How To Safely Catch Trout In Warm Water
By Greg LaBonte

Review: Fishing for trout and salmon in waters that exceed 67F is extremely dangerous for fish health and there is a high probability the result will end in death of the fish. Death comes so easy to trout in warm temperatures because of metabolic limitations that exist within cold water fish. Please share this article in hopes of educating anglers, knowledge is power and can lead to healthier fisheries.

*All facts in this article are supported by peer reviewed scientific literature cited at the end


            Clickbait title. Hopefully I got you. You CANNOT safely catch trout in warm water. I’m sure you’ve seen something like Figure 1 below. I’m sure you’ve heard to leave trout alone in warm water. Maybe you’ve heard to up your tippet size to decrease the length of time you are fighting the fish. I know you’ve heard keep em wet bub. This is great, but why is it bad to catch trout after a certain temperature, what is happening physically to them, and is there any science behind it? Let’s take an in depth look at what happens physiologically to a trout or salmon while being caught in waters over 68F.

Before the fight


            Let’s set the stage. It’s 7AM, July 15th, somewhere on a river in Maine. Air temp is 75F, water temp is a warm to the touch 69F. A brook trout is lazily sitting behind a rock in a slow current at the tail end of a small pool. At this temperature, growth is not really an option for brook trout1. Rather, they are simply trying to survive. Elevated temperatures induce stress responses in the trout endocrine and cellular systems. Plasma cortisol (central role in glucose metabolism and response to stress) are elevated, heat induced proteins like heat shock protein 70 (HSP70 is a stress induced protein that helps stabilize other proteins that are negatively impacted by heat as well as regulating several metabolic functions) are at elevated levels in the gills, upwards of 50 times normal conditions1. These metabolic responses to high temperatures require a lot of energy, so fish are using all their energy to just stay alive and putting no resources towards growth. Needless to say, the brook trout is not happy. There has been a plethora of studies that show brook trout are most happy between 55-60F (Baldwin, 1956; Hokanson et al., 1973; Dwyer et al., 1983) and the lethal temperature for brook trout is right around 78F (Fry et al., 1946; Fry, 1951; Wehrly et al., 2007). It should be noted, that comfortability and temperature vary with each species of trout or salmon, but most follow a similar trend, differences distinguished by only a few degrees.

            So, before your fly has even touched the water, in the aforementioned scenario, the brook trout is using most of its energy and internal resources to simply stay alive. Every bug it consumes, every calculated flick of the tail, all vital for survival. These physiological responses to warming water are what dictates the livable range of trout, that’s how important temperatures are!

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Fish On!


            It’s 7:15AM, you rig up your rod, you get on the river, wet wading because the water feels great! You’re not having much luck with a dry fly even though bugs are buzzing around, so you switch to nymphing. You hook up with a nice brook trout on your first cast and the fight is on! The fish begins to try and swim, consuming energy, utilizing metabolic processes not much different than our own. As the fish flexes its muscle during the burst of activity, anaerobic metabolic processes are taking place, building extremely large concentrations of lactic acid waste in the muscle 2. Large doses of lactic acid/waste leads to the acidification of fish blood, which disrupts many fish metabolic processes. Acidification of fish blood decreases the fishes ability to deliver oxygen to tissues, which in turn requires fish to expend even more energy to compensate physiological responses3. We as anglers note this as the fish tiring.

            As the fight continues, the maximum aerobic metabolic rate of the fish is lower due to the warm water 4, which means fish can’t consume enough oxygen to complete processes like reversing the acidification of their blood. In turn, and we’ve probably all experienced this, you end up reeling in what feels like dead weight. The fish has given up, exhausted it’s muscular potential, and you are dragging a fightless fish on top of the water into your net.

In The Film By Maine Fly Guys

The Release

You get the fish to net, a beautiful 16” brook trout, pop the hook out, pick it up, examine it momentarily, snap a quick photo, and back in the water to revive it. After a few seconds, the trout lazily kicks its tail and swims back to its hole. You’re relieved and thrilled that the trout is not dead and happily back where it belongs. But, it’s not happy is it? Blood (or plasma) glucose levels and plasma osmolality (electrolyte-water balance) are at elevated levels 5, adrenaline is still pumping through the fish, which probably gave it that one last kick to get away from you, and metabolic processes are even more heightened than they were when the fish was lazily swimming in it’s pool this morning at 7 AM. Not to mention the fish is desperately trying to restabilize it’s blood levels and remove lactic acid waste to start the healing process. It’s likely the fish you caught and released won’t be able to do all these things and eventually will succumb to fatigue and parish.

            All this because the water temp is 69F. Effective and proper catch and release practices have been shown to help trout go on living normal healthy lives after being caught, as long as the water temperatures is somewhere between 40 – 63F 6,7. However, I am yet to find a published study going back as far as 1800 that states catch and release is effective at returning trout home safely in temperatures above 67F, perhaps that’s because it’s not possible.  

What Can I Do?

            What can you do to help? Spread the word, knowledge is power. I truly believe nobody who is practicing C&R wants to do harm to the fish, they probably are simply unaware. Making people aware, educating anglers, that’s the best way to help. Always carry a thermometer with you! They are cheap and easy to use, why not? Finally, every state has warm water species, they can tolerate physical stress and process metabolically in warmer temperatures, go explore a new fishery!

            Maine currently has no educational material or fisheries management strategies set in place to educate anglers on the danger or protect native trout and salmon from fishing pressure during the hottest and driest months of the year. Educational material or fishing laws are common practice in fishing meccas such as Colorado (fishing closures), Montana (fishing closures), Oregon, and Idaho. Let your voice be heard, reach out to your local representatives or fish biologists and let your voice be heard! 

  Works Cited

1.     Chadwick and McCormick 2017

2.     Wood et al. 1983

3.     Rogers 2020

4.     Norin et al. 2016

5.     Gresswell et al. 1986

6.     Marnell and Hunsaker 1970

7.     Joubert et al. 2019   


About the author

Greg LaBonte studied and/or researched Maine fisheries for 8+ years and currently teaches Ecology/Evolution and other biological topics at The University of New England.

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