Trout Thermodynamics

Now that the summer heat is peaking I have begun thinking about where to go for fish. I’m an old bass fisherman who recently converted to a trout fly fisherman. Bass, of course, are warm water fish. They’re not skittish and will eat almost anything thrown at them (got a dead gerbil in your kids room?). Trout on the other hand are picky eaters, are very skittish, and are cold-water fish, especially brook trout. While brown trout and even some rainbows will live in water that gets into the high-70s or even low-80s in some instances (though they feed less as their metabolism slows), brook trout like it cold. So conventional wisdom is that, as the water warms up, the brookies move toward the headwaters where it is likely cooler.

Aha! But I live on the coast of Maine and we have several streams that support ocean-run brook (and some brown) trout populations. These fish are known as “anadromous” meaning they spend part of their lives in salt water but breed in fresh water (in fact the State of Maine is doing an interesting study of these fish using input from local anglers, since they don’t seem to fully understand yet why some trout in the same stream aren’t anadromous).

In any case, in Maine in the summer the absolute coldest water (not counting the deepest part of lakes and the high mountain streams) is the ocean. The Gulf of Maine (defined by Cape Cod on the south and the southern end of Nova Scotia to the north) is c-c-c-cold since the warm gulf stream passes it by. As an example, the latest northeast coastal water temperatures show a ten degree difference between Portland, ME and Woods Hole, MA. Nonetheless, look at the difference on that same chart between Woods Hole or New London averages versus Boston’s average (despite the closeness of the cities). The five degree difference over such a short distance is because Boston is on the Gulf of Maine.

So, to get to the point, I wonder if the trout in local coastal streams here in Maine don’t actually migrate downstream as the water warms. Of course, that would mean these coastal streams, which are partially tidal near their mouths, have a sort of “reverse” temperature structure, being colder toward the mouth. My plan (if I get around to it) is to map the temperatures some afternoon from the mouth of one up to near the headwaters (they’re fairly short streams – 10 miles at the most). One might ask: why bother? Well, I got a late start this year and am desparate to catch some trout around here before my schedule gets more structured with the start of classes in the fall. Plus the data might actually be useful to someone. I’ll have to check with some colleagues in the Biology department at work. Perhaps they already know the answer to this. Perhaps you already know the answer to this. If you do, then post it!


One Response to “Trout Thermodynamics”

  1. quantummoxie Says:

    Someone from Kansas posted a question about this where it was originally posted over are Blogger. Here’s the link: and here follows my response (to the best of my knowledge):

    Q: Why can’t trout live in the warmer waters of Kansas?
    A: Well, generally trout prefer colder water. In fact they are categorized as a cold water fish. Depending on the specific species of trout, they can tolerate water temperatures generally up to about 60 with some species tolerating warmer temps (even into the 80s). In fact there is even a species of trout in Louisiana. It is entirely possible, however, that trout populations are thinning since global water temperatures are on the rise.

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