Tasmanian giant freshwater crayfish
|Dry museum specimen|
|Modelled distribution of
Magenta: species likely to occur
Light pink: species may occur
Purple: translocated populations
The Tasmanian giant freshwater crayfish (Astacopsis gouldi), also called Tasmanian giant freshwater lobster, is the largest freshwater invertebrate in the world. The species is only found in the rivers below 400 metres (1,300 ft) above sea level in northern Tasmania, an island-state of Australia. It is listed as an endangered species on the IUCN Red List due to overfishing and habitat degradation, experts estimate there are less than 100,000 in the wild. The specific epithet gouldi commemorates the naturalist John Gould.
Decaying wood and its associated microbes form the main diet for the freshwater crayfish, though diet varies with age and they also eat leaves and animal flesh, including small fish, when available. A. gouldi is very long-lived, surviving for up to 60 years, it has previously been reported to attain weights of up to 6 kilograms (13 lb) and measure over 80 centimetres (31 in) long, however in recent years the majority of larger specimens are 2–3 kilograms (4.4–6.6 lb). When fully mature, the species has no natural predators due to its large size, smaller individuals can be prey of platypus, river blackfish and rakali.
Biology and ecology
A. gouldi are omnivorous crustacean, primarily eating decaying wood and its associated microbes as well as leaves, insects and animal flesh, such as small fish, when available. Juveniles tend to hide in shallow water where they are less at risk from their large predators including other crayfish, fish, platypus and rakali. Adults have no natural predators and dwell under submerged logs in deep pools where they appear to tolerate each other despite being aggressive elsewhere, the males are territorial and maintain a harem of up to several females. The species is long lived and known to live up to 60 years of age and attain weights of up to 6 kilograms (13 lb), however in recent years the majority of larger specimens are 2–3 kilograms (4.4–6.6 lb).
Colour varies considerably among individuals, with adults ranging from dark brown-green to black or blue. Males can be identified by their larger pincers compared to females. The dispersal and migratory patterns of A. gouldi are largely unknown, a 2004 study found the species appears to have periods of relative inactivity restricted to a "home-pool" for 1-10 days interspersed with movements involving travel over relatively large distances, including one crayfish moving over 700 m in a single night. A. gouldi is known to walk over land.
Tasmanian giant freshwater crayfish have extremely slow maturation rates, with females reaching sexual maturity at around 14 years of age at a weight of approximately 550 grams (19 oz) and males are thought to reach maturity at around 9 years and weigh approximately 300 g. Females mate and spawn once every two years in autumn after a summer moult, producing 224–1300 eggs proportional to its size. Gestation of the eggs takes about nine months, with females carrying the eggs on their tail through winter. After hatching in mid-summer, the hatchlings of about 6 millimetres (0.24 in) attach to the female's swimming legs and will remain with the mother until a few months later in autumn. A long reproductive process means that females spend much of their life attached to their eggs and hatchlings.
Distribution and habitat
Formerly, the species was distributed from the Arthur River in the west and eastwards across northern Tasmania, where it was found in all rivers flowing into Bass Strait, except for those of the Tamar catchment. Today, distribution of A. gouldi is patchy and limited to less disturbed areas. Large declines in numbers or localised extinctions are thought to have occurred in the Welcome, Montagu, Rubicon, Don, Brid, Boobyalla, Pipers, Ringarooma, Duck, Little and Great Forester Rivers and Claytons Rivulet. The species has been introduced into the North Esk catchment (St Patricks River) and the Derwent catchment where populations have become established. A. gouldi inhabit rivers and streams at elevations of approximately 20–300 metres (66–984 ft) above sea level with upper limits of 400 metres (1,300 ft). Approximately 18% of the waterways in which the species habitat is predicted to occur are protected in a formal reserve.
A. gouldi inhabit densely canopied slow-moving rivers of high water quality with high dissolved oxygen content, little suspended sediment, and water temperatures between 5.2–21 °C (41.4–69.8 °F), although relatively low temperatures are preferred. Adults need still, deep pools with submerged decaying logs and overhanging banks to shelter beneath, juveniles prefer shallow, faster-flowing stream habitats with distinct cavities to hide under and higher portions of bolder substrate and moss cover.
Threats and conservation
The principle causes for the population declines of the Tasmanian giant freshwater crayfish has been previous overfishing, continued illegal fishing and habitat disturbance by agricultural, forestry and urban activities. Population surveys and behavioural research are being undertaken in order to be able to provide improved habitat management and protection. Land clearing typically requires approval with a Forest Practices Plan and 10m streamside buffers. Until recently, buffer zones only prohibited machinery operating near waterways with harvesting and burning permitted up to the stream edge. A. gouldi is protected under the federal government Environment Protection and Biodiversity Conservation Act 1999 and Tasmanian state government Threatened Species Protection Act 1995 which prohibit the fishing or handling of the species without permit. The effectiveness of some current conversation efforts are not fully determined and are subject to review in the 2016 Recovery Plan.
The relative ease of catch, slow maturation, infrequent breeding and the removal of young when females are caught make A. gouldi highly susceptible to human fishing pressures. Previous regulations allowed recreational fishing during angling season, with restrictions on the size and number of catch. In 1998, the species was listed as "vulnerable" under Australian law and an amendment to the Inland Fisheries Act 1995 made it illegal to catch or handle A. gouldi without a permit, carrying a maximum fine of A$10,000. Past fishing pressures are believed to have had a significant impact on the populations of the freshwater crayfish, however a degree of illegal fishing is known to continue and has potential to significantly threaten the remaining populations. The level of illegal fishing is not fully known, but is evidenced by prosecutions, the presence of bait lines and anecdotal reports.
Habitat disturbance for A. gouldi includes the removal or destruction of native riparian vegetation, bank erosion, removal of snags, stream flow alterations such as culverts and farm dams, siltation and toxic chemical runoffs.
The clearance of riparian vegetation causes the destabilisation of waterway banks which impacts the burrowing habitats for A. gouldi and increases sediment runoff into waterways. Increased sediment levels arising from agricultural and forestry related land-uses have been correlated with decreased abundances of freshwater crayfish. The increase in turbidity impacts the ability of the crayfish to effectively transpire oxygen through its gills. Sediment depositions arising from upstream forestry operations have been observed to impact in-stream habitat for considerable distances downstream of up to 10km. Loss of riparian canopy cover allows more light to reach the water and has a negative impact on habitat by increasing water temperatures.
While Tasmanian river basins have been found to be generally less impacted by flow alterations than other catchments across Australia, some river basins (such as the Mersey and Pipers–Ringarooma river basins) are heavily impacted by hydro-electric schemes, the use of in-stream barriers reduce the dispersal potential of the crayfish. Water extraction for irrigated agriculture and urban water use is of concern to a lesser degree, however there has been an identified lack of contingency plans in the event of reduced environmental flows in waterways. Anecdotal reports indicate that low environmental flows caused the death of giant freshwater crayfish in several catchments in the north-west and north-east of Tasmania in 2006–2007
In 1994, a large spill from a holding dam at a pyrethrum extraction plant caused a major kill in the Hogarth Rivulet and the main channel of the Great Forester River. Reports from locals and fisheries officers suggested that there was little life left in much of the main channel, and the incident is believed to have severely harmed any populations that were in the waterways at the time of the spill.
The 2016 Tasmanian floods, which killed 3 people, raised concerns about the future for the Tasmanian giant freshwater crayfish after up to 100 carcasses were found washed up along the banks of the Leven River on a property in North West Tasmania, likely caused by the high water flows during the flood. Previous population surveys in the area had revealed already low numbers. There are further concerns that the floods displaced snags from waterways which are a vital part of the habitat for the species.
This article incorporates text from the ARKive fact-file "Tasmanian giant freshwater crayfish" under the Creative Commons Attribution-ShareAlike 3.0 Unported License and the GFDL.
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