Key words: re-snagging, fisheries enhancement, habitat, cost/benefit, Native Fish Strategy
Trunks, branches and root masses (known as large woody debris or snags) have a significant role in the species composition, primary production and habitat structure of many freshwater systems. Although the extensive removal of snags from the Murray Darling Basin (MDB) in the last 150 years for riverboat navigation, water conveyance and infrastructure protection was recognised at the time of this study as a threat to native fish species; it was not known whether native fish populations would increase following ‘re-snagging’.
Broad aim and specific objectives: The aim of this project was to conduct a scientific trial of re-snagging on the Murray River downstream of Yarrawonga and devise a cost-benefit model to assist river managers with future re-snagging of large lowland rivers.
Methods: Monitoring was undertaken of three native fish – Murray Cod (Maccullochella peelii), Trout Cod (Maccullochella macquariensis), and Golden Perch (Macquaria ambigua) – across three river reaches (Fg 1). One of these reaches was the target of re-snagging efforts(Fig 2), while the other two acted as controls. Monitoring of populations of these species took place before and after re-snagging was undertaken. Experiments were undertaken over seven years between 2007-2013 to enable comparison of changes between each area.
Electrofishing was used to collect information on length frequency data at each site. Age structure was inferred from length frequency data collected, and validated through collection and analysis of otoliths (fish ear bones). Electrofishing and mark-recapture methods were used to measure changes in population size over time, and also provided a secondary source of data to estimate annual population growth. Fish over 200mm were tagged with Passive Inductive Transponder (PIT) tags and a subsample was also fitted with radio-transmitters to provide additional information on fish movement. Fishery dependent data collected through an angler logbook program provided Catch per Unit Effort and species composition data.
Findings: The results from this long-term project clearly demonstrated that native fish respond positively to the addition of snags. All three native fish targeted used the re-snagged sites within 12 months of the re-snagging, and all size classes were represented in those sites. Native fish also responded positively to the density of snags in a site.
The optimal location for snag placements depended on the target species. If re-snagging activities are targeting Murray Cod then maximum benefit is likely to be obtained by re-snagging within 15m of the bank. Although re-snagging this inner area will also deliver benefits for Trout Cod, the re-snagging of mid river areas will preferentially but not exclusively benefit Trout Cod in large waterways such as the Murray River. The creation of habitat in depositional zones of meanders had lower rates of utilisation in comparison to other zones.
Two models were developed to allow cost-benefit analysis to be undertaken, one that examined costs and benefits at the landscape scale and the other at a local scale. The benefits of habitat created through re-snagging are influenced by the size of the existing resident population of fish and the extent of connectivity with alternate source populations.
Lessons learned and future directions: Increased scientific understanding of the importance of snags to native fish and river health has led to significant efforts to reverse the loss of this vital habitat. The most obvious way to achieve this is to put the snags back in the streams. Although conceptually simple, there is a lot of science and engineering behind the placement of snags. Consideration must be given to the type, size, shape and quantity of snags needed, as well as their position and orientation in the river. This level of detail is necessary to ensure that the structures are placed to maximise outcomes for fish communities.
Another component of this program has been the establishment of a data set that will provide estimates of the growth rates and population statistics of Murray Cod, Trout Cod and Golden Perch. This has already been used to assist with advancing the management of Murray Cod in the Basin.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy and The Living Murray initiative.
Contacts: Dr Jarod Lyon, Arthur Rylah Institute for Environmental Research. Tel: + 61 3 9450 8678, Email: email@example.com.