At one section in the upper reaches of the Margaret River, approximately 10% of marron collected were identified as hybrids, 40% as Hairy Marron and 50% as Smooth Marron. The lower and middle reaches of the Margaret River have reduced habitat quality and are subject to water extraction, factors which may have influenced the ability of the Smooth Marron to replace the Hairy Marron.
In Western Australia, a recreational license is required to fish for marron and rules apply to fishers including bag and size limits and the use of legal fishing gear (i.e. currently only marron with a 76 mm minimum carapace can be taken and there is a daily bag limit of ten marron per fisher). Females with eggs or attached live young and undersize marron are to be returned to the water. In addition, the use of fine mesh nets, boats or diving gear to catch marron is illegal.
In late 2002, in order to protect the Hairy Marron, the Western Australian Department of Fisheries prohibited recreational marron fishing from the Margaret River above the junction of 10 Mile Brook. Illegal fishing, which targets mature individuals of marron, is also likely to threaten the Hairy Marron when it occurs in the middle and upper reaches of the Margaret River, as poachers make no distinction between Smooth and Hairy Marron.
Introduced fish such as the Mosquito Fish, Gambusia holbrooki, and the Redfin Perch, Perca fluviatilis, are currently found in the Margaret River and other catchments of south west Western Australia. There is evidence that the Refin Perch has a negative impact on marron populations either from increased competition or predation. Yabbies, Cherax albidus, are also common in the south west of Western Australia, having been introduced in the 1930’s from the eastern States. This species competes directly with marron for food and habitat and may also have a competitive advantage due to their earlier sexual maturity (typically less than 6 – 9 months) and more aggressive behaviours.
Water volume and flow are also considered critical to marron. Sediment entering the river has a negative influence on in-stream habitats due to its smothering effect (e.g. it reduces pool depth, increases pool temperatures in summer, lowers dissolved oxygen content, and buries snags and rocky outcrops) and is considered to also adversely affect marron populations.
It is difficult to determine trends in total numbers for the Hairy Marron. Information provided by Austin and Ryan (2002) indicates that at one location on the Margaret River, the proportion of Smooth Marron has increased from approximately 10% of the total marron population (i.e. including both C. tenuimanus and C. cainii) to approximately 80% of the total marron population between 1985 and 1992 (i.e. an approximately 70% proportional reduction in Hairy Marron over 7 years). Sampling at the same site in 1998 showed that no Hairy Marron were present (i.e. a possible 100% reduction over 13 years).