It’s one of the strangest and rarest mating rituals -- thousands of slim, silverfish come completely out of the water to spawn and San Diegans can catch the spectacle.
It only happens on beaches between Northern Mexico and Southern California and only during full moon and new moon cycles between March and August.
Only then can locals see those little silvery fish come crashing onto San Diego County beaches.
Here's how it works: after riding the waves to the beach, the females dig into the sand -- tail first -- and bury themselves until only their heads are showing.
That's where they lay up to 3,600 eggs each and wait until males arrive to deposit their milt and fertilize the eggs.
Then, the adult fish ride the tide back into the ocean while the eggs stay buried in the sand to incubate for two weeks.
During the next high tide, waves come onto the beach, rouse the tiny, young fish and sweep them into the ocean.
Scientists predict a probable two-hour window when spawning occurs. The times vary at different beaches along the coast, with San Diego County runs occurring about five minutes earlier than runs in Los Angeles.
The California Department of Fish and Wildlife creates a calendar each year with predicted times grunion runs will occur. For this season's expected runs, visit here.
Open season runs allow for fishing and occur in March, June and July while observation-only runs occur in April and May.
To participate in open season runs, participants must be 16 years old or older with a valid California sports fishing license.
Grunion sightings may occur at Oceanside Pier View North Beach, Oceanside Harbor Beach, Del Mar Beach, La Jolla Shores, Mission Beach, Ocean Beach, Coronado and Coronado Shores Beach and Imperial Beach but sightings are not guaranteed.