Tides in Biology Tides in Biology Grunion leave the water at night to spawn on the beach in the spring and summer months two to six nights after the full and new moons. Spawning begins after high tide and continues for several hours. As a wave breaks on the beach, grunion swim as far up Spawning occurs from March through August, and occasionally in February and September. Peak spawning period is between late March and early June. Once mature, an individual may spawn during successive runs at about 15-day intervals.
Females can spawn as many as six times during a season.
Mature females lay between 1,600 and 3,600 eggs during one spawn, with the The eggs are deposited during the highest tides of the month and incubate in the sand during the lower tide levels, safe from the disturbance of wave action. The eggs are kept moist by residual water in the sand. The eggs hatch during the next high tid You can watch grunion eggs hatch by collecting a cluster of eggs after a grunion run and keeping them in a loosely covered container of damp sand in a cool spot for 10-15 days.