All the Fish We Cannot See

November 22, 2023

The ocean has a way of upending expectations. Four-story-high rogue waves peak and collapse without warning. Light bends across the surface to conjure chimeric cities that hover at the horizon. And watery wastelands reveal themselves to be anything but. So was the case for the scientists aboard the USS Jasper in the summer of 1942. Bobbing in choppy seas off the coast of San Diego, California, acoustic physicist Carl F. Eyring and his colleagues, who had been tasked with studying a sonar device the navy could use to detect German submarines, were sending sound waves into the deep. But as the echoes of their tests came reverberating back, they revealed a puzzling phenomenon: everywhere the ship went, the sonar detected a mass nearly as solid as the seafloor, lurking about 300 meters or more below the surface. Even more mysterious, this false bottom seemed to shift over the course of the day.

People had their theories—shoals? faulty equipment?—but apart from registering the anomaly, scientists let the mystery slide. (There was, after all, a war on.) It wasn’t until 1945, when oceanographer Martin Johnson dropped nets into the Pacific to take a closer look, that the culprit was definitively unmasked: a vast cloud of marine animals, most smaller than a human hand, that moved from the deep ocean to the surface and back every day.

Since the 1940s, scientists have been unpicking the mysteries of this vertical migration and the creatures taking part in it. Until recently, research focused on zooplankton and other small organisms, neglecting their larger predators. But technological advances have made it possible to look up the food chain in that migrating cloud to examine the variously stealthy, strange, and slimy fish of the mesopelagic zone, the marine “twilight” area between roughly 200 and 1,000 meters deep, where the last rays of surface light penetrate before dissipating into the absolute dark of the sea below. And that research has started to reveal something as significant—and mysterious—as the migration’s first documentation by scientists: by moving carbon through the ocean, migrating fish in the twilight zone, which are thickly distributed enough to have once fooled a sonar, may also play an important role in stabilizing the climate.

Researchers are now attempting to quantify how much carbon the fish in one of the planet’s most unexplored ecosystems are cycling from the ocean’s surface to the deep sea. The inaccessibility of these fish makes this work more challenging than conventional fisheries research, but there’s an urgency nonetheless: the same technological progress that makes it possible to study mesopelagic fish is also making it more enticing to harvest them, even as climate change threatens to reshape the ecosystem.

Mesopelagic fish are not yet commercially exploited, though projects are underway to investigate the marketability of the species and the best methods for harvesting them. Scientists working in this area are, therefore, in the rare position of being able to assess potential impacts from fishing before they happen. But with so many unknowns, the question remains: can they do it in time—for the fishes’ sake and our own?

Full article at Hakai Magazine