Torres Strait

The Torres Strait lies between Australia's Cape York Peninsula and Papua New Guinea and was named after the first known European explorer in the region, Spanish navigator Luis Baés de Torres (1606).

The Torres Strait is a shallow (mostly 10-20m depth) body of water 100 km long and 250-260 km wide (east-west) formed by a drowned land ridge extending from Cape York to Papua New Guinea. There are more than 100 islands in the Strait, which stretches over a total area of 35 000 square kilometres. Of these, approximately seventeen are inhabited. Reefs are generally aligned east-west, streamlined by the high velocity tidal currents that pour through the inter-reef channels.

Torres Strait has been described as the most extensive, ecologically complex shelf in the world, offering a multitude of habitats and niches for the great diversity of Indo- Pacific Marine fauna and flora. Eleven species of seagrass have been recorded in the Strait providing a relatively diverse array of species.

Seagrass communities occur across the open sea floor, on reef flats and subtidally adjacent to continental islands. A well defined line of large reefs runs northwards from Cape York, including the Warrior Reefs with extensive seagrass-covered reef flats. Mixed species occur on these flats; most commonly of the genera Halodule, Thalassia, Thalassodendron and Cymodocea. The large expanses of open water bottom are covered with either sparsely distributed Halophila or mixed species (Halodule, Thalassia and Syringodium) communities. Lush Halophila ovalis and Halophila spinulosa communities are also found in the deep waters (>30m) of the south-western Torres Strait.

Although Torres Strait has some of the most extensive seagrass meadows in northern Australia, and supports populations of threatened species and critical fisheries resources, little however is known about the seasonal trends that these meadows undergo.

The dynamics of tropical seagrasses are heavily influenced by weather patterns, flood and cyclone events. Our ability to predict the consequences of such disturbance on different seagrass habitats requires ongoing collection of monitoring information on seagrass habitat condition useful for government agencies and communities to make management decisions regarding these important resources.

To provide this information and to help build stewardship of the islanders over this resource, Seagrass-Watch is raising the awareness of the importance of seagrass and assisting in monitoring the condition and trend of nearshore seagrasses in the Torres Strait.

Within seagrass canopy water temperatures

Average within seagrass canopy water temperatures are generally between 24 and 30°C, with highest mean temperatures in the middle of the year. Extreme within seagrass canopy water temperature (40.5°C) was recorded at HI1 on the 8th May 2007. Mean temperatures at the monitoring sites generally follow a similar pattern.

Thursday Island

Thursday Island is the administrative centre for the Torres Strait islands, which have been part of Queensland since 1872, is located 39 km northwest of the mainland tip of Cape York. "T.I." as it is usually known is only just over three square km in area with a population of around 3500. The traditional name for Thursday Island, which is still being used, is Waibene and is thought to mean "dry place", owing to the lack of a consistent supply of fresh water on the island. The traditional owners of this island and the surrounding islands are called the Kaurareg Tribe. Thursday Island was given its English name by Captain William Bligh after he was set adrift from his ship the Bounty following a mutiny in 1787.

Thursday Island is tropical and experiences a summer wet season and winter dry season. The wet season is usually between December and April with the average rainfall being 1717 mm per annum, the majority of which falls during the months of the wet. Mean daily temperatures range from a minimum of 22.7°C in July to a maximum of 32.1°C in October. During the dry season, the prevailing winds are south-easterly and water shortages occur some of the time. Thursday Island maintains an adequate year round water supply since the dam on Horn Island was built and water is pumped over via an underwater pipeline.

Fisheries resources of the Torres Strait are an important part of the economy and traditional way of life. The Torres Strait's population of around 8000 people are engaged mostly in fishing (i.e. crayfish and mackerel) and a declining pearling industry. Some of the other employment opportunities on Thursday Island include: tourism; light engineering; construction; non-government organizations; local council; Island Board of Industry Services and the State Public Service. Due to the high reliance on fishing in the area, habitats that support commercial and traditional fisheries such as seagrass, mangroves and algae are of critical importance to the region.

Seagrass meadows occur around Thursday Island and in close proximity to port facilities. A fine-scale baseline survey of seagrass habitats conducted in March 2002 identified seagrass as the dominant benthic habitat with 11 species and covering over 1500 ha (Rasheed et al., 2003). Extensive intertidal banks of Enhalus acoroides dominated meadows are found along the foreshores of Thursday, Horn and Hammond Islands and on the reef platforms around Madge and Holmes reefs. Subtidal meadows dominated by C. serrulata or C. serrulata/S. isoetifolium occur in Ellie Channel between Thursday and Horn Islands. The channel between Thursday and Hammond Islands is almost entirely occupied by a subtidal T. ciliatum dominated meadows.

Back Beach (Thursday Island)

Monitoring: ongoing
Principal watchers: Andrew Denzin, Thursday Island High School, Tess Henry & Seagrass-Watch HQ
Occasional watchers: Thursday Island TAFE , Karlina SeeKee, Gillain Grayson, Gilianne Brodie, Marianne Nyegaard
Location: Back Beach
Site code: TI1

TI1 position: S10.58393 E142.20823 (heading 265 degrees)
Best tides: <0.5m (port Thursday Is, 58170)
Issues: occasional gleaning, trampling and boat traffic.
Comments: The seagrass meadow at Back Beach is an extensive intertidal fringing reef top meadow. On its landward edge it is fringed by mangroves which are used as a roost for a flying fox colony. Seagrass species diversity at this site is high. As many as seven species can co-occur in one quadrat. Species that regularly occur within this site are Halodule uninervis, Cymodocea serrulata, Thalassia hemprichii, Cymodocea rotundata, Halophila ovalis and Enhalus acoroides. Syringodium isoetifolium occasionally occurs in quadrats on the seaward edge of the site. Halophila spinulosa has also occurred from time to time. Octopus, crabs and large molluscs flourish at this location ideally demonstrating seagrass meadows as habitats of great biodiversity. Enhalus flowers and fruits are also a common sight at this location.

Status (Dec08):

Seagrass abundance appears to have increased gradually since monitoring was established
Seagrass appears to be showing a fairly typical seasonal pattern of abundance (higher in late summer than mid year) and was higher in 2008 than previous years
Species diversity at Back Beach is high, as 8 species were identified, including Enhalus acoroides, Thalassia hemprichii, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, Halophila ovalis, Syringodium isoetifolium and Halophila spinulosa
Species composition appear to fluctuate seasonally, driven by changes in Halodule uninervis and Cymodocea rotundata

Front Beach (Thursday Island)

Monitoring: ongoing
Principal watchers: Stacee Ketchell, Kristie McNamara, Shakira Weston & Seagrass-Watch HQ
Occasional watchers: Karlina SeeKee, Gillian Grayson, Gilianne Brodie, Carla Wegscheidl, Alan Keeling, Marianne Nyegaard, Lux Foot, Akila Barkus, Ashley Awang-Hewitt
Location: Front Beach across from the Federal Hotel
Site code: TI2

TI2 position: S10.58581 E142.21608 (heading 140 degrees)

Best tides: <0.5m (port Thursday Is, 58170)
Issues: Urban run-off, burial by sediment, trampling, boat traffic, anchoring, and careening of vessels.
Comments: This is a highly disturbed meadow due to its location at the receiving end of many storm water drains. During the wet season the storm water drains deliver large quantities of freshwater and sediment to this meadow. The meadow overall is quite patchy. The upper intertidal zone is occupied by Halodule uninervis and Halophila ovalis. Traversing the meadow seawards, Enhalus acoroides, Thalassia hemprichii and Cymodocea serrulata become more abundant. Within the Seagrass-Watch site, seagrass cover is sparse, with Halodule uninervis being the most abundant species present inshore with Enhalus and Thalassia occurring in the seaward quadrats.

Status (Dec08):

Seagrass abundance appears to have increased gradually since monitoring was established
Seagrass appears to be showing a fairly typical seasonal pattern of abundance (higher in late summer than mid year) and was higher in 2008 than previous years
Species diversity at Front Beach is high, as 8 species were identified, including Enhalus acoroides, Thalassia hemprichii, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, Halophila ovalis, Syringodium isoetifolium and Zostera capricorni
Species composition appear to fluctuate seasonally, driven by changes in Halodule uninervis and Halophila ovalis

Horn Island

The traditional owners of Horn Island are the Kaurareg people who call their island Nguruapai. The Island was given its English name by Matthew Flinders in 1802.

The island has a population of approximately 650 and is 53 km² in area. Horn Island is surrounded by coral reef and mangroves, with intertidal seagrass meadows between these two habitats.

Gold was discovered on the island at the turn of the 20th Century and the island served as an airbase during the Second World War. Today the island is the location of the main airport for the Torres Strait Islands.

Wongai Beach (Horn Island)

Monitoring: ongoing

Principal watchers: Kaiwalagal Indigenous Rangers & Seagrass-Watch HQ
Occasional watchers: Beccy Bowie, Kristie McNamara, Ina Mills, Shakira Weston
Location: Wongai Beach, approximately 150m SW of the main jetty
Site code: HI1

HI1 position: S10.59422 E142.24497 (heading 305 degrees)
Best tides: <0.5m (port Thursday Is, 58170)
Issues: terrestrial run-off during wet season, trampling, boat traffic, located alongside a breakwater, close to an area that was the site of a relatively recent diesel spill.
Comments: The meadow at Wongai Beach is similar to the meadow at Front Beach, Thursday Island, in that it has Halodule uninervis and Halophila ovalis occupying the landward edge of the meadow and Enhalus acoroides and Thalassia hemprichii seaward. However that is where all similarities end. HI1 has an extremely muddy substrate and in comparison has higher densities of seagrass and seeds. Zostera capricorni has also been noted at this site close to transect 3. Recently large numbers of small sea hares were observed on the site attesting to seagrasses’ function as a nursery area.

Status (Dec08):

Seagrass abundance appears to have fluctuated greatly since monitoring was established
Seagrass appears to be showing a fairly typical seasonal pattern of abundance (higher in spring/summer than mid year) and was higher in 2007 than previous years or 2008
Species diversity is lower than Thursday Island sites as only 5 species have been reported:Enhalus acoroides, Thalassia hemprichii, Halodule uninervis, Halophila ovalis and Zostera capricorni
In last 12-18 months, the species composition appears to fluctuate seasonally, driven by changes in Halodule uninervis and Halophila ovalis

Hammond Island

Hammond is a 10km long, 1,660 hectare island situated some 10 minutes by boat north of Thursday Island, in the Inner Group of the Torres Strait. It is a hilly island with mounds of basaltic rocks and noted for its natural, permanent, freshwater springs. Well known as "Kiriri", which is part of the Prince of Wales (Muralag) group of islands, which were traditionally owned by the Kaurereg people.

The Island is divided into four little residential areas. They are the Mission, Beach front, Village and Sebasio village. Water is supplied to the community from a underwater pipeline running from Thursday Island and also from local wells. A powerhouse, established in 1993, supplies electricity to the community. The population of Hammond Island currently is approximately 235.

Corner Beach (Hammond Island)

Monitoring: ongoing

Principal watchers: Hammond Island rangers and Seagrass-Watch HQ
Occasional watchers: Hammond Island Nursery
Location: Corner Beach, near Menmuir Point
Site code: HD1

HI1 position: S10.53990 E142.22532 (heading 110 degrees)
Best tides: <0.5m (port Thursday Is, 58170)
Issues: Terrestrial run-off during wet season, trampling, boat traffic.
Comments: The meadow at Corner Beach is similar to the meadow at Back Beach Thursday Island, comprising of Thalassia hemprichii, Halodule uninervis, Halophila ovalis, Enhalus acoroides, Cymodocea rotundata, and patches of Zostera capricorni. HDI1 has an extremely muddy substrate .

Status (Dec08):

Seagrass abundance appears to have decreased gradually since monitoring was established
Seagrass does not appears to be showing any seasonal pattern of abundance, although the dataset for this site is limited.
In last 12-18 months, the composition species composition appear to have remained stable.