullies of the many rivers draining into the estuary, a variety of wet forest species grow, such as dogwood, myrtle, sassafras and Huon pine. 

Wildlife in the landscape surrounding the marine reserve includes wombats, wallabies, a variety of small nocturnal marsupials, burrowing crayfish, and a delightful and colourful chorus of daytime birds, insects and frogs.