Cherax Tenuimanus or Marron were originally identified as a single species by Smith (1912). Two species of marron have now been formally recognised and described. The original species name ‘tenuimanus’, which once covered both species, is now restricted to the species that is endemic to the Margaret River, the Hairy Marron. The other, more common marron species, the Smooth Marron, now has the species name Cherax cainii. Cherax Tenuimanus are large freshwater crayfish which can grow to more than 380 mm in total length. They are one of the largest freshwater crayfish species in the world with specimens having been recorded in excess of 2 kg. The Cherax Tenuimanus or Hairy Marron has tufts of hair-like bristles on its carapace and other body surfaces. While adults are readily identified from the Smooth Marron (Cherax cainii), hybrids do occur and are more difficult to identify.
Cherax Tenuimanus are crepuscular or nocturnal, being most active for a few hours after sunset, especially around a new moon when water temperatures are above 18°C. Marron take at least two to three years to reach sexual maturity and are brooders with limited dispersal ability. Due to this breeding strategy, movement of both adults and juveniles is limited and is estimated to be in the order of several hundred metres with favourable conditions required (i.e. summer flood events) to assist the downstream movement of the species.
The Cherax Tenuimanus or Hairy Marron only occurs in the Margaret River in the south west of Western Australia. The species is currently listed as rare under the Western Australian Wildlife Conservation Act 1950 and is managed as 'critically endangered' (according to IUCN criteria) by the Western Australian Government. The upper reaches of the Margaret River, which contain the majority of the species’ known sites, are within managed State forest.
An interim Recovery Plan is currently being prepared for this species. In addition, the Western Australian Department of Fisheries manage the recreational marron fishery and have monitored marron populations for a number of years. Long term surveys (1970 – present) have been carried out by the Western Australian Department of Fisheries which show a decline in the abundance of marron in general and a decline in the range and abundance of the Hairy Marron in particular.
The decline in the Hairy Marron is considered to coincide with the introduction of the Smooth Marron to the Margaret River in the early to mid 1980’s. Monitoring of the species is based on surveys of recreational fishing licence holders and some field survey and therefore indicates a general trend of reduced abundance rather than absolute numbers. In late 2002, in order to reduce the impact of recreational fishing, the upper Margaret River, from 10 Mile Brook junction, was closed to marron fishing by the Western Australian Department of Fisheries.
In waterways of the south west, little is known of the ecology of marron and habitat requirement differences between the Hairy and Smooth Marron species. Habitat requirements may be similar for both species. Information on the current distribution of the Hairy Marron indicates that the species requires relatively good quality water and a diversity of habitat structure (e.g. they generally prefer sandy areas, particularly where organic matter accumulates and access to shelter and refuge sites) and may struggle to persist in disturbed habitats.
Smooth Marron appear to have a competitive advantage over the endemic Cherax Tenuimanus or Hairy Marron and concern exists that if competition from, and hybridisation with, the Smooth Marron remains unchecked, it is likely that the Hairy Marron will be completely replaced in the wild by this species. Cherax Tenuimanus brood stock is currently held at the Pemberton Freshwater Research Centre with the aim of producing individuals for reintroduction into the Margaret River.
The Cherax Tenuimanus is naturally restricted. The species is currently known from only eleven sites along a section of the Margaret River and occurs in an area less than 50 km in length. The majority of the species’ population is restricted to pools located in the forests of the upper reaches, within an area of less than 10 km2. Very few Hairy Marron have been collected recently from the middle or lower reaches of the Margaret River, where the Smooth Marron is the predominant species recorded. The Cherax Tenuimanus persists within the upper reaches of the Margaret River surrounded by forested land and is now thought to be mainly restricted to the upper reaches of this system. This section of the Margaret River (i.e. the upstream State Forest areas) support the last significant wild populations of the Cherax Tenuimanus and the relative abundance of Hairy Marron compared to Smooth Marron is up to almost 50% within some pools of this section of the Margaret River.