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Willow Root Mats: Exotic Habitat for Native Fish

Michael Wilson, Minal Khan, Heath Cameron, Paul Jaskerniak, Chandramali Jayawardana

Centre for Environmental Management, University of Ballarat, PO Box 663, Ballarat, VIC, Aust. 3353
Email: m.wilson@ballarat.edu.au

Native River Blackfish (Gadopsis marmoratus) in agricultural catchments in central Victoria preferred undercut banks as daytime refuges. In Birch Cr and Creswick Cr over 80% of River Blackfish sampled were occupying undercut banks. In aquaria, River Blackfish showed greater preference for artificial habitats that mimicked undercuts to those mimicking hollows and reedbeds. These observations led us to explore the morphology and development of undercut banks in these catchments. Willow lined banks contained complex bank structures that we label ‘overhangs’, to distinguish them from ‘undercut’ banks observed in native vegetation lined channels. The willow overhangs were an aggradation structure, which appeared to develop as willow root mats expanded into the stream channel. Undercut banks were considered a degradation structure that developed as flow eroded the bank below native roots. Below willow overhangs, channels up to 180 cm in length with entrances up to 20cm diameter penetrated into the bank. We introduced alternative habitats, including hollow logs, to pools in a multiple BACI design to test both River Blackfish loyalty to willow overhangs as daytime refuges and whether spawning was limited by hollows. No spawning occurred in alternative habitats and willow overhangs remained the preferred daytime refuge. This case study exemplifies the potentially divisive and complex management dilemmas that arise when an exotic species, in this case a Weed of National Significance, appears to be ‘doing good’ rather than ‘causing harm’ in terms of a target organism. Current willow management and research does not effectively deal with the inter-relationships that have resulted from catchment modification and exotic species introductions in agricultural zones.

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