Adapting to warm water
  |  First Published: March 2006

Successful fishing in the Canberra-Monaro district at this time of the year, usually our hottest and brightest, requires a good understanding of what really happens when the daily temperature rises above 30° and occasionally above 40° and the water is like tepid bathwater for weeks on end.

Redfin are interesting fish. Together with carp, they are the most numerous of the larger species in the region. They provide a lot of fun on lure, fly and bait and are great fish to introduce kids to the fun of fishing because apart from the fact that they sometimes will take almost any bait or lure on offer, they love things that move.

And that suits the kids just fine because have you ever seen a youngster just starting out fishing? All he or she wants to do is chuck the bait out, then wind it in. Chuck it out again, Dad, so I can wind it in! Now chuck it out again! Not good tactics for some fish but just great for redfin.

The problem with redfin, though, is that they are great breeders. They lay eggs around August, before local native or other fish breed, and when the eggs hatch the larvae get the best of the late Winter and early Spring tucker before anything else comes along so they have a great competitive advantage and grow quickly.

Pretty soon, however, the population builds up to such a size that it outstrips the food supply and you have a huge population of stunted fish.

The way anglers deal with this is to catch and kill as many of these fish as possible, hoping to bring the population into line with the available food supply and generate a residual population of larger fish.

This is great in theory but rarely achievable in practice. The fish are just too resilient, too good at breeding and there are just too many to cull.

Nature, however, is much more effective, in that it has produced a virus called Epizootic Haematopoetic Necrosis, or EHN for short, which kills redfin. This virus, first isolated from redfin in Australia in 1985, causes a high level of mortality among the species and is common in all of our local waters as well as being widespread elsewhere in NSW and Victoria.

The virus is present all through the year but develops a full-blown disease only when you get the right environmental conditions – hot weather and a high population density in the host species.

The disease is easy to recognise in the field. Redfin, especially smaller specimens, come to the surface exhibiting pronounced disorientation, twisting and turning and often staying upside down for short periods before swimming back down again. When they die they float to the surface and are washed ashore by wind.

Sometimes the death rate is so high that tens of thousands are seen along the shoreline. Birds preying on the sick and dead fish seem unaffected but the picture is not so rosy for some other fish species. The virus has caused mortality in Macquarie perch, silver perch, trout, especially rainbow trout and mountain galaxias and the decline in these species throughout much of their range in recent years is most probably linked to this interesting pathogen.

To date the virus has been found only in Australia and does not seem to affect humans, so even sick fish are considered safe to eat. It's interesting to note that of all the fish affected, only the redfin seem to bounce back. That's the benefit of being a highly effective breeder and a tough little bastard of a fish.


By this time of year natives capable of breeding in our region, namely Murray cod, small forage fish and residual populations of protected or partially-protected species such as trout cod, Macquarie perch, two-spined blackfish and silver perch, have finished their breeding cycle and are feeding solidly.

Most are in established locations and will stay there right through the warm weather, although occasionally there is a minor upstream or downstream movement if there is heavy rain or prolonged drought.

Having evolved in Australia's hot climate, the fish mostly relish the warm conditions and are at their best during the brighter part of the day. That's when they take lures, baits and flies most readily. When it gets too hot, however, as for example when the temperature goes up into the 40°s, they often lay up in the middle of the day and show a strong periodicity, feeding more in the early mornings, late afternoons and at night. Murray cod in particular like to feed at night during mid-summer.

To catch the fish you must match their habits. Go deep for much of the day, then really deep in the hottest part of the day as the fish seek cooler water and shelter from high light intensities.

It's worth remembering that fish do not have eyelids so they can't blot out high light intensities as we do. That's why they hide in the shade of snags or go deeper in the water column,

With lures, work on the basis that the deeper you go, the noisier or flashier the lure has to be to compensate for the reduced visibility at depth. That why you use spinnerbaits, big spoons, flashy deep-divers, those with the most visible colours at depth or noise-makers such as rattling lipless crankbaits.

If you are interested in what colours are most visible at depth, remember that red turns to a shadowy black first and chartreuse last, so if you want high visibility down deep, pick chartreuse lures. If they are not chartreuse then paint them that colour and see what happens!

At night, use noisy live baits such as shrimps or yabbies which produce small emissions of peizoelectricity each time they move their leg or claw joints and which fish can detect and home in on.

Alternatively, try smelly self-berleying baits such as wood grubs and bardi grubs which ooze attractive goodies into the water when pricked or cut for use.

Lure fishers at night should try big splashy and noisy baits such as spinnerbaits, buzzbaits, fizzers, surface poppers and Spaddlers designed to attract fish from the lower depths. Or the lures can be rigged with chemlights, which we have found to be most effective when placed not right on the lure but on the line 5cm to10cm ahead of the lure or partially hidden inside as in the Custom Craft Lumo Diver.


Trout, having evolved as a cold-water species in the Northern Hemisphere, are the fish most disadvantaged by high Summer temperatures and light intensities.

At lower temperatures of perhaps 15° to 18° they are in their comfort zone where they can most efficiently extract oxygen from the water. As the temperature rises this extraction becomes harder and harder to achieve and the fish naturally are less interested in bait, flies or lures. As the water temperature climbs towards 22° to 25° oxygen extraction can become so difficult the fish become stressed and may die.

To beat the dual challenge of light and temperature the fish adopt several strategies. They may seek out shade, found for example along a riverbank lined with trees, in weed beds, under snags of rocks or logs or even under man-made structures such as bridges. Alternatively, they might seek out more highly-oxygenated water such as the rapids at the head of a pool. In lakes they simply go deep.

When the fish are unduly stressed they are unlikely to feed, preferring to wait for more comfortable conditions. That's why the prime feeding times become early morning, late afternoon and night, with little feeding in the middle of the day.

To catch trout in hot weather you become an early bird or a night owl. Trollers can extend their fishing day by going deeper, with lead-core line down to about seven metres or a downrigger to perhaps 30 metres. Daytime bait fishers can move from shallower to deeper water as the sun gets up and fly fishers can progress from floating lines to sink-tip and full-sink lines.

It's all to do with depth but remember that under the most extreme conditions it is likely that nothing will induce the trout to bite. That's why you will see me during the heat of the day snoozing under a tree, waiting for the prime time to fish. A good lesson to take note of.

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