Fish-Habitat Relationships in the lower Darling River

"May 2010".

Project Number: Darling River Fish – Habitat Investigation - M/BUS/301.

MDFRC item.

34 pages. Finalised September 2010.

This project builds upon an earlier MDFRC study ‘The impact of drought on water quality and fish communities within refuge pools on the lower Darling River’ (Wallace et al. 2008). That project examined the ecology of deep pools and adjacent shallow run habitats in the lower Darling River during periods of low flows (connection) and cease to flow (disconnection) when the only available habitat for native fish in the lower Darling River that remained when the river ceased to flow were deep, isolated ‘drought refuge’ pools. Wallace et al. (2008) observed in their study that that the distribution of large bodied native fish during connection (when the river was flowing) extended beyond refuge pools, and that some species exhibited a preference for shallow run habitats rather than deep pool habitats. The ecological mechanisms influencing the distribution of fish during the connection phase of the lower Darling River hydrological cycle are the focus of the current study. As such, this project examines two questions: 1. Does the spatial distribution and complexity of available habitats (snags, shallow runs and pools) influence the spatial distribution of large bodied fish? 2. Are there particular habitat features which define the character of fish populations in the lower Darling River? Structural Woody Habitat (SWH), commonly referred to as ‘snags’, are known to be important to the ecology of fish for a number of reasons. SWH provides shelter from predation, shelter from sunlight, spawning sites (e.g. for Murray cod) and delineates territories. This study categorized all discernable SWH at those sites studied by Wallace et al (2008). Characteristics examined here were complexity, orientation, depth, width and length, and planform area of each discernable SWH unit (‘individual snag’) at three study reaches on the lower Darling River. Study reaches consisted of macrohabitat types; namely deep pools and shallow upstream and downstream runs adjacent to deep pools. Each SWH unit was categorized within the above SWH features using high definition side-scanning sonar. Overall, 234 individual snags were categorised. Fish were actively sampled by electrofishing, with SWH characteristics recorded instantaneously with sonar. Thus the habitat features selected by each fish at its time of capture was observed. This study limited its scope to large bodied fish species only, as these are readily encountered using electrofishing, whereas small bodied species (i.e. gudgeons, smelt) are more cryptic and not as efficiently sampled using this active technique. A total of 1,098 fish from 6 species were encountered during this study. Bony herring was the most abundant species, followed by Murray cod, carp, golden perch, goldfish and silver perch, in that order. Overall, bony herring showed a preference for open-water, deep pool habitats, and were not strongly associated with SWH. Carp showed no significant habitat preference, inhabiting a broad range of SWH and were also associated with open area habitats, while Murray cod and golden perch exhibited preference for highly complex SWH at relatively shallow depths. The common notion that Murray cod favor deep, sluggish water was not observed here: Murray cod were most frequently encountered at depths of 0.7 – 1.7m in the shallow run habitats: only 4 Murray cod were encountered in the deep pool habitat type, as compared to 171 in the shallow run habitats. The structural habitat characteristics selected by Murray cod and golden perch as observed in this study will inform future fish-habitat rehabilitation works across the Murray Darling Basin.