Most anglers remember to check the weather before heading out to see if they’ll need a jacket, but the air temperature isn’t the only thing that affects fishing. The water temperature, something most people only gauge as warm, cool, or cold as they stick their hand in, plays a huge role.
From fish behavior to hatches, there are a handful of aspects that are directly affected by water temperature. Unless you know to consider temperature changes, it might not be obvious why things on the water happen when they do.
Although some people do choose to carry a thermometer to precisely measure temperature changes as they happen, even knowing a general pattern of how things change as the water warms and cools is beneficial to an angler.
Although the species in a given body of water aren’t going to change in real time to fluctuations in water temperature, it’s fairly common for species patterns to change along the length of a river. Many rivers, like the Delaware and the South Platte, are known for having robust populations of warmwater species like smallmouth bass, as well as coldwater species like trout. Many rivers that start small, perhaps at a spring or at high elevation, have much colder water near the origin. As they flow down and grow in size, they can morph into much warmer rivers, hosting a completely different set of species.
Knowing which species to target is a crucial aspect of fishing, so having an idea of how warm the water is, and therefore which species it can hold, is also important. On the South Platte, for example, trout are the primary species only a bit west of Denver, while bass and carp dominate in the city and below. Although the foothills and the city itself don’t have many apparent weather differences, the water temperature changes significantly. Knowing to throw something big and buggy in town, after fishing a tiny mayfly only a few miles upstream, can be the difference between catching fish and not.
Here’s a quick chart of some common species and their preferred temperature ranges. Keep in mind, it seems as though no two references ever really agree on a black-and-white preferred temperature range. Instead, use this as a guideline for which fish prefer warmer waters, and which prefer cooler waters.
|Species||Preferred Water Temperature|
Fish behavior is another major aspect to consider when looking at water temperatures. This could be feeding or spawning behavior, and both are relevant to fishing.
Feeding behavior will likely change throughout the day to very small changes in temperature. As prey items change, fish behavior changes as well (more on this below). Additionally, many fish are known for taking a bit of a siesta in the heat of the afternoon. Mornings and evenings, when things are a bit cooler, are prime time for many species.
On a bigger scale, changes in water temperature are the primary decider on when fish begin to spawn. For spring spawners like rainbow trout and largemouth bass, the spawn begins when the winter water hits a certain temperature range, often still significantly colder than their preferred summer temperatures. Knowing when to start looking for feeding or spawning fish is a great way to time your fishing just right and avoid less productive times.
Below is a chart of the approximate temperatures that some popular species prefer for spawning. Actual temperatures will vary by area. Remember that even though many species will be aggressive and easy to catch during the spawning season, it’s best to leave actively spawning fish alone. If you do catch one, release it quickly and in the same spot you found it.
|Species||Preferred Spawning Temperature|
Additionally, seasonal changes in water temperature can make or break the fishing. In 2018 here in CO, many waterways were closed because they were too low and warm for fish to handle any extra stress. Oxygen levels are lower in warmer water, so knowing that fish may be more hesitant to play along when it’s scorching hot is useful for those who enjoy summer fishing. Along the same lines, many species slow way down in the winter months as temperatures plummet. This leaves spring, early summer, and fall as some of the best times to catch fish.
One of a fly fisherman’s most important bits of knowledge is when certain insects hatch. On a large scale, this knowledge tells us things like the fact that midges and BWOs may be found nearly any time of year, while we won’t really start looking for caddises until Mother’s Day or later.
Hatches tend to be more specific than the preferred temperature of a given fish species. If you look up a fish’s preferred temperature, you may get a range of 10 degrees or more. If you look up the temperature of a certain insect hatch, you might get a range of only a few degrees.
Even on a day-to-day basis though, temperature matters. During the summer in the Rockies, I’m hoping for hot days and cool evenings. This combination tends to lead to heavier evening caddis hatches than cool days do. If the water stays chilly all day, it’ll be slow in the evening.
Finding insect hatch temperatures isn’t difficult. There are a ton of hatch charts and guidebooks out there, and many specify the best temperatures to target for the hatches in your area.
This doesn’t mean the safety of anglers, although frigid water in winter is a serious concern for them as well. Instead, this refers to the safety of fish.
During extremely hot summers, waters heat up and the oxygen content decreases. Although all fish species have an upper temperature limit, the ones most likely to actually hit theirs are the coldwater species like trout, since their limits are much lower than warmwater species. Most people agree that 67-68° is the highest water temperature you should consider fishing for trout.
In late summer, fish can end up living at the top of their temperature range for weeks at a time. In these periods of high stress, the act of being hooked, fought, and landed can be a death sentence for a fish even if the angler uses a barbless hook and handles it correctly.
It’s important to remember, as with all these points, that water temperature matters more than air temperature. Yes, warmer days will inevitably lead to warmer water than cooler days will, but don’t mistake cool air temperature for cool water temperature at a given time of day. During these hot spells, water will cool overnight and then gradually warm throughout the day. Since it retains heat, the water will stay warm into the evening or early night, when it will then start to cool again. This means that even if the evening itself is cool, fish may still be stressed from warm water. On these hot days, it’s best to fish the morning when the water will be at its coolest.
Targeting the early to mid-morning window will not only provide great fishing most of the time, but also keep the fish safe from overexerting and using up too much oxygen. Of course, this matters more for catch-and-release anglers, since stressing a fish out doesn’t matter much if you’re keeping it for dinner. That said, even catch-and-keep anglers end up releasing lots of fish due to size and species regulations, so they should still pay attention to temperature.
Keeping these tips in mind will allow you to use water temperature to your advantage while you fish. If you choose to carry a thermometer for precision, that’s great. But, even having a general sense of how changes in temperature affect fish and insects should give you a leg up on your next trip.