Assessment of snag-habitat and fish community relationships in the Pomona Priority Habitat Reach, Lower-Darling River

"March 2010".

Project Number: Aquatic Ecological Communities (Pomona Reach) - M/BUS/262.

MDFRC item.

39 pages.

This project examines two questions: 1. Do the density, spatial distribution and complexity of available habitats (snags assemblages) influence the spatial distribution and abundance of large and small -bodied fish in the Pomona Priority Habitat Reach (PPHR) of the Lower-Darling River? 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 within four replicate 1 kilometre reaches of the Lower-Darling River near Pomona classified as containing Low (2 X 1km reaches) and High (2 X 1km reaches) snag densities. Characteristics examined here were snag density (abundance), complexity, orientation and planform area for each discernable SWH unit (‘individual snag’) at the four study reaches at Pomona and Bellevue on the Lower-Darling River. Each SWH unit was categorized using high definition side-scanning sonar. Overall, 145 individual snag units were categorised across the four reaches. The structure of fish communities occupying the reaches of differing snag density were examined by electrofishing throughout each entire reach in conjunction with passive sampling through the deployment gill and small fyke nets.. This suite of techniques was considered appropriate to sample the full range of species across all size classes within each assemblage. Additional targeted electrofishing at 10 snags within each reach was conducted to assess the SWH features in each reach selected by individual fish at the time of their encounter. This sampling regime was conducted during spring and autumn on two occasions over the two year study period (2008 and 2009). A total of 3,742 fish from 13 species (9 native and 4 exotic) were encountered during this study. Bony herring was the most abundant large bodied species, followed by goldfish, carp, golden perch, and silver perch, in that order. Additionally, three redfin and one freshwater catfish were recorded. No Murray cod were recorded throughout this survey. Carp gudgeon was the most abundant small bodied species, followed by Australian smelt, un-specked hardyhead, Gambusia, flathead gudgeon and Murray-Darling rainbowfish, in that order. Higher densities of SWH had some influence on the structure fish communities in the Pomona Priority Habitat Reach (PPHR). In particular, the abundance of most species was greater within the high snag density reaches as compared to low snag density reaches. Despite the differences in abundance of snags between high and low density reaches, the characteristics (complexity etc.) of snags was shown not be significantly different overall, thus the density of snags was considered more influential in the structure of associated fish assemblages present within study reaches. Overall, golden perch and bony herring were more abundant in the high density SWH reaches relative to the corresponding low density SWH reaches. Carp populations were not significantly different in their relative abundance between high and low snag density reaches.

Both native and exotic large bodied species appeared to exhibit a preference for highly complex snags oriented generally perpendicular to the direction of flow, which provide a variety of refuges and flow-relief sites, as well as locations from which predators can ambush prey. These snags represented the majority of SWH units in the high density reaches, being the general characteristics exhibited by fallen trees. Patterns of abundance exhibited by native and exotic fish species across high and low snag density reaches during this study suggest that the native fish community within the PPHR is influenced by the availability of snags, and also by factors not observed during this investigation. For example, while Murray cod are known to be present in the study area, they were not recorded in the electrofishing of netting surveys, despite the number of snags per kilometre and the overall character of snags from the high snag density reaches resembling other sties in the Darling River where Murray cod are abundant (Sharpe et al. 2009). As such, the number and character of snags observed in the PPHR was not considered to be limiting the Murray cod population there, rather the population is considered to be suppressed. While there are numerous factors likely to influence the status of large bodied fish populations in the PPHR, those considered most relevant and worthy of further consideration are: A general lack of fast flowing habitats A general lack of heterogeneity of flowing habitats A general lack of temporal flow variability Historical overfishing and angling pressure. An enhanced understanding the causative factors suppressing large bodied native fish populations in the PPHR is prerequisite to maximising future rehabilitation efforts.