With much of the NSW interior and Australia’s east coast from Victoria to Queensland feeling the effects of heavy rain and flooding, Fisho thought it timely to look at what these significant environmental events might mean for our fisheries, and ultimately our quality of fishing.
Like many fishos, I’ve long thought inland floods in late spring meant good native fishing in future years was assured. But is that the case? It has been widely perceived that springtime floods are required to trigger successful spawning in native fish such as Murray cod and silver and golden perch. So as some NSW western flowing rivers run metres higher than they have for years, can inland fishos expect improved fishing for natives in these waterways in years to come?
A little research soon highlights the fact floods can spell both good and bad news for river-based fisheries. It’s all in the timing. According to biological research conducted on the Murray Darling River system, food washed into rivers from flood plains enhances the conditions for native fish and greater recruitment can result. On the downside, if such floods occur at the same time spawning is underway, fish eggs, embryos and larvae can be swept up in the rising waters, resulting in higher mortality rates.
Scientific evidence also suggests that Murray cod spawn in spring every year, whether floods occur or not. Rising water temperatures and increasing daylight it seems are generally more reliable cues for breeding. According to native fish research, if a spring flood also coincides with this period, however, it often triggers breeding. Warming river temperatures provide ideal conditions for native fish reproduction – 20 degrees C is usually quoted as the threshold for spawning – although there are indications that Murray cod breed at lower temperatures in Victoria.
Golden and silver perch are known to favour rising water levels before spawning will occur. Both species are recorded as conducting long spawning migrations upstream to areas behind flood peaks. Goldens too are known to prefer higher water temperatures to cod when spawning, preferably between 23 and 26°C.
While our native fish are built to deal with times of flood, introduced species such as European carp – a hardy pest that’s proven difficult to eradicate in Australia – apparently don’t fare too well in flood waters. Reports of regular large scale carp kills have been reported as occurring on large rivers such as Sydney’s Hawkesbury in the past. Notably the Hawkesbury is also home to good populations of bass and estuary perch, which thrive in spring floods, providing these fish with easier migration throughout the system.
From this it can probably be taken as read that floods are generally a good thing for future native fishing, so what about our coastal river systems? With many east coast catchments now pouring megalitres of rain run-off into the sea, will long term fishing benefits be had as a result? It’s been a longheld belief that a good “flush out” can do wonders for a river system, especially those that have been largely “stagnant” for some time. The resulting abundance of food that enters the system, or becomes concentrated, as a result of a flood will trigger resident fish to feed or reproduce, while the going is good. Coastal river mouths during a flood tend to be fishing hotspots, especially for anglers who target big predatory fish such as jewies that cruise the edge of muddy water where fresh run-off meets the saltwater zone ambushing baitfish and other food items caught in the mix.
A similar phenomenon forms the basis of barra run-off fishing in the Top End at the tail of the wet season. Many charters rely on a successful run-off period each year for boosting charter numbers. For interstate fishos, timing a trip to coincide perfectly with prime run-off fishing can be difficult.
Fisho’s resident fish biologist Dr Ben Diggles says that in simple terms, floods are often a boon for such predatory saltwater fish.
“The simple story is the floods concentrate fish that were up the estuaries down at the mouths of rivers – making them concentrated and possibly easier to catch,” he says.
Unfortunately though, floods too can bring about disastrous environmental events in the form of acid sulphate run-off, which has been the cause of many large scale fish kills in river systems along the east coast in the past.
“The hard story nowadays is the acid sulphate flushes, as well as sedimentation, herbicides, pesticides, excess nutrients and other substances that come down the rivers today cause algal blooms and kill off fish larvae, oyster reefs and seagrasses. Sometimes we even see kills of adult fish (which means all the larvae are sure to perish)”, said Diggles.
“Once upon a time, rainfall equalled more fish in a year or two’s time, now that is not certain at all,” he concluded.
Have you personally seen the positive/negatives of a flood on your local fishery? Write a comment and let us know your thoughts.
