When I plunge into San Francisco Bay in spring, I’m swimming through a cool green fish stew. This is the season when microscopic diatoms are proliferating. Masses of these jewel-like, silica-walled plankton are turning sunlight into sugar, providing the base for an incredible production of life. The diatoms feed the animal plankton that feed the infant shrimp that feed the juvenile California halibut and Pacific herring. The strands of eelgrass lengthen with the days and shelter them all.
Unlike the hikers on land who enjoy vistas of poppies in bloom, we swimmers can’t see the explosion of vernal life all around us. The ingredients in this soup are hidden from the human eye by the very density of organic activity: The water is cloudy with the plankton and sediment that make the estuary so hospitable to baby fish and other animals.
My goggles give me a view only of murk. The water is so turbid, I often can’t see my hand at the end of my arm. I long to view the fish all around me, the way I can when I’ve snorkeled in warmer, clearer seas. But the only way I know they’re there is secondhand—when the hovering terns and dive-bombing pelicans let me know they can see goodies that I can’t.
So to meet some fish up close and personal, I went out in Alviso Slough in the South Bay with UC Davis researcher James Hobbs and his team. They regularly count fish around the Bay for the multiagency Wetlands Regional Monitoring Program.
Our boat putt-putted slowly in the olive-green channel, dragging a net that snagged a varied catch. The team tallied and measured the fish in white plastic trays before returning them to the water. This gave me a chance to see some of my fellow Bay denizens face to face: Yellowfin gobies almost as long as my hand lay on their broad bellies and stared up at me with froggy, ugly-cute faces. Silvery northern anchovies, with their permanently downturned mouths, looked dismayed. Shiny American shad shed glittering scales in an attempt at distraction. Staghorn sculpin made a faint growling sound and stuck out their gill flaps like little antlers, trying to look scary. But the endangered longfin smelt, semi-translucent in their juvenile state, with a brain you can see through their flesh, lay almost motionless, seeming vulnerable.
Swimming in the Bay
A series by Susan Kuramoto Moffat on what you can learn about our local waters from diving in. See her Summer 2025 Issue column for resources on how to get started swimming outside.
And continue the journey with her seasonal columns on Bay waters:
We didn’t catch any leopard sharks, bat rays, striped bass, sturgeon, or Chinook salmon, but Hobbs tells me these bigger fish share the Bay with us swimmers, too.
Next time I swam, it was a strange feeling to know that so much underwater wildlife is so near me, yet invisible. The fish know just where I am, while I’m like a blindfolded person at a crowded party. Their five senses are far better adapted to the water than the sorry equipment of this terrestrial mammal in a bathing suit.
“The fish know just where I am, while I’m like a blindfolded person at a crowded party.”
Fish smell well. And their highly developed olfactory systems are especially important in low-visibility waters. As a human, I can sense strong odors—say, the fishy breath of a seal nearby–but I can’t get a whiff of actual fish underwater, let alone navigate home by scent like salmon or identify friend or foe.
My sense of touch is adequate to tell me if there is a strong current or if a creature directly bumps me, but it’s nothing like the haptic equipment of fish that blurs “feeling” and “hearing.” A set of sensors on their flanks called the lateral line alerts them to predators and prey. This row of pores detects pressure and motion in the water and translates it to the fish’s nervous system through tiny hairlike structures similar to those in our ears.
Human hearing is not well-adapted to the water, and my earplugs make me even deafer. But fish hear, in a sense, with their whole bodies. In addition to the lateral line, some fish use their swim bladders to convey sonic information to their brains. And their otoliths, or “ear stones,” serve to catch sound that would otherwise pass quickly through a fish’s body because of the high water content of their flesh.
I can only taste with my tongue, and it’s not much good for navigation. Many fish, though, have taste buds on the outsides of their bodies, and the flavor of their environment helps them decide where to go.
My eyes are designed for the refractive index of air, not water, but fish have spherical lenses and better low-light vision that help them see underwater.
My human limitations create a peculiar phenomenon when I swim. My body is engaged—every inch of my skin is intensely sensing the water—but my blindness gives my mind room to wander. Exercising my imagination, visualizing the fishy world around me, is one of the great pleasures of swimming in the Bay.
