Sex on the Beach – Experiencing a Grunion Run, First Hand

To the uninitiated, a grunion run can be considered the marine equivalent of a snipe hunt. Send some sucker out to a deserted beach in the middle of the night and tell him to wait for high tide when thousands of fish will magically appear on the beach. Yeah, right!

Except that, in the case of the grunion, it’s true!

Every year from March through August, the California grunion (Leuresthes tenuis) completes what has to be one of nature’s most remarkable life cycles. By the tens of thousands, the grunion (singular and plural are the same) swim into the surf zone, catch a wave onto the wet sand, and beach themselves to spawn. Once the spawning is finished, they catch another wave back into the surf zone and disappear.

Although the behavior may seem strange (grunion are the only fish known to spawn out of the water), the grunion’s primal urges are so attuned to the lunar cycle that grunion “runs” can be predicted as much as a year in advance.

Simply put, the grunion have evolved two truly remarkable strategies for the survival of their species. First, they lay their eggs out of the water so the eggs will be safe from other predatory fish. And secondly, they lay the eggs high enough on the beach so that they will not be disturbed during the incubation period, which is about two weeks.

In Southern California, we generally have four tides each day—two high tides and two low tides, each a bit over six hours apart. But over the course of a month, the height of the highs and lows changes based on the pull of the moon and the sun. The highest tides occur at the full moon and the new moon. The lowest tides occur at the first quarter and second quarter. And the nighttime high tide is always higher than the daytime high tide. Because of these factors, California grunion always breed at night on the tides that follow the full moon and the new moon.

Within two or three days of the new or full moon, the grunion come ashore in droves to lay their eggs. The females, who can lay as many as 3000 eggs every two weeks, ride as far up onto the sand as possible. They drill themselves into the sand tail-first (just the head of the female is exposed), waiting for the first male who happens by. (The phrase “suitable male” is apparently not in the grunion lingo.) The male curls around the female, deposits his milt (sperm) into the hole she’s dug, and then they both wriggle back to the ocean. Although the actual mating process takes less than a minute, grunion can survive out of the water for a while and may spend a couple of minutes on the beach.

Once the eggs are laid, they incubate in the sand for almost two weeks. Since the grunion have laid their eggs just following the full or new moon, each succeeding high tide is slightly lower than the tide when the eggs were laid. It’s not until the approach of the next series of highest high tides (two weeks later) that water will again reach the eggs, just when they’re ready to hatch and begin life in the sea.

Basically the pattern is that a new or full moon occurs, the grunion come ashore to lay their eggs, high tides recede for a week to their low level, then start building back up for another week, and the new grunion hatch just before the next new or full moon, which also coincides with the highest high tides. The new or full moon passes, and the cycle starts again with another grunion run.

Going to watch the grunion mate (hence, the title of this article) is really a lot of fun. It’s important to pick the right beach because grunion themselves are choosy about where they mate. (It’s also important to dress warmly, take a light, a dive light is perfect, and bring an extra pair of shoes and socks, in case your feet get wet.) Cabrillo Beach in San Pedro is a pretty reliable place to go and also home to the Cabrillo Marine Aquarium, which puts on programs each month the grunion run.

The key thing is to be patient. A grunion runs starts with individual males, the scouts, coming in to check out the beach while the rest of the horny school waits offshore. And supposedly the grunion are rather skittish and it’s easy to scare all the fish off by being too noisy or using your light too much.

Once the scouts have deemed the beach to be okay, the grunion start arriving in small groups. In a few minutes, what had been bare sandy beach is teeming with thousands upon thousands of writhing grunion. Some will be involved in the mating process, some will be on their way back to the ocean, and some will just be hoping to get lucky.

The run itself can last for an hour or two and will repeat itself over the course of three or four consecutive nights. The activity is generally highest on the first evening of the run, tapering off slightly each night after that. It’s thought that an individual grunion may breed a couple of times each evening, as well as on subsequent nights. But with so many grunion coming ashore, it’s impossible to track an individual to confirm this.

Perhaps the most gratifying thing to understand about grunion is that while you’re watching this awesome spectacle, it’s not like watching a salmon where, where all the spawning fish are going to die. All the grunion that come onto the beach go back into the water and the next morning, there isn’t a clue as to what had happened the night before.

If you’re interested in observing grunion during the 2001 season, you can call the Cabrillo Marine Aquarium (310/548-7562) to obtain their program information. If you’d like to learn more about the grunion or to get the complete schedule for 2001, visit the grunion page on the California Department of Fish and Game website at www.dfg.ca.gov/mrd/grnindx3.html.

COPYRIGHT © 2019 CALIFORNIA DIVING NEWS. ALL RIGHTS RESERVED.Website hosted and managed by Make Me Modern.