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By Julianne E. Steers
If you grew up here in Southern California, there’s a good chance you’ve heard rumors of a midnight fish freak called a grunion running around at least once during your childhood. If you were lucky you stayed way past your bedtime and ventured out to the coast to take a peek at this wild activity. If you are from elsewhere, chances are you thought it was total urban folklore. Well, that’s not a myth; there really are fish throwing themselves on the shore under the silvery moonlight, all for the survival of their species. Grunion (Leuresthes tenuis) are famous for their remarkable breeding behavior which amazes spectators when they experience a race firsthand.
In phase with the moon, the races have a lunar periodicity occurring in coordination with new and full moons at higher tides. As the tide peaks, the nocturnal coastal rave begins and the rhythmic design emerges. A wave of grunion stirs on the shore; females dig in soft sand to lay their eggs, while males fly to fertilize their brood before they all retreat to the sea. The eggs are left high and dry to incubate until the next highest tide about two weeks later.
An unusual life cycle makes the grunion extremely vulnerable to ecological changes. As our climate changes, so does the habitat of the natural world. The impacts of an ever-growing human population combined with the climate intensify the problems facing grunion populations. Rising sea levels and climate-induced erosion on the coast have reduced the habitat available for these beach fish. The loss of this critical spawning habitat will have a direct negative impact on every future generation. When paired with seas that heat up exponentially, fish receive a boost for their survival. The increase in temperatures determines the sex ratio in the grunion. In particular, a warmer temperature at the start increases the proportion of males in the school, which translates into reduced recruitment of the next generations and long-term demographic trends. What to do with a brood of singles when there are fewer singles on the coast? Each needs the other for the survival of their species.
If finding a mate was the only concern, grunion must also fight against the increasing acidity of our ocean. The acidification of the oceans is an important consequence of the excess carbon dioxide in the atmosphere. Since these effects occur underwater, we neither see nor feel them, but our finned compatriots certainly do. Much of the carbon dioxide in the atmosphere does not stay in the air, but dissolves in our ocean. In the case of these fish, acidification can reduce their growth during crucial larval stages.
The adaptability of the California Grunion to long-term changes in their habitat remains to be seen.
Julianne Steers is a marine biologist and conservation photographer. She has extensive experience in ecology and has researched, dived and explored the local ecosystem and beyond to support the natural world.
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