Are We Wrong About “Safe” Temperatures for Trout Fishing?

Written by: Phil Monahan

We fly fishers pride ourselves on being good stewards of our resources, and graphics like the ones above circulate every summer to remind us that warm water is detrimental to the health of trout. Although the numbers may vary a bit, the accepted wisdom seems to be that we should stop fishing for trout as the water temperature approaches 70 degrees Fahrenheit. As I type this, the Battenkill gauge (which is several miles upstream from where I normally fish) lists the temperature as 69.3°, so I’m sticking to the cooler headwater streams.

However, a new study from Keep Fish Wet suggests that our self-satisfaction may be misguided. In a new article, “Fishing for Trout in Warming Waters—Science-Based Angling Thresholds,” Jamie Madden argues that the angling threshold, a temperature at which fish mortality begins to increase exponentially, is lower than previously thought. Madden—who recently earned her Master’s degree from Carleton University in Ottawa, studying under Keep Fish Wet science advisor Dr. Andy Danylchuk—pored over the scientific literature on the topic, and here’s the big takeaway:

The results of my literature review suggest the angling threshold of 68°F/20°C is too high for all the species I researched.

Well, isn’t that a turd in the punchbowl?

Be aware that this new literature review is not suggesting that we stop fishing a much lower temperatures, but it is arguing that we should all be more mindful. Madden argues that the best approach is a conservative one, until the science catches up with these suggested findings:

I encourage anglers to be more conservative and start being aware of the impacts of temperature of fish much earlier, specifically at 61°F /16°C for rainbow, steelhead, cutthroat, and brook trout, 66°F/ 19°C for brown trout, and 54°F/12°C for bull trout . . . . For now, I encourage anglers to pay attention to the fish in their hands, be observant, be conservative when the science is behind, and be advocates for the use of current science-based best practices when targeting trout.

I realize that these findings will be contentious and that the responses from many anglers is going to be “That’s bulls*#t!” And I understand this skepticism. I have always felt that the findings of fish-mortality studies don’t match my on-the-water experiences, and I’ve been involved of many such discussions over my long tenure as an angler and a journalist. We’ve all caught fish that bear the scars from many earlier encounters with flies and lures, and popular fly-fishing rivers, where thousands of fish per day may caught on a busy weekend, don’t seem to be littered with dead trout.

But I also know that anecdotal evidence isn’t science, and I encourage you to read Madden’s article.

I am not a scientist, and I haven’t read all the studies that she cites. But I do trust that she undertook this project with rigor and sound scientific reasoning. So I’m going to take her findings and her recommendations seriously, redoubling my commitment to treating fish as carefully as possible when waster temperatures rise about 61° and following the Keep Fish Wet Principles. Ultimately, what’s the downside to doing even more to protect and preserve the fisheries that we love?

Hopefully, there will be more studies in the future that will help us, as anglers, do our best to keep our fisheries healthy in an increasingly warming world.