Margaret River Hairy Marron

The Margaret River Hairy Marron (Cherax tenuimanus) is a large freshwater crayfish that is endemic to the upper regions of the Margaret River in south-western Western Australia. It is one of the largest freshwater crayfish in the world, reaching over 38 centimetres in length and 2 kilograms in weight. It has tufts of hair-like bristles on its carapace and other body surfaces, which distinguished it from the common Smooth Marron (Cherax cainii) and resulted in its recognition as a separate species in 2002. The Hairy Marron has black pincers and an olive-green or brown shell, with areas of red or purple on the underside of females. They survive on a diet of small organisms and detritus and, unlike other freshwater crustaceans, they are comfortable on land for short periods of time and do not need to burrow to escape drought.
The Hairy Marron, together with Gilgie (a smaller freshwater crayfish), have been an important part of the food culture of the local Noongar people for thousands of years. Today, however, its is estimated that the population has decreased by up to 90%, with only 10,000 individuals remaining in the river. Before 2002 there were a large number of Smooth Marron trans-located to the Margaret river in an attempt to boost declining populations. It is now known that the Smooth Marron, together with other introduced species such as the Mosquito Fish (Gambusia holbrooki) and the Redfin Perch (Perca fluviatilis), have a negative impact on Hairy Marron populations, either from increased competition or predation.
Yabbies (Cherax albidus) are also common in the area, having been introduced in the 1930’s from the eastern states of Australia. This species competes directly with the Hairy Marron for food and habitat, and may also have a competitive advantage due to their earlier sexual maturity and more aggressive behavior.  Water volume and flow are also considered critical to Hairy Marron population stability. Sediment entering the river has a negative influence on in-stream habitats due to its smothering effect. Sediment build-up reduces pool depth, increases water temperature in summer, lowers dissolved oxygen content, and buries ‘snags’ and rocky outcrops that would usually serve as shelter. Loss of vegetation is also thought to be a leading factor in population decline.

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