Fly fishing for trout that reside in lakes can be challenging.  With warm summer days comes warm water temperatures.  And, most fly fisherman know that means trout go deep – not ideal from a fishing perspective.

I came across an interesting article that discusses the physical reasons for why trout go deep as water temperatures warm.  The challenge for trout is really one of lack of oxygen – the warmer the water, the less capacity it has to hold dissolved oxygen.  At the same time, as water temperatures warm, the fish’s metabolism rises.  A trout must balance its metabolic rate against availability of oxygen so it tends to move to water temperatures where the two are in balance.

While trout have varying tolerances for water temperature, typical surface water temperatures in the 70s F (21-26C) are uncomfortable for them and they will typically seek cooler locations – meaning deep down.  At 75F (24C) the maximum dissolved oxygen capacity of undisturbed water is 3ppm (parts per million). This is the limit below which trout will not survive.

In general trout metabolism and access to oxygen (and hence predisposition to feed) peaks when water temperatures are between 55 and 65F.  Having said that not all species are equal in this regard.  For instance brook trout and lake trout have a lower tolerance for warm water than rainbow trout which have a lower tolerance than brown trout.

That’s my rough interpretation of the biology. So how does this play out for a fisherman?  It seems to me, the issue is more relevant to catch and release fishing.  Obviously if you plan to keep the fish, its mortality is greatly at risk anyway. Assuming catch and release, here are a couple general rules that I try to follow.

• Avoid fishing for less tolerant species when water temperatures are high.  Even if they have moved deep and you manage to catch them, excessive play may be harmful to them.
• For more temperature tolerant species, play and release them quickly – preferable without a lot of out of water time.

So after all that – what did I do?

Bob Jurmain and I decided to head out in search of lake rainbows yesterday.  Air temperatures had cooled somewhat the last little while so that was encouraging.  We also picked a lake where we know subsurface water temperatures are consistently cold through the sumer.

Our start was pretty leisurely getting out on the water by 9:30am.  It was a calm, sunny day with cool air temperatures. Surface water temperatures were around 22C (72F) and down around 25 feet the water temperature was close to 15C (60F) and that’s were the fish were located.

Rigging of choice (or necessity) was full sinking lines, short leaders and streamers.  We let the line sink until we estimated we were close to the 20-25 foot depth and then began stripping in.  And the fish were pretty cooperative.  Both of us were on to fish within a few minutes of putting in.  None were huge, consisting of this year’s stockers and some hold overs but they were energetic. When caught they were quickly released.  As the morning wore on the fishing slowed so we stopped for lunch which was a combination of sandwiches and a couple kept rainbows Bob smoked over his travel stove.  They tasted pretty good too.

Fishing was even slower after lunch although we did manage a few more fish just before we packed it in around 4pm.

In the interest of quick release the tin foil trout were the only ones photographed.  It was a good to get out.  A couple more weeks and the fishing should be in fine form.