Hex Hatch Tracker
Live Hexagenia limbata emergence probability. Water temperature, moon phase, wind, and sunset conditions scored nightly during hex season.
The Science of the Hex
Hexagenia limbata is the largest mayfly in North America, with a wingspan approaching two inches. The nymphs live burrowed in the silt and sand bottom of the AuSable for up to two years before crawling out on warm summer evenings to emerge as duns, dry their wings, and fly to the trees. The emergence happens after dark, typically between 9:30 PM and midnight, and draws the largest brown trout in the river to the surface in a feeding event that is audible from 50 feet away.
The triggers are temperature-dependent. The nymphs will not emerge until water temperature climbs through 62°F toward 65°F. Air temperature at dusk needs to be above 60°F. Calm evenings produce the heaviest emergences; strong wind suppresses them. Darker nights fish better because trout feed with less caution when they cannot see the leader. A new moon night in late June with 65°F water and no wind is the combination anglers plan their entire year around.
The hex hatch on the AuSable Holy Water between Grayling and Mio is ground zero for this event in Michigan. The gravel-and-silt bottom provides ideal nymph habitat, the cold spring-fed water reaches emergence temperature later than other rivers (protecting the hatch from starting too early), and the catch-and-release regulations have allowed the brown trout population to reach trophy size. Fish over 20 inches are caught annually during the hex.
Where to Be
Position matters more during the hex than at any other time on the AuSable. You are fishing in the dark, wading water you cannot see, casting to sounds. Know your water in daylight.
The Flies
Hex patterns are big, visible even in the dark, and impressionistic. The fish are eating by sound and silhouette, not by inspecting your tying. Tie or buy these in advance: the shops sell out.
What You Need
The hex is a night game with specific gear requirements. A headlamp with a red filter is essential: white light spooks fish and ruins your night vision. Heavy tippet (2X-3X) is necessary because you are setting the hook on sounds, not sights, and a 20-inch brown on 5X in the dark is a lost fish. Bring a large net with a long handle. Wading boots with felt soles grip better on the AuSable's rounded gravel in the dark. Bug spray is not optional: the mosquitoes during hex season are legendary.
Arrive at your spot at least 45 minutes before sunset. Wade to your position in daylight and mark the landmarks you will navigate by in the dark. Once the hex starts, do not wade. Stay in your position and let the fish come to you. The sound of a large brown trout eating a hex dun off the surface is unmistakable: a deep, deliberate slurp, not a splash. Cast to the sound. Strip-set. Hold on.
Reading the Hex
Not all rises during the hex are feeding fish. The splashy, dramatic rises are usually small fish or fish that missed. The largest browns feed with a slow, confident sip that barely breaks the surface. Listen for the rhythm: a big fish will set up in a feeding lane and eat every few seconds with metronomic timing. That is the fish you cast to.
If you hear rises everywhere but cannot get a take, you are likely fishing the dun when they have switched to spinners, or vice versa. The dun phase happens first as the insects emerge and float on the surface drying their wings. An hour or two later, the spinners (adults that have mated in the air) fall back to the water to die. A dun pattern rides high on the water; a spinner pattern lies flush. Carry both and switch when the takes stop.