May is not peak season in the Keys because the moon cooperates, the worms swarm, or anybody says so in a magazine. It is peak season because the adult tarpon population is physically present — aggregated, staged for spawning, and locked into a residency window that tracking data now describes with surprising precision. What the average angler calls “timing” is really just finding that standing population, on a day when the wind and tide let you actually see fish. The rest — the hatch, the moon phase, the choice of fly — matters only after the population question has been answered in your favor.

The work that anchors this claim is a peer-reviewed acoustic-telemetry study published in Marine Ecology Progress Series by Lucas Griffin and colleagues at Bonefish & Tarpon Trust. Tagged adult tarpon showed a tightly clustered spring–summer residency in the Keys, with a median residence period of about 40–60 days and a mean around 51. Arrivals concentrated in April once sea-surface temperatures reached roughly 26°C (79°F). Peak occupancy fell in May at 26–28°C. Most departures happened in June between 27 and 29°C. Individual fish arrived with high repeatability — the same animals, on a similar schedule, year after year — which the authors interpret as a photoperiod cue, not a temperature one. The population-level threshold around 79°F just happens to be what May delivers.