The fish that promote ‘orgies’ on California beaches in rituals ordered by the Moon

In the middle of the night, on a beach in Southern California, in the United States, a citizen scientist observes thousands of fish having sex.

“Thousands of them, rebels, all making noise,” he notes. “It seems like some kind of Mad Max post-apocalyptic marine.”

This unique mating ritual is known as the California kingfish race.

Unlike most other species, this small silvery fish spawns on land, after launching itself from the ocean onto the sand. And they only spawn during the full moon or new moon, because they need the high tide to reach the beach.

Since 2002, these rituals have been recorded by “California kingfish watchers” – citizen scientists who volunteer to observe the fish on 50 beaches in the US state.

They forward their observations to scientist Karen Martin, from Pepperdine University in Malibu, California. She has been studying the species for decades and more than 5,000 people have already collaborated on her project.

“We couldn’t do it without them,” she says. “There’s no other way to get this kind of data. It’s really remarkable.”

Counting the fish is almost impossible. They intelligently avoid nets and are not attracted to bait on hooks.

“‘Normal’ population assessment methods don’t work for this species,” explains Martin.

The California kingfish is only found along the Pacific coast, mainly between Punta Abreojos, in Baja California (Mexico), and Point Conception, in central California (USA).

The females swim as far as they can and launch themselves out of the water onto the sand. They wave their tails to dig a hole and release the eggs. The males follow them and fertilize them.

The eggs remain buried in the sand until the next wave is high enough to hit them, which typically takes about 10 days. Then they hatch.

But this behavior poses risks for the California kingfish – and not just because it is easy prey when spawning in the sand.

“Everything people do on these beaches impacts the environment,” according to Martin.

Collecting data on this species is a challenge. But the implementation of regulations to protect them has brought some successes.

The State Department of Fish and Wildlife created the first regulations protecting California kingfish in 1927. Scientists had observed that the fish was caught in enormous quantities when it came out of the water. People used nets made from sheets to catch fish en masse.

Restrictions have been introduced on the use of equipment during the closed season (April to June) when fishing is prohibited. As a result, local residents could only use their hands to fish.

In the 1940s, marine biologist Boyd Walker (1917-2001) observed every fish run in La Jolla, California, for three years. His dissertation mapped the range occupied by the species, its mating habits and developed a method for counting fish, today known as the Walker Scale.

It is this scale that Martin’s observers use to compile their reports.

The scale ranges from W0, which means “no fish or just a few individuals”, to W5, which indicates “fish covering the entire beach, layered with several individuals, impossible to see the sand between the fish”.

W5 is a rare event, seen in only about 1% to 3% of observations per year. But citizen scientists evaluate the number of fish on the coast, the duration of spawning and the length of beach covered by the fish.

Data collected from the beaches most popular with California kingfish demonstrate a general decline in their population.

Martin’s study, published in 2019 using citizen science data, concluded that between 2002 and 2010, the average rating on the Walker scale was W2, i.e. 100-500 fish at the peak of the run period.

Between 2010 and 2018, the rating dropped to an average of W1 – less than 100 fish at peak. Additionally, the average W0 (“no run”, with little or no spawning) was recorded in 2014 and again in 2016.

“Despite local concentrations, California kingfish are not abundant,” says environmental scientist Dianna Porzio of the California Department of Fish and Wildlife.

“Although the data [dos observadores de peixes] have limitations, the conclusions demonstrate a reduction in the number of California kingfish spawning in a large part of the southern part of the state in the last decade”, according to her.

Data from citizen scientists led to greater protection for fish.

“Data is useful for several reasons,” explains Martin. “Knowing where California kingfish congregate helps us find eggs and track human impacts on the species. We discovered that beaches were being swept where the fish nest.”

In 2020, the Department of Fish and Wildlife increased restrictions on fishing for California kingfish, normally done for individual consumption. The restrictions directly mention data obtained by Martin’s fish watchers as evidence of the population decline.

On several beaches in the southern part of the state, beach sweeping protocols have been introduced to protect California kingfish eggs. And the data also led to the discovery that the species is expanding north.

“We were able to identify their occupancy range,” says Martin. “Now, we know it is found north of San Francisco and not just in the center of the state.”

The program also created an army of California kingfish defenders.

“People have become great protectors of the species,” she says, laughing. “Some of them even approach other people who disturb the fish or catch too many fish, explaining the importance of respecting the rules.”

This is an important example of how members of the public can be trained as citizen scientists, with notable impact, according to Martin.

But Martin insists on highlighting the value of public participation.

“People are excited to know they’ve made this sighting and that their knowledge is useful – that someone cares about what they saw,” she explains. “And it makes people appreciate what’s in their environment, knowing about these wonderful wild animals.”