The Barmah-Millewa Forest is a highly significant wetland on the Murray River, which has been significantly altered by the river’s highly regulated flow regime. Environmental flows are commonly targeted at enhancing native fish populations by attempting to improve spawning and recruitment. However, our understanding of the relationship between the flow regime and its influence on the early life history of the MDB’s fishes is poorly known. This project aimed to assess the impact of water management (particularly environmental flows) in the Barmah-Millewa region on fish breeding and recruitment. Specifically, this project aimed to:
- describe the distribution, timing and abundance of larval fish communities in Barmah-Millewa;
- determine approximate spawning periods and peak spawning times
- Establish the importance of a range of off-channel habitat types as nursery habitats;
- determine if flow conditions influence spawning triggers and/or survival of larvae and juveniles;
- improve our understanding of the importance of floodplain inundation and habitats for native fish recruitment; and,
- if appropriate, aid in the modification of existing environmental watering strategies and management to optimise native fish recruitment.
The project was initially conducted over a five year period (2003/2004 – 2005/2006), then funded by a third party resource for two years. Research funded by the Native Fish Strategy re-commenced at the same location in 2010/2011 to capture the results of a predicted large-scale flood.
The Barmah-Millewa Forest region was found to contain a high diversity of native fish and is a significant area for native fish conservation. The majority of native species resided and recruited in a range of forest habitat types (creeks, wetlands, lakes, floodplains and the main channel river), during both flood and non-flood conditions. This study recorded breeding and adult residence of a number of significant species that have either not been recorded in the Forest for some time, or have been recorded in low numbers, such as Unspecked hardyhead (Craterocephalus stercusmuscarum), Murray-Darling rainbowfish (Melanotaenia fluviatilis), Southern pygmy-perch (Nannoperca australis) and Trout cod (Macculochella macquarensis).
This project demonstrated that flooding can influence spawning and recruitment of Golden perch (Macquaria ambigua), Silver perch (Bidyanus bidyanus), Murray cod (Maccullochella peelii) and Trout cod; but their response varies between the four species. Golden perch and silver perch increased their spawning activity in the main river channel during the flood of 2005–06 compared to the previous two seasons. Murray cod and Trout cod appeared not to increase their spawning activity in the flood year, but rather increased the abundance of young-of–year resulting from the flood year compared to the previous year.
A variety of native fish species (generally smaller species) used floodplain habitats to spawn and recruit, however this occurred during both flooding and non-flood seasons. Most native fish were found not to require overbank floods to stimulate spawning, but many species did alter the timing and extent of their spawning period in the 2005 flood season. The flood year did however indirectly increase the abundance of juvenile Southern pygmy perch found in the Forest. Alien Carp, Goldfish and Oriental weatherloach had increased recruitment associated with the 2005 flood event.
Flooding also played an invaluable role in maintaining habitat quality and connectivity in wetlands and creeks for a variety of fish residing and recruiting on the floodplain. Indirectly it also probably provided a boost of nutrients and prey items in returning waters to permanent waterbodies such as the main channel and wetlands.
Large scale flooding in 2010/2011
Flows through Barmah-Millewa Forest in 2010/11 were larger and of a longer duration than had been seen in the Forest for at least 14 years. Flooding in this season also gave rise to a significant hypoxic blackwater event (extremely low dissolved oxygen) which affected the forest, and the Murray and Edwards River’s downstream of the Forest for many months. The blackwater flood event reduced the spawning and recruitment of the majority of native fish species. Species composition of the Barmah-Millewa forest changed with four species (Trout cod, Dwarf flat-headed gudgeon (Philypnodon macrostomus), Southern pygmy-perch and Redfin perch (Perca fluviatilis) ), that had been recorded in previous surveys in the Forest absent in 2010/11. Unspecked hardyhead, which typically make up a third of the total juvenile fish numbers in the river, were virtually absent during 2010/11. In general the blackwater conditions that arose during the flood had a negative impact on native fish in the Barmah-Millewa region, which made any assessment of the resilience of the fish community to drought difficult.
Implications for native fish
The initial phase of this project found that flooding is an important aspect of the life history of some native fish, either as a direct spawning cue, or by increasing the survival of their young. This research has demonstrated that it is possible to optimise and manage flows to improve native fish spawning and recruitment opportunities. The maintenance of the diversity of the Forest’s aquatic habitat types is the key to ensuring the conservation of native species within the Forest. The large number of rare and threatened species found in the forest suggests that the diversity of the Forest’s aquatic habitat types needs to be maintained to ensure conservation of the region’s fish fauna.
The results of the large-scale flooding in 2010/2011 provides a timely reminder to environmental water managers that flooding does not always deliver short-term benefits for native fish populations. Information on the positive and negative flow requirements are still poorly understood for fish and many other organisms, and are critical for the effective management of future environmental watering strategies.
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