Status of the Gunbower Island Fish Community, June 2005: Including Recommendations for Future Monitoring

"November 2005".

Project Number: Flooding enhancement of Gunbower Forest Monitoring Program Implementation: fish surveys - M/BUS/92.

MDFRC item.

Gunbower Island State Forest (Gunbower Forest) represents the Victorian component of the Gunbower and Koondrook–Perricoota Forests Significant Ecological Asset (SEA) — SEAs as identified in the Living Murray initiative. It is a floodplain system of almost 20 000 ha that includes a range of forest types and permanent and semipermanent wetlands. The wetlands are of international significance (Ramsar listed) and are valued particularly for their genetic and ecological diversity — part of this diversity is described by the fish fauna present. Gunbower Island is bounded by the River Murray to the north-east, and an anabranch, Gunbower Creek, to the southwest. It is broadly accepted that the reduction in flooding extent and duration on Gunbower Island, as a result of regulation of the River Murray, has substantially altered the composition of the fish fauna on the island. In an attempt to halt, and where possible reverse, this process, the North Central Catchment Management Authority (CMA) is managing ‘The Flooding Enhancement of Gunbower Forest’ project. A key component of this project is the introduction of a flooding regime that will be beneficial to the fish fauna of Gunbower Forest and Gunbower Creek. The key aims of this report were to develop an appropriate and repeatable sampling methodology for monitoring the fish fauna of Gunbower Island (wetlands and Gunbower Creek), to undertake sampling in accordance with this methodology, and then to report on collected data and the sampling program. The sampling program used was developed from the ecological objectives for fish monitoring listed in A Monitoring System for the Gunbower Forest (Crome 2004). Focused heavily on the relative abundances and evidence for breeding of a number of individual species, these ecological objectives guided the identification of targeted species and the development of testable hypotheses.

Sampling consisted of a one-off survey conducted in June 2005 at 12 wetland and creek sites suggested by the North Central CMA. The majority of fish sampling was undertaken using large and small fyke nets because low water levels and steep unstable banks in the Gunbower system prevented effective electrofishing (except at one site in Gunbower Creek). Recommendations for a future monitoring program of fish fauna of Gunbower Island were largely based on the results of the one-off survey. Eight native fish species and five exotic fish species were collected in the survey. Carp gudgeons were the most abundant native species (13 865 fish), followed by Australian smelt (157), flathead gudgeon (126) and flyspecked hardyhead (120). Crimson-spotted rainbowfish (26) and golden perch (7) were much less abundant and only one Murray cod and one dwarf flathead gudgeon were captured. Gambusia was the most abundant exotic species (1290), followed by common carp (36), goldfish (9), redfin (2) and weatherloach (1). Significantly greater numbers of fish were collected in wetlands (13 879 fish) compared to creek sites (1762 fish). This was due primarily to the large numbers of the small-bodied carp gudgeons and gambusia collected from wetlands. Of the exotic species captured using fyke nets, most were from wetlands: all goldfish, weatherloach and redfin, 94% of total common carp catch and more than 99% of gambusia. Despite these differences in abundances, multivariate analysis did not reveal a difference in community composition between wetland and creek sites. For the large-bodied native species, Murray cod and golden perch, there was no clear evidence of a recent breeding event. No juvenile Murray cod were collected and the smaller golden perch collected may have been stocked. In the absence of a readily accessible and inexpensive technique for discerning stocked fish in the field, this will remain a limitation to assessing evidence for breeding of important angling species. A sampling methodology that targets larval fish on Gunbower Island may overcome the above limitation. In contrast, there was clear evidence of breeding by most of the small-bodied native fish collected on Gunbower Island. Length-age analyses of fish collected indicated that Australian smelt, flyspecked hardyhead, crimson-spotted rainbowfish, flathead gudgeon and carp gudgeon (complex) spawned and successfully recruited during the previous breeding season.

Three of the native species collected in the present survey are currently listed as ‘threatened’ under the Victorian Flora and Fauna Guarantee Act (1998): flyspecked hardyhead, crimson-spotted rainbowfish and Murray cod. Both flyspecked hardyhead and crimson-spotted rainbowfish were collected in reasonable abundance and demonstrated evidence for breeding, suggesting that Gunbower Island may be an important habitat for mid-Murray populations of these threatened species. Power analysis indicated that the monitoring system applied in this survey was not suitable for testing hypotheses that describe changes in the abundance of individual fish species. In the vast majority of cases, a reasonable increase in sampling effort (i.e. netting effort and number of sites surveyed) is not expected to improve total catch consistency or abundances such that power estimates are substantially increased. We recommend, therefore, that the ecological objectives for fish monitoring contained in Crome (2004) be altered so that the effects of a modified flow regimes on the fish fauna of Gunbower Island can be ascertained. We suggest that the ecological objectives should be broadened to consider: a) all small-bodied fish species as one group, b) total native fish abundance and species richness, c) total exotic fish abundance and species richness, and d) using a weighted ‘fish index’ as the basis for hypothesis testing. It is also recommended that future sampling efforts are conducted late in the fish breeding season (ideally March) so that evidence for breeding can be more accurately detected using larval and early juvenile sampling techniques. We also recommend that small fyke nets and boat electrofishing are the most appropriate techniques for sampling small and large fish, respectively. This report describes fully the adaptive management of a hypotheses-based fish monitoring program. Incorporation of suggested changes into a long term monitoring program will provide a sound basis for the assessment of changes in the abundance and breeding activity of the native and exotic fish fauna of Gunbower Forest.