Identifying a Mayfly Hatch | Fly Fishing Insider Podcast

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Identifying a Mayfly

Mayflies, known scientifically as Ephemeroptera, are a crucial insect for the avid fly angler to recognize due to their significance in the aquatic food chain. Adult mayflies are easily identified by their unique wings, which stand upright like those of a butterfly, a slender tapered body, and two or three long, thread-like tails. The nymphal stage of the mayfly, which lives underwater, varies in appearance. For example, the slender and streamlined "Baetis" nymphs contrast with the flatter, leaf-like "Heptagenia" nymphs. Observing these physical characteristics alongside an insect field guide can help in pinpointing the exact species.

Mayfly Adult Dun - Mayfly Hatch

Understanding the Mayfly Hatch

The term "hatch" in fly fishing terminology refers to the period when aquatic insects emerge from their underwater nymphal stage to become winged adults, otherwise referred to as duns. With mayflies, this process is absolutely fascinating. The nymphs will start migrating towards the water's surface, and once there, they'll break through their exoskeleton to reveal the subimago or 'dun' – an immature winged formed adult. You can observe the duns floating down the water looking like miniature sailboats taking a gentle cruise. These duns will then fly off to nearby vegetation where they'll undergo another molt, shedding their skin one last time to become the fully mature spinner or 'imago'. Recognizing the hatch involves noticing an increase in bird activity over the water, fish breaking the surface to feed, and the presence of floating duns on the water. Shortly thereafter the adults return to the water to mate which results in their passing.  Although some stay in this dun-like matting form longer than others, the lifespan is short, 24 hours or so, and the end of life is referred to as a spinner.

Matching the Mayfly Hatch

The essence of fly fishing during a mayfly hatch is to "match the hatch," which means selecting an artificial fly that closely resembles the emerging or adult mayflies. Keen observation is crucial. For instance, if you notice trout taking insects just below the water's surface, they might be feeding on emerging duns; thus, an emerger pattern would be ideal. A big difference in the feed is gulping, sipping, and splashing.  When feeding below the service you’ll often see larger splashing feeds as the fish are actually chasing rising emergers through the water column, thus breaking the surface. Keen observation here can be critical and often if they are taking adults they will take emergers. If the trout are seen gulping down adult mayflies from the surface, then a dry fly resembling the adult stage would be effective. The silhouette, size, and color of the chosen fly should mimic the natural insect as closely as possible. For example, during a "Pale Morning Dun" hatch, a light-colored, size 16 dry fly might be the ticket.

This is a favorite of mine for fishing emerger patterns.  The CDC Emerger can be deadly!

I refer to my memorization tool for selecting patterns. The tool follows the process of selection by utilizing the acronym “OSSCP.” Observation, Silhouette, Size, Collor Presentation.  Observation and Presentation bracket and hold everything together by being the most powerful elements.  Then, in order of importance, it is Silhouette, Size, then Color to finalize your decision-making process. You’ve likely heard this phrase before, “the flies are for the angler!”  Meaning that the real importance is the observation of what they are eating followed by something that looks relatively similar in the right location. The actual fly itself only needs to resemble it and not be an exact replica.  

Two classic adult dun patterns.  First the Parachute Adams full adult and second the Mayfly Cripple

Parachute Adams - Adult Dun Mayfly Cripple - Adult Crippled Dun

Factors Affecting the Mayfly Hatch

Several environmental factors can influence the timing and intensity of a mayfly hatch. Water temperature is paramount: most mayfly species prefer a specific range for hatching, often between 50-65°F (10-18°C). Weather conditions can also play a part. Overcast days can extend hatching periods, while a sudden cold snap might delay or reduce hatch intensity. Being aware of these factors, along with knowledge of local hatch charts or logs, can help an angler predict when a mayfly emergence is likely to occur, enhancing the chances of a amazing day of fly fishing.

Wet Nets!

Christian Bacasa, Host of the Fly Fishing Insider Podcast

By Christian Bacasa
Host of the Fly Fishing Insider Podcast

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