Assessment of fish recruitment in Barmah-Millewa Forest during flooding and non-flooding

Key words: recruitment, native fish, Barmah-Millewa, environmental flows, Native fish Strategy.

Threats and Impacts: The Barmah-Millewa (B-M) Forest is a highly significant wetland on the Murray River, whose natural wetting has been 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 (survival of fishes from eggs to reproductive stage). However, our understanding of the relationship between the flow regime and its influence on the early life history of the Murray-Darling Basin’s (MDB’s) fishes is as yet inadequate.

Broad aim and specific objectives: This research project aimed to assess the impact of environmental flows on fish breeding and recruitment in the B-M region (figs 1-3). Specifically, this project aimed to:

  • describe the distribution, timing and abundance of larval fish communities in B-M;
  • 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.

How the research was carried out: The project was initially conducted between 2003 and 2011 to capture the results of a predicted large-scale flood. Sampling took place during breeding season each year, for water quality, zooplankton and fish eggs, larvae and juveniles.

Murray River near Ladgroves Beach.  (Photo courtesy Alison King)

Figure 1. Murray River near Ladgroves Beach. (Photo courtesy Alison King)

Navigating Lower Budgee Creek floodplain in flood.  (Photo , courtesy  Alison King

Figure 2. Navigating Lower Budgee Creek floodplain in flood. (Photo , courtesy Alison King

Findings: This study recorded breeding and adult residence of a number of significant species that have either not been recorded in B-M Forest for some time or have only been recorded in low numbers, such as Unspecked Hardyhead (Craterocephalus stercusmuscarum fulvus), Murray-Darling Rainbowfish (Melanotaenia fluviatilis), Southern Pygmy-perch (Nannoperca australis) and Trout Cod (Maccullochella macquariensis).

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 the response of each species varied. 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 did not increase their spawning activity in the flood year, however the abundance of ~ 1 year old Murray cod and Trout cod resulting from the flood year increased 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 indirectly increase the abundance of juvenile Southern Pygmy-perch found in B-M Forest. Common Carp (Cyprinus carpio), Goldfish (Carassius auratus) and Oriental Weatherloach (Misgurnus anguillicaudatus) also increased recruitment following the 2005 flood event.

Flooding also improved habitat condition and connectivity, and probably provided a boost of nutrients and prey items.

Large scale flooding in 2010/2011

Flows through B-M Forest in 2010/11 were larger and of a longer duration than had been seen in the forest for at least 14 years. This also gave rise to a significant hypoxic blackwater event (extremely low dissolved oxygen), which affected the B-M 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. 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 were 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 B-M region, which made any assessment of the resilience of the fish community to drought difficult.

Young-of-year Trout Cod  (Photo courtesy Alison King)

Figure 3. oung-of-year Trout Cod (Photo courtesy Alison King)

Key messages/Implications for native fish

Flooding was shown to be important to some native fish, either as a direct spawning cue or increasing the survival of their young. This research has demonstrated that it is possible to manage flows to improve native fish spawning and recruitment opportunities, however blackwater impacts showed that flooding does not always deliver benefits for native fish populations.

Stakeholders and Funding bodies:

This project was conducted by Alison King, Zeb Tonkin and John Mahoney from the Arthur Rylah Institute for Environmental Research, PO Box 137, Heidelberg, VIC, 3084.Telephone: (03) 9450 8600

Contact: Associate Professor Alison King, Charles Darwin University, (+61) 8 8946


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