Grunion

By laying its eggs in damp beach sand above the reach of ordinary waves, the grunion places the developing eggs somewhere that aquatic predators cannot reach them, giving the eggs a far safer place to incubate than open water. The trade-off is the risk and difficulty of the adults briefly stranding themselves on land, but grunion have evolved precise timing and body movements to minimise that risk, coming ashore only on the specific nights and tide stages when a wave will carry them up the beach and a later, higher tide will return to release the hatched young roughly ten days afterward.

The eggs are laid several centimetres deep in sand that stays consistently damp, which keeps them from drying out, and their position above the normal high-tide line means they are not disturbed by everyday wave action during the roughly ten days they need to develop. The embryos become fully formed and ready to hatch but then enter a holding state, waiting for a mechanical trigger. When a high tide during the next spawning period washes over and agitates the sand, the churning water and reduced oxygen prompt the eggs to hatch within minutes, releasing the young fish directly into the water that will carry them back to sea.

Grunion spawning is tightly linked to the lunar cycle and the tides, occurring on the nights during and just after the full moon and the new moon, in the spring and summer months, beginning one to several hours after the night's highest high tide. Because tide times and moon phases are known well in advance, marine agencies in California publish predicted grunion run schedules each year, listing the specific dates and time windows when runs are most likely, so that people can visit beaches to watch the spectacle and researchers can plan monitoring, though the fish do not appear on every predicted night.

In California, grunion may be caught by hand during open season under a set of regulations, including a closed season covering part of the peak spawning period, a requirement to use only bare hands rather than nets or holes dug in the sand, and a fishing licence for adults, all intended to limit the impact of the popular grunion hunts. The species has not been formally assessed by global conservation bodies, but it is treated as a species of concern because its beach spawning habitat is vulnerable to coastal development, mechanical beach cleaning that disturbs buried eggs, loss of sand, artificial lighting, and pollution, and long-term monitoring suggests populations have declined.