About Sydney Harbour

Sydney Harbour is unarguably Australia’s most iconic estuarine system, it is renowned for its natural beauty and this beauty emanates from the complexity of inlets formed by the drowning of a river valley. Sydney Harbour hosts a diversity of marine habitats and associated biodiversity. More than twice the number of fish species have been recorded from Sydney Harbour (550) than for the entire coast of the United Kingdom (200). This estuary is also one of the most modified estuaries in the word. It is home to a growing urban centre of 4 million people, > 50% of the foreshore has been armoured, 90% of the catchment is urbanized or industrialized and it is infamous with regards to the extent of metal contaminants in benthic sediments and macroalgae.


The Harbour is a drowned river valley which formed during the sea level rise approximately 10,000 years ago. The entrance is approximately 3 km wide with a depth of up to 30 m. From the entrance to Sydney Harbour, the estuary opens up to form Port Jackson, and then extends into three main branches, Middle Harbour to the north and the Parramatta and Lane Cove Rivers extending south, then westward away from the heads. The Parramatta River, Lane Cove and Middle Harbour are major tributaries joining the main estuary channel. The estuary, which has a complex shoreline and topography, is approximately 30 km long, with a surface area of about 50 km2 and a total catchment of 500 km2.

Download the Sydney Harbour – A systematic review of the science 2014 technical report here.


The bathymetry of Sydney Harbour is complex, comprising of dredged channels for shipping, a number of deep holes (approx 28-45m) separated by shoals with depths of 3-5m. The estuary consists of a number of large, shallow, bays adjacent to the main channel which represent a large reservoir for tidal waters. The present-day estuary comprises six environmental or sedimentological units, including the harbour entrance (marine flood-tide delta sands), lower estuary (sands), central estuary (muddy sands) and upper estuary (muds) and the off-channel embayments (muds).

Check out the OzCoasts site to fully explore the complex bottom of Sydney Harbour.

Tides and Currents

The poleward flowing East Australian Current (EAC) off the coast of Sydney provides what is generally considered a nutrient deplete sub-tropical water mass. Current speeds offshore can be up to 1.5 m/s in as little as 65m of water, meaning that water flowing past the entrance to the harbour is continually being renewed and replenished. At a depth of 100m offshore, oceanic temperatures range 12 deg C to -25 deg C in February. Temperatures are generally more mixed in winter ranging 16-20 in June, with salinity ranging 35.2-35.6 psu’s (or Practical Salinity Units).

There are three dominant wind patterns affecting Sydney Harbour, the strongest winds (occurring 17% of the time) are from the south. The most frequently observed direction was from the northeast (22% of the time), although not as strong as the winds from the south. The third most common wind pattern was from the west, occurring primarily during winter (18% of the time).

Subtotal Reefs

Most of the research on natural subtidal reefs in the Harbour has been focused on habitat-forming macroalgae, particularly Ecklonia and Sargassum spp., and/or the organisms these support. Despite the occurrence of macroalgal dominated reefs in the Inner Harbour, most studies have been done in Middle Harbour, near the entrance of Sydney Harbour. In these reefs, beds of Ecklonia support very diverse assemblages of green, brown and red understory algae, sessile and mobile invertebrates, such as sponges, bryozoans, cnidarians, annelids, molluscs and crustaceans, and fish. Of 575 species of fish recorded in Sydney Harbour over 60 % inhabit subtidal reefs indicating the importance of rocky reefs to fish diversity in the Harbour.

In addition, Ecklonia provides habitat to many mobile and sessile epibiota. For instance, the canopy-dwelling sea-urchin Holopneustes purpurascens inhabits the thalli of Ecklonia and the surfaces of kelp fronds often are colonized by filamentous algae, bryozoans and hydroids. Sargassum spp. beds also support diverse assemblages of organisms, particularly isopods and amphipods.

In contrast, despite being one of the main types of subtidal reef habitat in the Harbour, barrens have not been studied. On the open coast, barrens are generally dominated by the sea-urchin Centrostephanus, the turbinid snails Turbo turquatus and Australium tentiformis and several species of limpets, such as Patelloida alticostata, P. mufria and Cellana tramoserica. Urchins and limpets maintain most of the substratum covered by encrusting coralline algae (> 80 %) and keep covers of foliose algae low (< 10 %).

The Rocky Shores of Sydney Harbour

In Sydney Harbour, intertidal shores are usually horizontal and/or gently sloped sandstone platforms which is very similar to most shores in NSW. They have relatively little exposure to waves and have a tidal range of 1–1.5 m. There are, however, some areas with vertical natural rocky shores of about 15-20 m long. Intertidal boulder fields, although not particularly common in Sydney Harbour or in the NSW coast, support a great diversity of organisms living on or under the boulders and are, therefore, an important component of the rocky shores.

Studies have found a total of 127 taxa on rocky shores around the harbour, however with great variability in the diversity of species at the different heights of the shore and from place to place. The low part of the shores in Sydney Harbour can be characterized, however by large covers of foliose algae, the tubiculous polychaete Galeolaria caespitosa and/or the ascidian Pyura stolonifera, while the mid-shore assemblages are generally dominated by the presence of the oyster Saccostrea commercialis, limpets, barnacles and encrusting algae.

Horizontal or gently sloped rocky intertidal habitats in Sydney Harbour also support important biogenic habitats such as oyster beds (Sydney rock oyster, Saccostrea glomerata), the turfing algae Corallina officialis and the tube-forming polychaete G.caespitosa. The distribution of these habitat-forming organisms on intertidal shores in Sydney is naturally patchy, forming mosaics on rocky shores. Such habitats are extremely important and support a great diversity.

Our Marine Plants

In general, mangroves and saltmarsh are restricted to the intertidal margins of sheltered bays and inlets of the Middle Harbour, Lane Cove River and Parramatta River arms of the upper harbour. Conversely, seagrasses are found subtidally in the lower reaches of the harbour.

The spatial extent of saltmarsh in Sydney Harbour has declined significantly since colonisation, and the area mapped from aerial photographs in 2005 was less than 20 km2. It is difficult to identify small patches from aerial photographs, and the actual extent is probably closer to 40 km2. The largest contiguous patch of saltmarsh remaining in Sydney Harbour occurs in Newington Nature Reserve (approx 6 ha), but over 70% of the 757 patches that have been mapped are small (< 100m2) and isolated.

In contrast, mangroves have increased their distribution, being relatively uncommon until the 1870s. Their mapped extent has continued to increase between the 1940s and the 2000s, with the current estimate being nearly 200 km2. In many places in the harbour, mangroves have displaced saltmarsh habitats. Extensive areas of mangroves are today found around Homebush Bay, and the upper reaches of the Parramatta River.

The Open Water

Open water is a major habitat in estuaries and marine embayments. The contribution of this habitat to sustaining biodiversity and ecosystem function is well recognised through its role in the transport, dilution and transformation of dissolved and particulate materials that impact estuarine ecology , provision of habitat for planktonic foodwebs, facilitation of life-stage transitions for meroplankton and fishes , and as corridoors for movements of higher trophic levels such as fishes, sharks and mammals.

The species that occupy these habitats span orders of magnitude in terms of size (microorganisms to mammals; 10-6 to 100 m), and spend at least part of their lifecycle in the water column with little direct interaction with the benthos.

By area, open water habitat is relatively abundant in Sydney Harbour, comprising about 50 km2 or about ten times the area of rocky reef, mangroves, and intertidal flats . This pelagic habitat can be affected by diffuse and point source discharges into the waters of the Harbour. Offshore sewage discharge creates limited detectable disturbance, resulting in minimal accumulation of heavy metals in the tissues of benthic organisms both outside and inside Sydney Harbour. However there has been little investigation of the magnitude and impact of external sources of nutrients and pollutants delivered through the harbour entrance.

Our Beaches and Sandy Bottoms

No comprehensive surveys of soft bottom benthic communities of Sydney harbour have been undertaken, but some indication of the diversity is given by Australian Museum collection records. To date, 308 species of worms, 118 echinoderms, 1375 molluscs and 672 crustaceans have been recorded from Sydney Harbour, although these numbers are certainly a significant underestimate of true species richness since many areas of the harbour are poorly sampled. This compares with Botany Bay (392 worms, 462 crustaceans, 782 molluscs), Port Hacking (195 worms, 179 crustaceans, 607 molluscs) and the Hawkesbury River (328 worms, 304 crustaceans, 703 molluscs) based on Australian Museum records. These other locations have fewer habitats than within Sydney Harbour and would suggest that a considerable amount of benthic diversity remains to be documented. This is an enormous range of animals, however we have little knowledge of many of the other animal groups that can be commonly found in Sydney Harbour, such as sponges, lace corals and cnidarians!

Sydney Harbour Fish Guide

This field guide is an interactive website, enabling you to:

  • Become more familiar with the rich fish diversity in Sydney Harbour and its surrounds
  • Identify specific species that you have encountered while enjoying the Sydney Harbour environs
  • Contribute new information to the guide, including sightings and photographs

The field guide currently focuses on fish, but over time with the help of SIMS researchers and public contributions the coverage will extend to include corals, seaweeds, crustaceans, echinoderms, sponges and other, often remarkably beautiful and bizarre marine invertebrates.

To get the most out of the field guide, register now and start exploring the hundreds of species that are currently documented. A help system for using the guide is also available.

This fish guide is an extension of the Gaia Guide system of field guides.