FOR those who thought that the only problem with the Murray-Darling system was drought and that the only recipe required to fix it was to “just add water”, recent rains have shown this to be wishful thinking. At the time of writing the first decent rains in a long while have been entering the system, but all this has done is fuel yet more algal blooms (and in some areas blackwater events) that have again killed off entire fish populations over many kilometres of river and adjacent waterways billabongs, wetlands and dams. When waterway systems like the Murray-Darling are so heavily modified by water abstraction, river regulation and agriculture in adjacent catchments, and while other parts of our river catchments are also heavily damaged by bushfire, our rivers need more than just water, and more than just environmental flows. They also need water with oxygen in it!
Of course, water without oxygen is fine for watering crops, cattle and other livestock (provided the algal blooms are not due to cyanobacteria which generate microcystin toxins, which can kill animals that drink contaminated water), but a living river system requires much better water quality than that. In order to support an aquatic ecosystem that ends with (hopefully native) fish at the top of the food chain, sufficient oxygen is not negotiable, it just has to be there, which is why governments and environmental groups such as OzFish Unlimited were quick to employ “last resort” interventions in the form of installation of aerators in several areas of the Murray-Darling system last year to try to establish some isolated areas suitable for the surviving fish. But oxygen is only the start of what fish need. In order for fish populations to recover from these very sad and unprecedented fish kills in the Murray-Darling and other rivers damaged by bushfire, they need successful spawning from broodstock fish, a healthy planktonic food chain with algae and zooplankton to feed the larval fish (in the absence of pollutants – pesticides, herbicides – which can damage the plankton and the fish larvae themselves), and the juvenile fish then need access to nursery habitat on order have a place to live, find food and hide from predators like birds or other fish.
In the short term, after loss of so many very large, old broodstock fish, we may indeed have to rely near totally on the efforts of dedicated fisheries staff and fishing volunteers who have saved broodstock native fish from algal blooms, desiccation and the ashes of bushfire runoff and got them into various hatcheries around the country.
From these institutions, we can only hope that future breeding and restocking programmes will be successful to provide our rivers with some native fish recruitment in the hope that the affected fish populations will rise phoenix-like from their current ashen state. But even having water in the rivers, sufficient water quality and assuming that the various difficult environmental flow and fish passage problems can be solved, restocking these rivers with hatchery reared fish will in itself not be enough.
Unfortunately, degraded river systems with damaged catchments and cleared bankside vegetation tend to be turbid, heavily silted, and generally lack the habitat required for small bodied native fish species to thrive. Conventional fish habitat restoration efforts have usually targeted the needs of larger fish by replacing “large woody debris” back into the water as snags, installing “fish hotels” and other such large structures that benefit adult native fish.
However, while these undoubtedly provide habitat for adult native fish, recent research suggests that smaller fish species and juveniles of larger native species may not benefit very much from these large scale fish habitat restoration efforts.
In a paper entitled “Habitat preferences and habitat restoration options for small- bodied and juvenile fish species in the northern Murray–Darling Basin”, fisheries researchers from Queensland studied the habitat preferences of seven native fish species, including juveniles of Murray cod (Maccullochella peelii), golden perch (Macquaria ambigua), and silver perch (Bidyanus bidyanus), as well as adults of four species of smaller fishes (rainbowfish (Melanotaenia fluviatilis), olive perchlet (Ambassis agassizii), on-specked hardyhead (Craterocephalus tercusmuscarum fulvus) and carp gudgeons (Hypseleotris spp.).
The researchers gave each fish species a choice between sandy habitat and submerged plants, emergent plants and rocky rubble. The majority of fishes (Murray cod, golden perch, carp gudgeon and olive perchlets) preferred structure over open sandy habitat, but the silver perch, un-specked hardyhead and rainbowfish did not avoid open sandy habitats. The Murray cod preferred rocky rubble habitat over all other habitat choices, which suggested that restoration efforts using complex rock piles would provide excellent nursery habitat for juvenile Murray cod, while golden perch and carp gudgeons also preferred the rocky habitat.
The research also found that given a choice between a bare sandy bottom and areas with submerged or emergent plants, only silver perch did not prefer the plants, suggesting that in turbid water situations (with poor sunlight penetration which prevents underwater plant growth), restoration using rocky rubble or floating attached plants in shallow areas could be viable restoration options in areas too turbid to establish submerged plants. The researchers concluded that it is vital to think through the entire nursery habitat needs of small-bodied baitfish as well as the juveniles of the usual angling target species throughout their entire lifecycle (not just provide homes for the adults) whenever habitat restoration interventions are required. And they will be, on a very large scale, if we are to see the re-emergence of native fish populations in many parts of inland Australia from their current nadir.
