Page Content
Characteristics of the Australian Marine Environment
- Marine ecosystems and regions
- Biodiversity and fisheries interactions
- Critical habitats
- Climate effects
Marine ecosystems and regions
Characteristics of Australia's marine environment - based on Environment Australia's Interim Marine
and Coastal Regionalisation for Australia (IMCRA) demersal provinces.
Australia’s marine environment is generally characterised as being low in nutrients and, therefore, low in biological productivity. This is due to low runoff from an ancient landmass, a paucity of extensive nutrient-rich upwellings, and two southward-flowing currents (the Leeuwin Current in the west and the East Australian Current in the east), which bring low-nutrient waters from the tropics.
This general description, however, tends to mask the extremely diverse marine environments around the continent. The map demonstrates this diversity. It is based on the Interim Marine and Coastal Regionalisation for Australia (IMCRA), which describes nine core demersal regions (‘provinces’) of the continental shelf. Between the provinces are zones of transition called ‘biotones’. These provinces and biotones are based on a classification of demersal-fish-species diversity and richness, and reflect characteristics of the climate, which ranges from warm-tropical conditions in the north to cold-temperate conditions in the south. Beyond the continental shelf break the regions have not been clearly defined, and currently seafloor topographic features (for example, flat plain, plateau, and deep shelf) are used to divide this area of Australia’s marine environment into regions. Superimposed on these demersal provinces and biotones (but not shown in the map) are pelagic provinces and biotones. They are based on pelagic species on the continental shelf and, beyond that, on water-mass types classified according to water column properties, including temperature, salinity, and nutrient and silicate composition.
The IMCRA nests smaller-scale (‘mesoscale’) bioregions within these provinces. Their physical and biological characteristics further highlight the wide range of climatic conditions, oceanography, geology and geomorphology, as well as the diverse fish and other animal and plant communities, which make up the marine environment around Australia.
In considering the major environmental issues relating to the fishing industry, this diversity within the marine environment around Australia should not be overlooked. For example it is important not to generalise and conclude that practices regarded as acceptable in one region or fishery will be tolerable elsewhere. The effects of fishing, even from the same fishing method (for example, trawling), may be quite different from area to area around the coast.
Biodiversity and fisheries interactions
Australia has the world’s largest area of coral reefs, the third largest area of mangroves and some of the world’s broadest species diversity of seagrasses, marine microalgae and marine invertebrates. In Australia’s southern waters, 80% of known species are endemic—the result of a long period of geographic isolation and their exposure to severe climatic conditions. In Australia’s northern waters only 10% of known species are endemic.
Little is known of the conservation status of most of Australia’s unfished marine species, although some limited data on birds, mammals and reptiles are available. All seabirds, marine mammals and marine reptiles are protected under the Environment Protection and Biodiversity Conservation Act (EPBC Act).
Of 110 seabird species found in Australia (including offshore territories) approximately three-quarters are locally resident, with the rest being migratory. Most populations of shore birds and seabirds appear to be stable, although thirteen species of albatross are listed as threatened because of their low numbers and exposure to natural and human-induced risks, including fishing.
Australian marine mammals include one species of dugong, eight species of seal and sea lion, and 43 dolphin and whale species. The effects of fishing on the status of the seal and sea lion populations are currently under investigation. Any measures of the impacts of the fishing industry on dugong populations need to take into account the permitted harvests by indigenous Australians.
Thirty of the world’s 50 species of sea snakes are found in Australia. Approximately 50% of the Australian species are endemic. Eighteen species of sea snake have been recorded as bycatch in the Northern Prawn Fishery. Six of the world’s seven species of sea turtles are found in Australian waters and are protected under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Turtles are occasionally caught incidentally to prawns in the Northern Prawn Fishery and by longline fisheries. The introduction of Turtle Excluder Devices in the Northern Prawn Fishery has significantly reduced the incidental catch of turtles. In addition, human impacts on turtles include speedboat collisions, loss of habitat, feral-animal predation of eggs, and subsistence/indigenous harvesting of both adult turtles and eggs.
Much less is known about fishery interactions with microorganisms, algae, invertebrates and non-target fish. It is estimated that marine-invertebrate species in Australia number in the tens of thousands. Several invertebrate species are known to be rare and locally restricted. Examples include cowries in Western Australia and Queensland, and the viviparous seastar Patiriella sp. localised to Tasmania and South Australia. The reservation of habitat is considered a key approach for their protection.
Around 3600 of Australia’s estimated 4500 fish species have been described but, other than for some of those targeted in commercial fisheries, the status of most is not known. Species most at risk from fishing are those that have low reproduction rates, restricted distribution and exposure to loss of habitat. For example, Syngnathidae (sea horses and leafy sea dragons) and Solenostomidae (ghost pipefish) have been identified as being particularly vulnerable to fishing-related impacts. Some populations of target-fish species are also increasingly threatened by overfishing. A number of fish species are now protected in specific areas around Australia or are fully protected.
There is concern that deteriorating water quality is affecting the composition of coastal marine communities. Marine biodiversity may also be sensitive to the introduction of foreign organisms. Whilst it is known that a number of these exotic marine organisms are parasites, or out-compete resident endemic species, the effect of many introduced species is unknown (see Marine Pests).
Critical habitats
Sensitive marine environments, valuable as fish habitat, have become degraded through a variety of polluting and extractive activities, especially near centres of human populations. Habitats becoming degraded around the country include saltmarshes and mangroves, seagrasses, macroalgae beds, and benthic sedimentary communities. Because of their proximity to the land, nearshore habitats are also exposed to influences such as sedimentation, turbidity, and nutrient enrichment. Some impacts from such influences are fundamental to the productivity of coastal regimes, but it is now widely recognised that eutrophication resulting from excessive nutrient inputs has severely affected the quality of nearshore waters.
Another issue of concern is the impact of trawling and shellfish dredging on seabed habitats and benthic ecosystems. The situation in offshore, benthic communities has been less intensively documented than that in coastal habitats. A few deep-seabed sites have been studied—for example, the northwest shelf and the continental shelf adjacent to the Great Barrier Reef, and to a lesser extent the Tasmanian seamounts. Nearer to shore there is still relatively little understanding of benthic communities. Fishing-generated changes to the seabed differ in different areas according to the characteristics of the gear, the site and their interaction. In some environments, change is only observable after the repeated passage of trawls or dredges. Additionally, natural environmental variability may generate significant change, further emphasising the need to adopt precautionary management approaches.
Climate effects
El Niño – Southern Oscillation (ENSO) effects result in lower-than-average rainfall over Australasia and are receiving wide publicity in Australia. Drought and bushfires in Australia and storms and flooding in South America have been attributed to strong El Niño conditions during 1997–98. ENSO events have strong effects on wind patterns, ocean circulation and water temperatures, and it is not surprising that the success or failure of several fisheries is closely linked. The Peruvian anchovy (sardine) fishery, for example, landed over 1 000 000 t in 1996, a La Niña year—when conditions are the reverse of El Niño conditions. With the return of El Niño conditions in 1997, the Peruvian sardine catch was under 37 000 t.
Patterns of El Nino and La Nina occurrences
By affecting rainfall and nutrient runoff, ENSO events influence the productivity of fisheries in northern Australia, such as the Northern Prawn Fishery. ENSO effects are strongest at the equator but are dissipated at higher latitudes. The influences on fisheries along the eastern Australian seaboard are apparent but not clearly understood. Fishers have reported massive shifts in species composition in their catches and in species availability along the southeastern coast. Oceanographic conditions that affect fish abundance and fishing success include water temperatures, current strengths and productivity. Those conditions can affect fishing directly (for example, albacore tuna have a preference for waters of 17–19°C), or indirectly through effects on the abundance of prey species and spawning success. Oceanographic conditions off southeastern Australia are influenced by the interaction of the warm (>16°C) East Australian Current running southwards along the coast, and the cooler (<14°C) west wind drift south of Tasmania. Pulses of warm, tropical water dominate conditions off southeastern Australia. Those strong, seasonal patterns interact with longer-term effects like ENSO events. Tropical water has reached as far south as northeastern Tasmania during La Niña periods (for example 1989–90), resulting in the appearance of tropical species such as yellowfin tuna in the area.[glossary]