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  • Writer's pictureTony Entwistle

When the Going gets Hot!

Improving Success When Water Temperatures Climb

The importance of water temperature on trout feeding behaviour can not be understated. Anyone who has attended one of my First Steps classes will also recall, that along with the availability of food, a trout's feeding activity and ability to metabolise that food is significantly controlled by water temperature.

The survival range for a brown trout is from 0°C – 25°C.

- Growth limits for brown trout are from 4°C – 19°C

- Optimum growth range is 7°C – 18°C

- Optimal temperature for maximum growth is 14°C

- Stress temperature range is 19°C – 25°C

- Lethal temperature range is 25°C+

Brown trout feeding behaviour slows down substantially after 18°C and on many rivers, will come to a halt around 21°C. Through November into early December, water temperatures were cool enough that feeding activity didn't fire up until around 1100 - 1130. However, from now on you should be looking to hit the water early, to avoid missing the start of the feeding period, which is when the trout are hungriest and the most aggressive. As they fill their stomachs feeding activity declines as the day progresses. By early January, as river levels drop and temperatures climb in the hot weather, the best fishing period will be over by 1400 - 1500.

The following graph, collected from over 2000 trout landed in the upper South Island, illustrates the prime fish-catching temperature zone, and how the number of trout caught drops away quickly at temperatures over 18°C.

During a hot summer (like this one), from early-January right through until mid-March, you will need to adjust your fishing behaviour to take into account the limiting effects of water temperature. This will have a huge impact on your success or failure, especially on many of our more accessible local rivers here at the top of the South Island.

On hot, clear days water temperatures on many local rivers will vary in range through as much as 4 - 5°C ... as illustrated by this recent temperature graph of the lower Wairau River. The Marlborough District Council provides some excellent information on its website, including river flow and temperature data, at

This graph shows the recent daily rise and fall from January 10th to mid-morning on January 17th. Note that the coolest temperature each day is around 0700 - 0730. The maximum temperature shift during the week was 4.8°C and on three days the temperature peaked at over 23°C (the level that triggers a complete shutdown of feeding for brown trout).

For the first five days, the temperature started at a low of around 18.5°C, which is already nudging the stress-temperature range of 19°C – 25°C. My earlier catch records show that the opportunities to catch fish are almost gone by the time the temperature hits 21°C, which was reached by midday on four out of five of the first five days. The prime opportunity for catching trout during this week was the four to five-hour window from just after dawn, to midday at the latest. Those anglers who weren't on the water until mid-morning or after lunch, were probably better to have stayed at home and mowed the lawns. The two prime days to get excited about getting on the water  this week, were the 16th and 17th, when a weather change on the east coast dropped the temperature range back into the 17.3°C - 20°C range.

The Motueka River also experiences very similar temperature profiles. Wouldn't a temperature logger on the Motueka River be a wonderful asset for anglers😊? Interestingly, Motueka trout seem to have adapted somewhat to slightly higher average water temperatures than on many other rivers. From experience, I have found the Motueka River to produce a few more fish at slightly warmer temperatures, at times producing fish right through until 22°C.

On the 13th of January, (the hottest day on the Wairau graph) a friend and I arrived on the Motueka Rv to find that the starting water temperature at 0730 was 20.3°C. As the sun touched the water a few trout started rising sporadically to an early morning mayfly hatch close to willow cover, and we both landed a couple of nice brownies. By 1000 the water temperature had moved to 21.5°C and we had stopped seeing rising fish. We quickly pushed on upstream to an area where a run of fast water flowed through a mix of riffles and deeper slots amongst the bedrock. We were then pleasantly surprised to get into a group of actively nymphing trout that willingly took our double-nymph rigs (size #16s on 5X tippet). For about 40 minutes, we had several double hookups and some fabulous fishing before the water temperature hit 23°C at 1130 and the fish promptly shut down. This does illustrate the need to be in the right place at the right time ... and at this time of year, that means early in the morning!

Not all rivers experience the extremes of the Motueka Rv and Wairau Rv, but I have found similar issues on the likes of the Pelorus, Rai, Motupiko, Mangles, Owen, and Buller Rvs (compounded on the Buller by warm water flowing off the surface of both Lakes Rotoiti and Rotoroa). Further south I have experienced the same on the lower Waiau Rv in Canterbury and the Opihi Rv in South Canterbury ... both freestone streams.

Luckily, there are some other ways to mitigate the difficulties of fishing in the extreme heat. Some rivers have naturally colder temperature profiles. Rivers that start higher in the mountains for instance, such as the upper Wangapeka, upper Wairau, Travers, and Sabine seldom get into the stress-temperature range. Other Nelson rivers are also fed by cooler artesian water from large underground springs. Where these tributaries run into a bigger river can be a 'hot spot' for fly-fishing. Trout will often congregate in these places, where the water temperature can be several degrees cooler for 30 - 100m downstream before it mixes with the warm water of the mainstream. Natural spring creeks also tend to feature a narrower temperature band each day, with only a 2°C spread. It is worth noting that rainbow trout can handle temperatures that are 1-2°C warmer than brown trout, which makes them an alternative target when the hot weather hits.

The moral of the story to improve your success during a hot spell, (especially if you don't have a good thermometer to monitor the temperature), is to get out of bed and fish early ... make the effort and you will reap the rewards.

Just a cautionary note when handling trout you intend to release when water temperatures are up over 19°C;

- minimise your contact with the fish - wet your hands - keep the trout in the water as much as possible - where possible, remove the hook and release the trout without lifting it from the water - if you want a photo, only lift the trout a short distance above the water for just a second or two.


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