Most rivers in NZ have an evening rise when trout seem to appear from nowhere to feed on insects. More fish seem to rise to flies on the surface after dark than during the day. Why is this so and can it be explained scientifically?

Dr Mike Ladle, a retired UK fisheries ecologist, spent many years studying this phenomenon and shares some of his insights below. It is fascinating stuff.

“The first thing to say is that there are two behavioural components to the activities of invertebrate trout food in rivers. Firstly there are those aquatic insects which emerge from the river as adults. There is often a peak in this emergence round about dusk, probably related to the favourable still air conditions which tend to occur at this time. This is partly responsible for the well documented ‘evening rise’.”

The second component is rather different and less well understood.

Perhaps the most obvious aspect of the rivers and streams is that water and suspended material, including animals, tend to flow in a single direction. This flow has long interested freshwater ecologists as large numbers of living organisms are often transported downstream in the water column. Since many of these creatures have limited swimming ability and the movement is apparently largely passive, the phenomenon has been called drift. Animals drifting in the water column are vulnerable to fish predation and this of course is of interest to anglers.

Drift became the subject of serious study after Muller (1954) proposed that compensatory upstream flight by adult insects was necessary to maintain populations. Study has shown that this does not seem to be the case. Another researcher Tanaka (1960) subsequently discovered that the drift of organisms peaked during darkness (called nocturnal periodicity) and this further stimulated research.

While most descriptions of drift concentrate on larger invertebrates it has also been described in some larval fish and amphibians. There is however, no drift phenomenon common to all species in the river. Drift consists mainly of bottom dwelling creatures and they spend very little time in the water column. Some types are particularly common. For example :-

Insects:

• Mayflies – Ephemeroptera
• Some Flies – Diptera (especially sandflies)
• Some Stoneflies
• Some Caddisflies

Crustaceans:

• Shrimps – Amphipods
• Slaters – Isopods

Scientists have reported many early life stages (especially caddis) in the drift suggesting that it may play some role in dispersal but drift is also common in later larval stages of many insects.The number of drifting animals is usually much greater by night than by day, often several orders of magnitude greater. In the graph below for Baetis (Mayfly) species the drift around midnight is 6,700 of insects per hour versus less than 100 during the day .

 

This phenomenon lead Waters (1965) to propose the following explanations of why animals may appear in the water column.

• Behavioural Drift – This shows a consistent pattern, usually with a pronounced increase after dark. In other words they seem to be entering the drift on purpose.
• Constant Drift – A continuous background of low numbers, most easily detected during the day.
• Catastrophic Drift – Effects of floods and other major adverse events. The image below shows caddis stranded after a major flood on the Tongariro. Catastrophic drift also provides a useful index of human disturbance in some cases.

In many ways behavioural drift is the most interesting form since it implies that drift is acting as an evolved strategy and that it has an ecological ‘purpose’.

Most drifting species show the nocturnal peak apart from the chironomid midge larvae which are usually reported to have no particular pattern and some water mites and caddis larvae that are reported to be day-active.

Nocturnal drift usually peaks just after dusk and then declines throughout the night, sometimes with a smaller peak just before dawn. Less frequently the pre-dawn peak is larger. This lead Muller (1965) to investigate whether light level is linked to behavioural drift. He manipulated the light levels that two species of Baetis received during a 24 hour period. Amazingly he found that:-

• It was possible to concentrate all the drift into a single hour.
• Continuous light resulted in virtually no drift and no rhythm.
• ‘Natural’ drift patterns persisted for 8 days in total darkness.

This clearly shows that light levels are an important factor in behavioural drift.

Links between the onset of drift and darkness provide a number of estimates of ‘thresholds’ below which drift commences. They are usually between 0.1-1.0 lux. A level of 5 lux was found to prevent nocturnal drift completely while levels between 1 and 2 lux reduced the nocturnal peak by 5-10 fold. Bright moonlight (about 0.2 lux) does not seem to reduce drift although the results of studies are somewhat conflicting.

Given this dicovery, it came as no surprise that the next question that researchers focused on answering was why behavioural drift was mainly nocturnal. Allan (1978) thought that behavioural drift occurred after dark as insects were deliberately avoiding predators that use vision to locate prey. He tested his hypothesis on brook trout and found that they only fed on large Baetis and he also showed the greatest size selectivity during the day. Allan predicted that the risk of drifting during the day would therefore be greater for bigger Baetis. This confirmed that nocturnal drift was more common for bigger insects and supported the hypothesis that the insects were avoiding predators.

This size-dependent version of the predation risk hypothesis may partly explain why smaller creatures such as water mites and chironomids don’t show much nocturnal activity”.

Photo: Tim Angeli

Summarising Mike’s discussion above:-

• Many insects drift during the hours of darkness which explains why trout feed so voraciously at the dusk change of light.
• Light levels of 0.1 – 1 Lux are best for nocturnal drift.
• Moonlit nights may or may not affect drift. This may explain why sometimes the fishing on moonlit nights can be erratic. It is likely that if the fishing is excellent then there has been a hatch of fly.
• Nocturnal drift usually peaks just after dusk and then declines throughout the night, sometimes with a smaller peak just before dawn.
• Some larger insects tend to drift during the hours of darkness.

This nicely explains the evening rise and why the hours of darkness can be the most productive time of the day to fish.

One thing for certain is that the big fish, especially trout, abandon caution and feed voraciously after dark when there is a hatch of fly. This is obviously because food is concentrated and it takes less effort to capture large quantities of prey. If your target is a trophy trout then fishing later in the evening, when light levels are between 0.1 – 1 Lux, should increase your chances of success significantly.