Grunion Dance: By the Light of the Moon

grunion and poem
photo by Kelly Scofield

There has been much interest in the full moon lately because of recent headlines about the ‘super blood wolf moon’ in January (coincident with a lunar eclipse) and the ‘super snow moon’ in February. However, California Grunion don’t care what we call the various full moons or the unnamed new moons in the early spring through fall months. It’s the combined gravitational pull of those moons and the sun that produce the highest tides of the month, and bring California Grunion ashore by the thousands to spawn.

You can watch these incredible spawning events, called grunion runs, at predictable times of the year in Southern California. The 2019 schedule of predicted grunion runs, produced by the California Department of Fish and Wildlife, is now available online to help you plan your night time excursion. These runs can occur for three or four nights following the full and new moons. While the runs are predictable, remember that there is no guarantee the fish will run on the night or the beach you choose. Luck plays a big part in catching one of these magical events.

The phases of the moon have a powerful hold on the lives of these small, silvery fish from beginning to end. Scientists think their “internal clock” may detect the tidal changes heralded by the full and new moons and trigger them to begin their runs. The exact trigger is not known, though one guess is they may be able to detect changes in water pressure caused by the rising tides.

For the most part, California Grunion live lives of quiet anonymity in nearshore waters. However, on full and new moons from early spring through summer, they abandon their secretive ways and swarm onto some of Southern California’s wide, sandy beaches (also down into Baja California).

Being somewhat discreet, they wait until the cover of darkness to come ashore, just after the peak of high tide. During the runs, they swim ashore with waves that wash high onto the beach. The females twist and wriggle tail-first into the moist sand until only their head is exposed, and then begin laying their eggs. One or more males will  wrap themselves around her and fertilize the eggs. With the next wave, the fish jump, leap, and wriggle back into the sea. This ‘dance of the grunion’ is one of the most unusual mating rituals found in fish.

Female California Grunion can return to ‘dance on the beach’ four to eight times per year and may lay over 3,000 eggs every two weeks. That means a single female has the potential to produce tens of thousands of eggs in her short, four-year life span. The eggs incubate under the sand for about 10 days, until the tides are once again high enough to wash out the fully developed embryos. The jostling action of the waves stimulates the embryos to hatch. The larval grunion enter the oceanic stage of their lives and start growing into adults. They will join in their own ‘dance on the beach’ the following year.

Some enthusiasts go out in the wee hours with buckets to collect these small fish with their hands (the only legal method for capturing grunion), then enjoy a tasty dinner later. Remember, possessing a valid sport fishing license is required. If you would prefer to help conserve California Grunion populations, you can observe and learn about these fish rather than hunt and collect them. Leave the females to deposit eggs tail-first in the sand, and do not disturb the incubating eggs they leave behind. This will allow the fish to complete the spawning portion of its life cycle, and contribute to future California Grunion spawning events and generations.

Either way, a night on the beach under the moon or stars can be fun. Be sure to visit the California Department of Fish and Wildlife’s grunion web page to learn more about these fascinating fish.

post by Leslie Kashiwada, CDFW Scientific Aid