How to Read Hatch Charts and Match the Hatch Year Round

Available for most popular trout lakes and rivers with a quick Google search, hatch charts can provide an angler with the basic knowledge of which aquatic insects will be present in a specific water and when the adult insects will be active. While all fly fishers love to fish the dry fly patterns highlighted by the traditional hatch chart, these rudimentary tools fall short in several ways.

1. Hatch charts only highlight the adult life cycles that account for .1% of most aquatic insects’ life cycle and 25% of the feeding focus of trout year round.
2. They don't make any allowance for a river that might be 400 miles long or the effect of elevation and temperature.
3. They don't tell you which flies or life cycles are active during every month of the year.

Before we delve into how to more fully understand a hatch chart, we need to take a quick look at what factors affect the growth of aquatic insects and the timing of their hatch. Aquatic insects and their hatches are very similar to gardening: there is a season to plant, a season to grow, and a season when the fruit becomes ripe. Just like elevation and temperature will affect how fast your tomatoes would grow in the Rocky Mountains vs. the Midwest, so elevation and water temperature act as the primary factors in the rate of growth of aquatic insects and the timing of their hatches.

When the hatch window (highlighted season) and the active temperature window (ex. 55-59 degrees F) line up, the hatch begins.

The lower in elevation you go, you will find warmer water temperatures earlier in the season and the result is that the emergence (hatch) of the adult insects will happen earlier than at higher elevations. As the season progresses, water temperatures at higher elevations will begin to increase, in turn triggering the hatch.

The hatch time period outlined in the hatch chart needs to be applied to the entire length of the river. As water temperatures reach the “active window”, the target species should begin to hatch. The hatch begins at low elevation and extends upstream as the water warms to the “active window”. This is a fluid progression as you move upstream throughout the chart-defined season which is usually highlighted across a series of months. Cold water springs, small tributaries joining the main river, rain, snow, or warm sections of river exposed to the sun will either delay or accelerate the progression of the hatch.

Within a given section of water, the hatch will start slowly, increase in intensity over a matter of days or weeks, and then taper off as the hatch moves upstream. Invertebrates are genetically hardwired to hatch in this staggered fashion to improve their chances of mating and passing on their DNA in the case that some catastrophic event were to wipe out a portion of the population (e.g. flash flood, fire, or erosion event).

Now let's apply this knowledge to read between the lines of the traditional hatch chart.

By using a traditional hatch chart and a basic knowledge of aquatic insect life cycles, you can begin to anticipate the types and sizes of fly patterns that will be effective throughout the year. During the "hatch window", the aquatic insects will be at their largest size and all life cycles will be active and fishable (e.g. Nymph, Emerging Nymph, Emerging Dun, Dun, Spinner, & Dead Spinner). After the hatch concludes, the next generation of nymph will emerge from their eggs and will be very small (our fly patterns in this season will be sizes 20-26). In the months leading up to the hatch, the living nymphs and our fly patterns will increase in size (e.g. sizes 16-18). This growth and transition to larger fly patterns throughout the year can and should be verified every time you are on the water with a quick sample using a seine, and then by tying on the appropriate sized pattern.

By utilizing hatch charts and learning to read between the lines, you will be more prepared for each trip to the water, become more confident in your fly selection, and ultimately catch more fish!