When under water, my eyes are usually trained on the bottom, so it’s especially dramatic when I sense the ambient light suddenly decrease and darkness settles in around me. Like a cloud crossing in front of the sun when I’m absently looking elsewhere, I just have to look up to see what cast the shadow. Instead of cumulus formations, under water I see clouds of sardines, all silvery streaks passing over me, which call to mind a moving partition. I inflate my buoyancy compensator and float up to the seemingly solid curtain above me. The shiny wall miraculously parts, then closes in around me, and I become engulfed in a fish bubble. I lose my bearings and become dizzy. Despite years of diving, I am continually fascinated by the complex social behavior that makes a school seem more like a single entity. About one-quarter of all fish school throughout their lives, and half of all fish join schools for at least some part of their lives — often as juveniles or during mating. For example, our local adult giant kelpfish (Heterostichus rostratus) school as tiny youngsters but go solo as adults, and the giant sea bass (Stereolepsis gigas) goes it alone as both juvenile and adult but schmoozes during mating season. Whether forming a small clique or a group too numerous to count, fish may school to travel, feed and rest, but mostly they school to avoid predators. Any group of fish of the same species and about the same size can join forces and make up a school, although I frequently see a minority fish species of similar size interloping a different schooling species, if only briefly. As I watch, I marvel how fish move in synch better than the most highly trained marching band. They always maintain the same distance from their neighbors and from all other fish in the school. Each fish is able to do this by zeroing in on visual clues and “freezing” the movements of their neighbors by swimming at the same speed as they swim and maintaining the same distance from each other. Any change in swimming direction and speed, no matter how small, is repeated in a chain reaction throughout the school. This accounts for the pulsed wave I see when the leaders suddenly change direction. Frequently, a dark stripe or spot acts as a visual tip. For example, sardines are easily identified by the rows of longitudinal black dots on each side and the backs of their bodies. By coordinating their eyes, nerves and muscles, schooling fish move with astonishing precision and regularity. Being in the midst of a school, I can truly understand how this behavior evolved as a way to ward off predators. When I am enveloped, I become temporarily confused and dizzy in the same way as any predator. Most predators look for their prey using visual tips when chasing smaller fish. They quickly and automatically compute size, colors, contours and movements around them, comparing these with information stored in their brains. Then, if something looks like prey, they attack. But when a predator approaches a school, the school contracts and the individual fish dissolve into one another so they appear as one gigantic shimmering mass (a superfish!) or even shape themselves into creatures unfamiliar to a predator. I’ve seen a school of fish shape themselves into what looked like a 500-pound giant sea bass. Another time I watched a school shape themselves into a 10-foot-long eel-like serpent. Amazing! Any fish that cannot keep up with the school or chooses to play hooky is easily targeted by that no-longer-dazzled predator. Besides safety, schooling fish are better swimmers than their nonschooling counterparts. All bicycle riders will appreciate that schooling fish draft off one another to reduce the drag of pushing themselves through the water. And, of course, having such close contact makes for the ultimate singles bar. Despite all these evolutionary sophistications, no animal can compete with human predators. Fishers’ knowledge of schooling and use of highly refined detection devices have made them super efficient, thwarting the very behavior that has proved so valuable in allowing many fish species to evolve and survive. Tattling devices aren’t necessarily on boats. The Internet has provided an easy way to out the locations of marine life. For those noticing a recent flotilla of fishing boats at night and early morning off La Jolla Shores, they likely arrived via scuba divers, who have been posting reports on dive club websites detailing their sightings of squid spawning in the canyon. With no place to hide and human fishing behaviors continuing to impact stocks worldwide, this pressure, along with the effects of global climate change, may drive more commercially fished species into collapse in the not-so-distant future. — Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? Email [email protected].
