Understanding Water Temperature: Notes from an Aquatic Scientist!

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© 2006 E. P. Macri Jr.

There is probably more nonsense and misunderstanding on water temperatures and trout fishing than most of the topics in fly fishing. Why? The first reason is that most fly fisherman for some reason believe that they are aquatic scientists. They write articles which they believe are true and this is accepted as gospel by every generation of fly anglers who repeat the mistakes over and over again.

Let's start off by talking a bit on how these so called limits of a temperature for trout are derived. They are actually derived when 50% of the fish die in a test! Yes, that is correct. The so called limits of trout in a stream are actually in many instances..lethal limits. To be perfectly honest, thermal tolerance of trout in streams is poorly understood.

Let's take some examples you may have seen in a books and magazines:

Brook Trout- 78.5 f. Brown Trout 79.5 f; Rainbow Trout 79.0

Of course, I've seen other charts and tables over the years which show different numbers. There are a number of reasons for this including the strain of the trout, whether they are hatchery trout; the type of test performed and variety of other conditions too numerous to mention. What is the optimum temperature for each species? There is no model that accurately predicts this because every stream is different.

For example, the brook, brown and rainbows that inhabit Big Spring Creek in Cumberland County, Pennsylvania (a limestone spring creek) for the most part live their entire lives in water temperature from 46 to 50 f. The trout's metabolism in these waters is geared to these temperatures and the trout grow all year round. Take the same temperature and move a few miles away to a freestone stream and those trout are barely active in such cold water!

So how does a fly angler ascertain which temperatures are best. Well to be honest carry a thermometer and put it into the stream (make sure the thermometer has been calibrated properly for accuracy) and read a temperature. Most anglers don't until the fish aren't hitting and then they say the stream is too warm or too cold to be productive during that time period.

Here are some of my favorite temperature ranges for each species (not lethal limits).

Brook Trout: below 65 f. I find the fish most active in most situations. Once again this is an average. If this is a spring creek you know it doesn't apply.

Brown Trout below 68 f and rainbows below 67 f. Yes, I know you catch trout on many streams when it' warmer but these are averages over the years when I believe the fish hit best.

Now here is the secret that few anglers understand about water temperature.  It's not the actual temperature but whether it's rising or falling to the optimum feeding temperature in that stream!

For instance, if the temperature in the stream is 66f the brook trout may not be that active. Sometimes when the temperature (especially true in freestone streams) is at a certain temperature for a long period of time the fish go into a "lull state." Now by the same token let's say that the stream temperature is 69f and begins to fall towards 66f the brook trout may go wild and feed. Or the temperature may be 62f and it begins to rise towards 66f the same thing may happen. This occurs for all species of trout.

Part of the reason is that oxygen plays a role in the metabolism and appetite and growth of the trout. Cold water holds more oxygen. It appears when there is a proper amount of oxygen saturation at a certain temperature this is what may trigger feeding!

Understand that this so called "set point" that is triggered by the rise and fall of the temperature changes! One week it might be 66f and the next week it might be 63f. Unless you are keeping temperature profiles of the stream (which isn't a bad idea) you should use your thermometer often and do some experimenting.

Let me give you one more example on how the temperature set point works. It's in the summer on a freestone stream. You are going to fish in the morning at your favorite stream. You get there at 7:00 a.m.. You fish for 3 hours with little to show for it. You meet another angler who said he got there at 6:00 a.m.. and he had about an hour's good fishing! What was the difference? The water temperature was 66f when the other guy got there but rose to 68f when you started fishing. You think that the 68 f is fine but once again it's the rise and fall that triggers the feeding. There are small windows in the summer in which the temperature may be right for only 30 to 60 minutes. The same thing occurs in the evening when the stream cools down in some areas after dark. Many anglers leave too early.

Understand how temperature affects trout in conjunction with the environment with the temperature triggers, and you will increase your success rate. Remember these few points and don't buy the temperature charts in most books in magazines. In a future article I'll have a discussion on which flies to fish at certain temperatures.