About the data

This site models Michigan's fly calendar as three thermal processes rather than one date table: insect emergence, egg availability, and forage. Here is what each rests on.

Evidence tags

• SScientific or agency: peer-reviewed papers, Michigan DNR reports and weir data, university checklists, USGS.
• AAngler: fly shop hatch charts, guides, magazine articles. Most Michigan-specific dates come from here.
• IInference: a reasonable extension we made when no source spoke directly to Michigan.

Hatch timing

Baseline windows are for the Au Sable and Manistee. We group rivers by the Michigan DNR weekly fishing report's five regions (Southeast, Southwest, Northeast, and Northwest Lower Peninsula, and the Upper Peninsula). Southwest rivers run about one to two weeks ahead, the southeast a little more, the Tip of the Mitt rivers in the two northern regions one to two weeks behind, and the Upper Peninsula two to four weeks behind. Each river carries its own offset; where a river's own chart gives a window, we use it instead. The scientific layer is thin: Hexagenia limbata has a real degree-day model (emergence at or above 20 °C water after roughly 1,800 to 2,030 degree days above 10 °C), and Michigan caddis are date-driven rather than temperature-driven. Everything else is guide consensus, and we say so.

Growing degree days

Air temperatures come from gridMET, a 4 km daily grid released CC0 and updated through yesterday, sampled at each river's centroid. We accumulate from January 1 at bases 32, 42, and 50 °F with the simple-average method and extend the series six to seven days with the National Weather Service forecast. Water temperature comes from USGS gauges where a river has one. Groundwater-fed rivers like the Au Sable and Pere Marquette respond slowly to air temperature, so we treat air degree days as a guide and let live water temperature override it. Calibrated degree-day thresholds per river do not exist yet for any Michigan hatch; building that record from seasons of local reports is the long-term project.

Eggs

Spawn timing and egg sizes come from Michigan DNR egg-take dates, Great Lakes fisheries literature, and Detroit River spawning studies. Suckers spawn near 43 to 45 °F with a peak around the second week of May; Chinook drop 7 mm orange eggs in late September and October; steelhead spawn in March and April. Egg fly colors and sizes follow what Michigan shops recommend for each.

Each river page lists only the eggs that river can carry. Salmon and trout eggs appear where that fish is documented on the river. Walleye, lake trout and redhorse come up from the Great Lakes, so their eggs appear only on reaches open to the lake, which we take to mean any reach with a steelhead or salmon run. That is a coarse rule: lake trout eggs matter mostly near river mouths, and the river pages do not yet say so reach by reach. Eggs keep drifting for weeks after the spawners are gone, going pale as they age, so for those months we list the washed-out colors instead of the fresh ones.

Fish presence

Species-by-river-by-month tables are anchored on DNR weir counts and stocking records and carry a confidence letter: A for weir or stocking data, B for multiple angler sources, C for inference. Runs shift with the year's weather; use the months as a guide.

Stocking

Each river page shows Michigan DNR Fish Stocking Database records since 1979 for the water bodies that feed that reach, pulled from the table behind the DNR stocking dashboard and snapshotted into the site a few times a year. Counts are fish planted, not fish surviving. “Rainbow trout” plants in Great Lakes tributaries are steelhead strains unless the strain says otherwise. Average lengths are converted from the DNR's centimeters.

Insect photos

Insect photographs come from iNaturalist observers who released them under CC0, CC BY, or CC BY-SA licenses. We prefer research-grade Michigan observations and widen the search only when Michigan has none. Each photo carries the observer's name, the license, and a link to the observation. If you see one of your photos here and want it removed or credited differently, contact us and we will fix it.

Until our own bench photos are shot, some pattern pages show Creative Commons reference photos of the same pattern tied by others, sourced from Wikimedia Commons and Flickr via Openverse, or shared directly by their photographer (Quinn, whose work appears on Fly Deal Flies), and labeled “reference photo.” They show what the pattern looks like, not the fly you will receive, and they never appear as product images in the shop.

Regulations

Gear rules are transcribed from the 2026 Michigan Fishing Regulations digest. Two reaches have conflicting sources and are flagged. Always confirm with the current DNR guide.

Questions or corrections are welcome. Start with the river or hatch page in question; every record lists its sources.