The California grunion are some of the most amazing creatures on Earth. Unlike most fish, they lay their eggs on land. No other marine fish goes to such lengths.
Grunion runs in California are legendary. Between March and August, during the spring tides, two to three days after the new and full moons, the grunions come ashore in the thousands to lay their eggs. They time their arrival right after the peak of the high tide. That way, they insure their eggs will have sufficient time to develop before the next spring tides arrive some 10-14 days later.
On the days mentioned above, large schools of male and female grunions gather in the shallow waters of the surf zone. At about the time of the high tide, a few male scouts run up with a wave and appear to check things out. If there are a lot of lights, people, or a flock of hungry seagulls standing by, then they move further down the beach. When the scouts decide the coast is clear, a wave of grunion rushes up on the shore, and the sexual frenzy commences.
First, the females dig their tails and bodies into the wet sand. At about the depth of two-thirds of their body length, they deposit their eggs, anywhere from 100 to 3,000, according to reports. The males, who outnumber the females, immediately wrap their bodies around the submerged female and spray her with sperm. There is no act of copulation among grunion. The milt (as fish sperm is called) runs down the body of the female and fertilizes the eggs.
One to several males may court a female; males continue to produce milt and fertilize eggs throughout the evening. Once the males are finished, they run off in search of another female. The females are left to fend for themselves, wriggling and struggling out of the sand, then flopping back towards the waves and into the night.
Grunion appear to be limited in range from about Morro Bay to halfway down the Baja peninsula. In Baja, grunion runs occur during the day because the high tides there occur earlier. Grunion runs in Southern California always occur at night.
Once the eggs are deposited, they develop very quickly. Within 10 days, a 1-millimeter, nearly fully developed larval fish is formed. When the water and waves disturb the eggs 10-14 days later, the eggs immediately burst open and the grunion swim out to sea.
How the grunions know when to arrive on the beach, and which beach is the best and where they go when they are not flopping around on the beach is one of the great mysteries of the sea.