For example, beach construction is prohibited during the spawning season to better assure that eggs will hatch. During a grunion run, there may be a few dozen or thousands that wash up and flow out during that hours-long episode of matings. If they haven't shown up anywhere during the first nights of the expected four-night window, then they probably will not appear until the next full moon or new moon period a couple of weeks later, Thompson said. Read listed some beaches that in previous years have had reports of good grunion running, including Imperial Beach, the Silver Strand, Coronado, La Jolla Shores and around the south side of Oceanside Harbor. To improve the odds of seeing grunion, he suggests not making noise and not shining lights in the water, because the fish probably sense environmental changes when choosing a beach to land upon.
Some people speculate they send out scouts.
In that scenario, if those first plungers don't return to the water, no more come ashore. This isn't a stupid fish here. This is probably a smarter fish than it looks.