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By Julianne E. Steers

If you grew up here in Southern California, odds are you heard rumblings about a midnight fish phenomenon called a grunion run at least once during your childhood. If you were lucky, you stayed up way past your bedtime and ventured out to the coast for a peek at this wild activity. If you’re from elsewhere, chances are you thought this was total urban folklore. Well, it’s no myth; there really are fish that fling themselves on shore under the silver moonlight, all for the survival of their species. Grunion (Leuresthes tenuis) are famous for their remarkable spawning behavior that amazes onlookers when they experience a run firsthand.

In tune with the moon, runs have a lunar periodicity occurring in coordination with new and full moons at the highest tides. As the tide peaks, the nightly coastal rave gets underway, and the rhythmic conception transpires. A wave of grunion wiggles onto shore; the females dig in to the soft sand to lay their eggs, while the males flit about to fertilize their brood before they all retreat back into the sea. The eggs are left high and dry to incubate until the next highest tide about two weeks later.

An unusual life history leaves grunion extremely vulnerable to ecological change. As our climate changes, so does the habitat for the natural world. Impacts of an ever-increasing human population combined with climate intensifies the issues challenging the grunion populations. Climate-induced sea level rise and erosion on the coast has diminished the available habitat for these beach-spawning fishes. Loss of this critical spawning habitat will have a direct negative impact on each future generation. When paired with exponentially warming seas, the fish receive a one-two punch to their survival. Increased temperatures determine the ratio of sexes in grunion. Most notably, a warmer temperature early on increases the proportion of males in the school, translating to reduced recruitment of next generations and long-term population trends. What is a bachelor brood of fish to do when there are fewer bachelorettes cruising the coast? Each needs the other for survival of their species.

If finding a mate were the only worry, grunion must also combat the increasing acidity of our ocean. Ocean acidification is a significant consequence of excess carbon dioxide in the atmosphere. Since these effects are happening underwater, we don’t see or feel them, but our finned compatriots certainly do. Much of the carbon dioxide in the atmosphere doesn’t stay in the air, but dissolves into our ocean. In the case of these fish, acidification can reduce their growth during the crucial larval stages.

What remains to be seen is the adaptability of California grunion to long‐term changes in their habitat.

Julianne Steers is a marine biologist and conservation photographer. She has an extensive background in ecology, and has been researching, diving and exploring the local ecosystem and beyond to sustain the natural world.

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