Widespread flooding throughout the Canberra-Monaro district and much of the rest of south-eastern Australia in March and April has had a calamitous effect on people's lives, with immense damage to property, livestock, crops, pastures and general infrastructure.

So it is timely to consider also what impact the floods have had on fish stocks.

Much of this is guesswork because there is little documented evidence on the behaviour of some species under flood stress but we can infer some reactions from anecdotal evidence and slim scientific studies.

Where there are extreme water flows, some fish, especially smaller specimens, are likely to be swept downstream, finding themselves in new habitats as the waters recede.

The nature of that habitat will determine whether they survive and prosper, are eaten by predators, whether there is sufficient suitable food and where water temperature and oxygen supplies are suitable.

Some fish later may make their way back upstream to their former habitat but for others the journey is too demanding or progress is blocked, for example when they are in pools isolated from the main waterway.

It depends to some extent on the time of the year. In the ACT, for example, Murray cod and golden perch normally make their way downstream to Burrinjuck Reservoir in Autumn and back upstream in late Winter and early Spring.

A flood in March would simply aid their normal downstream movement; a flood in, say, August would facilitate upstream migration.

A downside is where the fish move downstream from an impoundment such as Googong, which has no fish ladder, and can't get back upstream. The result is a loss to the Googong fishery and possibly an over-population of downstream fisheries.

Floods in Autumn and early Winter also could trigger spawning runs by brown and rainbow trout, especially in Jindabyne, Eucumbene and Tantangara. All received immense inflows and anecdotal evidence suggests there was an early gathering of brown trout near the mouth of the Eucumbene River after the March floods.

Floods are likely to have chemical and physical triggers for the migration and upstream movement could be facilitated by above-average water flows.

Some fish probably resist flood movement better than others. I've seen large Murray cod and golden perch in exactly the same location, usually on a favourite snag, before and after a major flood. Presumably they get behind a rock or a significant snag and resist the flow, possibly resting in the reverse flows that develop behind such obstructions.

Some fish are adversely affected. They can be irritated or killed by silt in their gills or they may develop infections by bacterial and other pathogens that become effective when the fish are stressed.

Some die from physical force. Golden perch in Lake Burley Griffin, for example commonly try to make their way downstream during a flood.

They pour through the floodgates at Scrivener Dam and are hammered against concrete energy-dissipating bollards. Many are killed or injured.

The local meat hunters are quick to take advantage of this, gathering in numbers to harvest the stunned or dead fish, oblivious to bag limits or other considerations.

Flood flows can have considerable effect on water quality. Rainwater, for example, commonly has an abnormal concentration of oxygen and nitrogen, absorbed from the atmosphere as the rain falls.

The oxygen initially would seem to be of benefit to the fish but there's always a snag. Floodwater also brings in large amounts of organic matter, which is quickly tackled by breakdown bacteria.

Bacteria use a lot of oxygen and they are better at getting it from the water than fish, so after an initial flush there is commonly a depletion of oxygen available to fish and they suffer accordingly.

Nitrogen can trigger algal blooms which can have an adverse impact on the environment. Some are toxic to humans and livestock; others grow to great numbers, die, attract the breakdown bacteria and exacerbate oxygen shortages.

How, then, should we be fishing during or immediately after a flood?

Obviously the most active flood areas are out of the question. Extreme water flows are dangerous for boaters and trees, logs and other debris increase that hazard.

Access to the water often is impeded by flooded roads, boggy tracks and inaccessible launching areas. On top of that, the fish commonly are not in the mood to bite.

As the water clears and flows subside, fish commonly rediscover their appetite. Initially, bait is generally the best option. For best results use stronger-smelling bait such as bardi grubs, scrub worms or PowerBait, or noise-makers such as yabbies and shrimps.

Later, as visibility improves, lures become effective. Start with flashy patterns such as spinnerbaits from Outlaw and Mudguts, spoons such as Imp, Wonder and Crocodile, Hogbacks and the like, then progress to noise-makers such as rattling minnows and deep divers.

Surface lures such as the Crazy Crawler, Jitterbug, Nightcrawler and Basscada often work better than subsurface lures.

Fly fishers often benefit from larger surface patterns such as Muddlers, grasshoppers, cicadas or wets such as beadhead and flashback nymphs, beadhead Woolly Worms and Woolly Buggers or beetles and other big flies with a bit of flashy material woven in.

In our part of the world much of the water is too turbid to fish during and for a long time after a flood. Urban and rural runoff is laden with silt and organic debris and lakes and streams quickly become unfishable. That rules out Burrinjuck, Wyangala, Googong and the urban lakes Tuggeranong. Burley Griffin, Ginninderra, Gungahlin and Yerrabi.

The big mountain lakes, however, are a different question.

They generally have a well-vegetated catchment and runoff, after the initial flush, tends to be relatively clear and clean.

During the giant March floods, for example, Eucumbene, Jindabyne and Tantangara remained clear enough for good fly, lure and bait fishing.

The feeder rivers, too, the Thredbo, Eucumbene and Murrumbidgee, also were the first to clear and were eminently fishable within days of the flood subsiding.

So when you are in doubt as to where to go, head for the high country and especially waterways with protected catchments such as in a national park or forestry reserve.