Does Beach Grooming Harm Grunion Eggs?

California Sea Grant Rapid Response Project
Researcher Karen Martin, Pepperdine University

 

August 28, 2002

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

grunions

The grunion, a slender silvery fish that looks a lot like a large sardine, is one of only a few species of fish to reproduce on land. Not only does it brave air to spawn, but it does so with an uncanny, clockwork adherence to the rhythm of the tides.

In spring and summer, grunion are believed to congregate just outside the surf zone, waiting for the phase of the moon, and the tides it controls, to cue them to “run.” Grunion runs occur a couple days after a full or new moon, when the tidal cycles are most extreme at night. There are two high tides a day in California. Grunion spawn only when the highest high tide occurs at night.

As the high tide begins to recede, exposing wet sand, grunion surf the shore break, purposefully washing themselves up onto gently sloping sandy beaches. Once aground, their writhing bodies begin a dance of life, or at least of reproduction. Females furiously wiggle into liquid sand while males curl themselves around the females’ upright bodies, depositing milt (male gametes) as females lay their eggs. With no time for small talk, the grunion stage a mad dash back to the surf.

The eggs, buried a few inches down, are incubated in the nest-like warmth of summer sand. It takes about 10 days for the eggs, which look like orange grains of sand, to develop fully. During this time, the tidal cycles shift. Sand is scoured away, and the eggs, now ready to hatch, are swept with the sand back out to sea.

The phenomenal delicacy, or precision, of the grunion life-cycle has prompted some to question whether beach grooming could harm grunion eggs, and hence impact grunion populations.

The City of San Diego grooms its beaches, usually in the wee hours of the morning, to remove trash left by beachgoers and tangled heaps of kelp washed ashore by the tides. During the grunion’s spawning season, the city curtails its grooming practices, avoiding sandy stretches regularly saturated by the tides. It is in these areas that grunion lay their eggs.

The concern is that heavy metal rakes, which are pulled by tractors, may uncover eggs, damage them, or compact the sand above them. In all these scenarios, loss of egg viability

grunions

could reduce grunion numbers.

To investigate whether beach grooming does in fact harm grunion eggs and whether the city’s modified grooming practices adequately protect grunion eggs, California Sea Grant recently funded a Rapid Response project led by biology professor Dr. Karen Martin of Pepperdine University in Malibu. Her project is being conducted in collaboration with the City of San Diego and Project Pacific, a San Diego non-profit organization that leads a “grunion school” at which local volunteers are taught how to monitor grunion runs at local city beaches.

For her project, which will end this summer, Martin is conducting surveys at four groomed, popular city beaches – Ocean Beach, Mission Beach, Pacific Beach and La Jolla Shores. With help from volunteers who keep her apprised of recent run locations, Martin goes out and locates grunion eggs, called clutches, precisely marking their locations via global positioning satellites. City workers are then directed to groom over some of the grunion eggs, leaving others untouched, for comparison later. Martin later digs up the eggs, visually inspecting them for damage. Some of the eggs are also taken back to her laboratory for further tests of their viability.

The results of Martin’s project will be presented to a subcommittee of the San Diego City Council in October. The full council will vote on whether to approve or reject her recommendations.