Warm Water Caution for Coldwater Fish
Updated: Aug 26
If you're targeting trout, salmon, or steelhead, you need to know the facts to make sure that fish that come off your line while being fought, and the ones you release, swim away and have the best chance at recovery and survival - we owe it to the fish to understand their needs, and lessen our impact.
It is scientific fact, not opinion, that the effects of warm water on coldwater fish are deadly, the deadly effects are even worse when fish experience additional stress.
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Fish need to filter out the oxygen found in water to breathe. While this is easy to do for coldwater species of fish (such as trout, salmon, and steelhead) when the water is cool, once the water is 68 degrees Fahrenheit (20 degrees Celsius) and the fish become stressed by being hooked and fought, their survival rate significantly decreases. As water warms, its ability to provide oxygen to fish significantly decreases.
Coldwater fish have adapted with the need to breathe the amount of oxygen that is found in cold water, warmwater fish have done the same with warm water. If you take a cold water fish and expose it to warm water, things usually don't go very well.
So, what is a responsible angler supposed to do, and how do we monitor this?
Lucky for us, small portable thermometers exist!
If you don't have a thermometer in your pack or vest, we highly recommend that you do, and fish by your thermometer. A fish's metabolic rate also changes with water temperature, and knowing what temperature the fish are being exposed to also helps you adjust your presentation. But, we don't always have to head to the river to find out the what a river or stream may be reading on the mercury. Our two most well known trout fisheries in Southern Ontario have real-time water temperature monitoring stations. There are a few stations that monitor temperature on the Credit River, and a number of stations on the Grand River that do the same. You can check water temperatures, and water temperature trends, even before you get out of bed in the morning. We've done this often, rolled over, grabbed our laptops or phones, checked the morning water temperatures on the Grand or Credit, checked the forecast for sweltering air temperature then either gone back to bed because they were too warm, or jumped out of bed, into our trucks, to find our way to the water.