What it's Like to Witness a Grunion Run

It was a balmy Southern California evening like any other when evolutionary biologist and wildlife photographer Neil Losin and I (a cognitive psychologist and science writer) got into our cars in West L.A. and drove to San Pedro. Thirty miles later, we were faced with what could only be described as massive, salty, sandy, beachfront orgy. And we had front row seats.

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Participants in the bacchanal were thousands of five- to seven-inch-long fish called California grunion (Leuresthes tenuis).

About three hundred curious human voyeurs had gathered on the Cabrillo Beach to watch the spectacle. The beach was open later than usual thanks to the Cabrillo Marine Aquarium, which organizes a handful of Meet The Grunion programs throughout the mating season.

These fish mating events are known as "grunion runs," and each run lasts four nights in a row surrounding the full and new moons in spring and summer. Each night, the fish ride the waves up onto the sandy beach, where they spawn for several hours, beginning just after high tide.

Onlookers were instructed by aquarium staff to keep their voices down and to walk around as little as possible. The noise and vibrations, understandably, can cause some performance anxiety in the fish, scaring them away from the beach. Suddenly, we were told that we could turn on our flashlights. The once calm, sandy beach had transformed into a mess of wiggling, squirming, silvery fish. What appeared chaotic was actually the grunion's highly choreographed mating dance, fine tuned through countless generations of evolution.

Once on the beach, the female burrows backwards into the watery sand with her tail to create a nest, burying herself until only her head is poking out of the sand. Then, she deposits her eggs about four inches below the surface. Meanwhile, males curve their bodies around the female's protruding head on top of the sand.

As many as eight males may gather around the same female, each releasing their milt, which flows down along the female's body until it reaches and fertilizes the eggs. After their release, the males flop down the beach and back into the water. The female twists herself free and catches the next wave back into the ocean, leaving between 1,600 and 3,600 eggs behind.

The eggs incubate for ten days buried in the sand, disturbed only by foraging shorebirds, beetles, sand worms, and other insects. The next high tide washes the newly hatched larvae back into the sea.

According to the California Department of Fish and Game, grunion runs occur from March through August, though sometimes as early as February and as late as September. Peak spawning occurs beginning in late March and ends in early June.

While grunion are found all along the California coast -- as far north as Monterey Bay, and as far south as San Juanico Bay, in Baja California -- the fishery has faced a critical loss of spawning habitat due to beach erosion, harbor construction, and pollution over the last one hundred years. In 1927, a regulation was passed that forbade fishing for grunion from April through June. After the fishery began to improve, the closure period was shortened in 1947 to just April and May. That closure is still in effect today, and has the provisions that licensed sport fishermen over age 16 may take grunion only by hand, and that they may not be trapped in holes dug into the beach.

For more information, including a calendar of expected grunion runs for the rest of 2012, see the California Department of Fish and Game website.

For information on the Cabrillo Marine Aquarium's "Meet the Grunion" program, see their website.

Their program costs $5 for adults and $1 for seniors, students and children. However, if you wish to skip the program, you can meet up later with the group and the aquarium staff on Cabrillo beach for free. Bring a sweater, clothes that can get wet and sandy, and a flashlight.

About the Author

I'm a doctoral student in Developmental Psychology at USC, and my research focuses on the evolution and architecture of the mind, and how different early experiences might affect innate knowledge systems. My blog, The Thoughtful A...
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