The Stream Doctor

October 20th, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. From W. Powell. Hi Doc: For twenty five years I've been fishing for trout during the winter and for more than half of that time I've had doubts about the survival rate of fish released when the water temperatures are so cold that the fish's metabolic functions are lower and the forage levels are severely decreased. Although I experience my most productive fishing from November through March, I would certainly rather discontinue the practice than kill fish. Have you any light to shed on this issue?

A. First of all, I'll relieve your concerns; enjoy your winter fishing. Here's why.

We're looking at several interacting factors here that influence the potential impact of winter trout fishing. Let me discuss these with you.

Evidence from some studies in southern Ontario revealed that during winter, trout remained thin despite having full stomachs. In other words, although food is plentiful (see below) and the trout feed regularly, their metabolic rate is too slow to assimilate the food and convert it to body tissue. Similar results have been found in California and elsewhere in Canada. This would indicate that trout, being in poorer condition during winter, might be more susceptible to fishing impacts.

On the other hand, I talked to two local fisheries biologists (one state and one university) about your question. The state biologist said that in the fisheries he supervises in this area, he sees no detrimental impact of winter fishing on local trout populations. The key word here is "populations." Biologists think in terms of overall fish populations when interpreting impacts, whereas individual anglers are probably concerned with impact on individual fish (is the one I just released going to make it or not?) as well as what impact they may be having on the overall population. His overall reaction based on his experience was to go ahead and enjoy your winter angling. The university biologist thought that factors favoring trout survival during winter were related to the fact that trout are more lethargic in colder water, fight less, and thus experience less stress than those fighting in warmer temperatures.

Another point in this respect is the problem of trying to answer your question in generalities without knowing where you fish or the type of water. Whereas the state biologist found no problems here, I expect that if I called enough biologists, I'd find one who did indeed find some negative impact of winter fishing on a particular population. In other words, different populations may respond differently. For instance, tailwater fisheries, which experience fairly uniform temperature conditions, will probably show no impact to seasonal differences in angling.

Colder water holds more dissolved oxygen; thus, oxygen saturation is the norm at this time of the year so this shouldn't add to any stress. Further, the very fact that the fish's metabolic functions slow down possibly indicates that they are less susceptible to stress at this time.

One point the state biologist did make, however, was this. He did some studies on individual fish survival from handling in winter conditions, particularly ice fishing. For ice fishing, the longer a fish was exposed to freezing air temperatures, particularly the gills, the less chance it had for survival. Thus, the less you expose the fish you catch to the air when it is freezing, the better their chance for survival.

Contrary to your belief that forage levels are low, this is usually the time of greatest abundance for immature aquatic insects. Remember, most insects emerge and deposit their eggs during the warmer months. The eggs hatch and the developing instars grow during winter, reaching their maximum size in spring and summer just prior to emergence. Thus, highest populations of immatures are present during the winter months, although they may be smaller in size initially. As populations develop, the nymphs and larvae get larger and fewer in numbers as natural mortality occurs.

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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