Temperature effects on Brown trout.

[whinging pom]

Just in case anyone ever looks at this for reference material i just found this link, its a handy paper on browns, migratory and salmon from hatching to adult


https://assets.publishing.service.gov.u ... lv-e-e.pdf

[whinging pom]

I’m sure they have always been nocturnally active, Chris. In summer months anyway. The largest browns I’ve caught have been around 2-3am. But what surprised me initially was that rainbows are active too. I must have spent 1/3rd of my trout fishing time after dark; it’s a good way of avoiding people

Generally speaking an hour after the evening rise slows down and it turns dark the fish turn on again. This activity slows around 1.30am. Then it’s time to get the biggest flies out and see what is active. About an hour before dawn there is another active period that runs through to just after first light.

Both browns and rainbows will come right into the shallows to feed. Water little deeper than their body height.

29B3B5A1-1D27-4A02-BDE0-53FD12D0355C.jpeg

This fish (NZ) was in very shallow water.

Cheers, Paul

Paul
I was asked at the AGM when I catch the big trout I said after dark mostly.

An older member came up later and asked me if I was using big sedges, I said I was and I used to use a semi automatic reel and twitch the lever to make them skit on the surface,
He was describing a big white fly they used to use,a technique that they called Bustard fishing, which was once common on more northern waters, I was really interested to find out more.
Then some Pratt interrupted us and I never found out the pattern!

When I’m walking back to the car in the gloom on a late summers night there’s usually big white moths fluttering around the bankside , I’m guessing it’s them that they are imitating,
Best pom

[Paul Arden]

I’ve read about that. Can’t remember where and it was a long time ago. Possibly John Goddard.

I first started fishing with sea trout flies; Teal Blue and Silver etc. Then I discovered unweighted black marabou lures. And finally the greased up muddler (basically a skated sedge). I’ve also experimented with lumo flies and mice. I’ve never been very successful with white. The darkest nights have always been best with no moon. However I have seen all-out rises on Lake Hebgen under a full moon! But usually nights so dark where you can’t even see your hands has always been best for me.

I haven’t been very successful here in the jungle. A few jungle perch on sedges. I think the Snakehead sleep but I’m also sure I’m missing a trick here, possibly with Gourmai. To my way of thinking if fish are active in the evening and the morning and not during the day, then they are almost certainly active at night.

Cheers, Paul

[Truchasconseca]

Hello.
Two years ago I changed my residence here in Spain and now I fish at 37 degrees Celsius in July at two in the afternoon and in water no more than one meter deep and the trout can survive in the middle of the mountain. I did not measure the temperature of the water but for me is great for those 37º outside. The life is adapted out there.

[whinging pom]

Hi
If they are Brown trout its well researched about the levels of dissolved oxygen needed and the effects of a range of temps on metabolism and recovery rates. I don't think they will have locally adapted to reduced oxygen levels. the research is not just UK, but the states and Australia and Canada which all have high summer temps.

As for the temp of the water compared with air there are so many variables especially in the mountains. whether there some deep dark ( plunge) pool around, whether the water has tree cover up stream or deep gorges sheltering the surface, and then there's the amount of organic matter affecting the other limiting factors; carbon dioxide and ammonia levels. And then waterfalls and cascades that can add to the dissolved oxygen.....

But what was the actual water temperature was when you were fishing?
it was 42c here last year around after nearly a week of 34c+ our water temp was 20.5c,( measure in an about 50cm depth )
Just because trout at the temp swim off when they are released doesn't mean they will recover condition after the stress. We cease fishing for Browns at a water temp of 19c.
If you want to be sure i suggest you pack a small digital thermometer in your fishing pack and dont rely on waterfalls.
all best
pom

[Boisker]

Not a scientific paper, they operate hoot owl in Montana, no fishing between 2pm and midnight.
2022 restrictions based on-
“Hoot owl restrictions are state enforced and they are automatically put in place if our rivers hit water temperatures over 70 degrees for more than 3 consecutive days. All the rivers are a little bit different due to different elevations, current speed, etc., but the main rules are pretty much the same across the board. After 2 pm you have to reel up and call it a day. This means early starts and early takeouts until it cools off.”
70f is roughly 21 C.

I carry a thermometer and always stop if the temp hits 68f (20C), but normally at this point in the UK, at least on my rivers, by the time the river hits that temp it’s incredibly low and I rarely fish; warm water and the river on its bones seems to high risk to the fish for my liking.
If hot and warmer water, if I do fish I start at dawn, when the temp will have dropped back significantly… a couple of years ago, the water was at 63f (17C) at dawn and back up to 68f by 8am…
So I guess if you want to fish when the weather is hot, dawn is the time.

[whinging pom]

Hi Boisker
that's an interesting take on it and on certain waters i can see the justification.

My taking a spot reading once or twice a day during the hot weather was our bench mark but I felt it was less than ideal for setting fishery policy. especially as we lost 7 weeks of the season last year based on this.( that made me very popular with some club members!!)

After talking to a few people around the UK and based on a study that I saw at a conference years ago about the effects of tree planting on bringing the temp of a river down that was monitored by instream temperature taggers over a number of years.
I got the club and instream thermometer called a tiny tag 2, it monitors the water temp.. ( not the radiant sunshine) every 15mins throughout the season on graph charts, Id put one up , but can never seem to manage to upload pictures on this forum. Its positioned under a road bridge inside a concrete building block.

One of the people I asked for advice monitors a spring where the temp throughout the year is a constant 10c. he has another sensor only 6 metres down stream from the source and the variation there in a day is 2c... what it would be 20 or 60 m down stream would be interesting?!

Another chap is a river keeper of a highland scottish free stone salmon river , that is quite shallow in places. His increments on that stream are set at every 3 hours .
So on the 24th July starting at 9am the water temp was rising to 16c,
by midday it was 23c,
3pm ...27c
6pm 25.5 c,
9pm 22c
midnight 19c
and at 3am it was 17c
6am a low of 16c
... the following day the range was 16c-27c-15c. A variation of 12c in one day!
Later in the week was a patchy cloud day and you can see a variation of 3c as the water reacts to the cloud cover.

Our stream is deeper and slower with more tree cover so Im not expecting such a range of variation in a day, as the scottish highland stream, but when the temps of summer come, ill feel better armed to make a judgement call and i'll be backing it up with dissolved oxygen readings this year.
all best
pom

[Paul Arden]

To add photos Pom, go to Full Editor & Preview, underneath the reply box there is a tab called Attachments, click add files. It has to be under a certain size so might need resizing locally.

While Essex has brown trout in a few places, mostly it doesn’t. I believed that in the past it held trout and the warmer summers that we grew up with had caused them to disappear. Many parts of Europe have seen a similar shift in species over the past 100 years

IMO we will undoubtedly see this trend accelerate with warmer longer summers. There may be some local adaptation but I don’t know of anywhere that trout have adapted to warm water.

Water temp is of course affected by night as well as day temps. Mountain rivers tend to be blessed by lower night time temps.

Cheers, Paul

[Boisker]

That’s interesting Pom and the temps sort of mirror what I was seeing, although I never took a high temp as I’d long left the river. I’m lucky as I have access to the Frome, which being spring fed chalkstream runs cooler.
I wouldn’t want to be in your position, one of the clubs I am in stopped fishing for a couple of weeks last season due to water temps, and people were constantly pushing to be allowed to fish.

It always amazes me how no-one is on the river early when the summer temps are high, I always have it to myself…it’s by far the most productive time to fish, I’ve had some amazing fishing just after dawn, and a but like Paul’s night fishing experience found fish with water barely covering their backs on the inside of bends. Without doubt one of my favourite times to fish, sun just starting to rise, cool, sometimes with that very early morning mist hanging on the water and hearing / seeing fish rising… perfect

[Paul Arden]

Those are big temperature jumps. Probably because I’m fishing a deep lake but I don’t see anything like those swings. Typically at this time of year the water is slightly below 31C at dawn and around 33C mid afternoon. Usually a little more than 2C difference. Air temp is 23C at 5am and 33/34C mid afternoon.

The lowest temp I’ve measured the water has been around 27/28C (Wet Season) and the highest slightly above 34C (heat wave). I note that the thermocline changes considerably during the year. From about 15’ depth to 35’+ but don’t quote me on that because a lot of my sounder screenshots are on my old phone.

Has little to do with trout of course because totally impossible for them to live at these temps. But I notice that the fish that have a capacity to breathe air start to do so at around 32C. During the heat of the afternoon we rarely see fish breathe, initially I had jumped to the conclusion that the fish had shut down activity, but it could in fact be a consequence of the production of oxygen due to photosynthesis of the algae during the brightest part of the day.

Sorry off topic – but still rather interesting!

Cheers, Paul