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Thermometers – do you use them?

12/13/07 – Thermometers are a must have for some and a “why bother” for others.  Thermometers will tell you the temperature but they won’t change the fishing, which is why a lot of people don’t bother with them.  But knowing the temperature can, and should, give you a good idea how to approach the water and what techniques you might want to use.

If I check the water temperature and get a reading between 50 and 65 degrees I’m a happy man.  Happy because my preferred target species is trout and trout like that range of temperature. When trout are comfortable they usually have the feedbag on.  It doesn’t seem to matter what time of day (or night) they are out there looking for a snack when the temperatures are right.

But when I get a reading of 70 degrees or higher I often don’t fish for trout and pursue Smallmouths, Pickerel or Perch (a much underrated fly rod species).  I mostly catch and release my trout and when I let them go I like to think they live to be caught again. 

However, when water warms and the levels of dissolved oxygen drop released trout often, in my opinion, suffocate or succumb to other temperature-influenced ills like lactic acid buildup or heat stroke. And this happens after they give you that warm-fuzzy feeling by swimming away. 

Fish seem to tolerate cold water better than warm water.  They hunker down and their metabolism slows until the warming that is bound to come.  So when the temperature reading is low (below 42 is my confidence cut off point) I seldom catch fish but when I do I’m fairly confident that released trout survive. 

Now I’m not saying trout can’t be caught and successfully released above 70 degrees.  Nor am I saying they can’t be caught below 42 degrees.  There are few hard and fast rules when it comes to fishing.  Yet, it bears mentioning that fish are cold-blooded and the water they are in dictates their metabolism. It is my belief that when the water temperature is above 70 or below 42 the fishing is slow to non-existent, at least when fly fishing for trout.

The bugs and trout often tell me when the temperature range is between 50 and 65 degrees.  They communicate that to me by way of a hatch and surface feeding. That’s an easy one – when I see surface feeding I know dries or emergers are the way to go. 

But – if I get to the water and there isn’t surface activity and the thermometer tells me the water is in the optimum range I’ll often tie on a dry fly even without surface feeding.  Why?  Because I know fish will move to a fly when they are comfortable.

When the water is in their comfort range trout more than move to a fly they often are cruising and aggressively seeking food, especially if there is no hatch.  Your fly may well be the trigger they need to strike.  But if I check the temperature and get a reading below 50 degrees I seldom tie on a dry and go searching.

Instead I tie on a nymph, maybe even switch to a sink-tip line and I go down after them.  Down as in bottom bumping.  The colder it is the less they move and so taking the fly down to them in the way to go.  I find a feeding lane that has slack water adjacent to it or the tail-out of a pool – that sort of water and I plumb the depths for them.  The colder it is the more I pound the bottom. 

If the temperature is just below 50 degrees I don’t feel the need to hook bottom every fifth or sixth drift.  If the water temperature is 44 degrees I don’t feel I’m in the game if I’m not hooking bottom frequently. The colder the water the less a fish will move so the colder the water the deeper and slower I fish.

So while not having a thermometer won’t stop me from fishing it helps when deciding how to fish or what to fish for. Remember too that the river or stream temperature changes during the day.  Often the morning water is 4 to 6 degrees cooler than the evening water.  Depending as I do on the information thermometers provide puts me firmly in the “must have” category with the discussion of thermometers comes up.

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