The Murray Spiny Crayfish (Euastacus armatus) is the second largest freshwater crayfish in the world and holds high cultural, economic and social values. Once widespread and abundant throughout the Murray River system, the species has suffered dramatic declines in range and abundance since the 1950s. Crayfish populations suffer from river regulation, loss of habitat and over-fishing. While commercial fishing for the species was banned in the late 1980s, a recreational fishery remains.

Adult Murray Spiny Crayfish. Photo Scott Raymond

The species is found in a wide range of habitats, from small upland streams to large lowland rivers, showing a preference for clean, fast-flowing water with abundant and complex structural (woody) habitat. Murray Spiny Crayfish plays a key role in the structure and functioning of river systems through the transfer of nutrients within foodwebs and movement of debris within rivers. The cray is long-lived (> 25 years), extremely slow-growing, late-maturing (8-9 years) and has limited mobility. These unique biological and ecological characteristics make the species particularly vulnerable to over-exploitation.

Kind of habitat near Barmah Millewa that Murray Spiny Crayfish love. Photo Scott Raymond

For almost 20 years, the Murray-Darling Basin Authority (MDBA) and research scientists from the Arthur Rylah Institute (ARI, DELWP) have monitored Murray Spiny Crayfish populations in the Murray River and its tributaries through The Living Murray (TLM) and Demonstration Reach programs. Data from these programs have been incorporated within an established crayfish population model to analyse changes in: 1) abundance and 2) changes in sex-ratios in response to different harvest pressures and harvest strategies.

Four harvest levels (0.1, 0.2, 0.3 and 0.4, representing 10, 20, 30 and 40% of crayfish captured and kept) were chosen to represent fishing pressures expected to occur in the wild. Harvest strategies included male only, female only and combined sex catch. Changes in population parameters were used to highlight the utility of current fishing regulations in the management and conservation of Murray Spiny Crayfish.

Corey Walker and friends out in the Arthur Rylah Institute boat catching crays. Photo Scott Raymond

Our research found that:

  • increasing fishing pressure resulted in increased risk of the population declining to a small number of individuals (increased risk of extinction)
  • crayfish abundance declined most rapidly under the combined male and female harvest strategy (current harvest regulations) compared with a male only strategy, and
  • sex-ratios became more biased with increasing fishing pressure as more females were captured and kept.
Female Murray Cray with eggs. Photo Scott Raymond

These findings provide valuable insights into the likely impacts of the current regulations and options to reduce impacts from harvest.  The current harvest regulations for Murray Spiny Crayfish in Victoria, include:

  • a restricted harvest season (July 1 – August 31)
  • a restricted harvest zone (Murray River and tributaries below Hume Weir downstream to Tocumwal)
  • bag limit (2/person/day, max. of 4 in possession)
  • harvestable slot limit length (HSLL – 100 to 120 mm, Occipital Carapace Length: OCL), and
  • a ban on possessing egg bearing females.

Based on our findings we suggest that:

  1. the current harvestable slot limit length (HSLL – 100 to 120 mm, Occipital Carapace Length: OCL) be increased to account for higher risk posed by current harvest regulations and
  2. a harvest strategy which allows only males to be caught and kept represents the least risk to the species persistence.

Watch the video to find out more about this work:

 

This project involves strong collaboration across a range of partners including:

  • Department of Environment, Land, Water and Planning (DELWP) and Arthur Rylah Institute
  • Murray-Darling Basin Authority (MDBA)
  • Goulburn-Broken Catchment Management Authority
  • Wangaratta Sustainability Network
  • Victorian Fisheries Authority, and
  • interested community members who continue to advocate for healthy crayfish communities across the southern Murray-Darling Basin.
Scott Raymond
Author

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