Swimming, Swirling, Schooling, Shoaling: How Fishes Move About

Taking photos of fish schools is one of my favorite underwater activities. It’s always challenging and getting good photos isn’t easy. Schooling fish tend to move away from you in unison, so it’s hard to get close. Only rarely do they let you into their midst.

Like birds in the sky, a fish school acts as one, changing direction in an instant. It seems a miracle individuals don’t collide. The school moves like a river, flowing around objects in its path, while each fish maintains a certain speed and distance from those around it. Any changes in the environment around them are quickly accommodated.
It was only recently I learned the difference between a school of fish and a shoal. Shoaling fish are simply swimming together. The photo of the blacksmith is a good example of that. As you can see, there is no coordination in direction or distance among fish. Salema is the best example of schooling. These fish are all moving in the same direction and are carefully spaced. And now that I’ve introduced “shoaling,” I need to tell you that fish experts rarely use this term and you are unlikely to find it in any fish identification book. Most use “school” to describe most, if not all, fish aggregations.
Of the more than 20,000 species of fish, about 16,000 school as juveniles and 4,000 as adults. Both freshwater and marine fish school — but why and how? The first question is the easiest to answer. Schools offer protection from predators because there is safety in numbers. It is also theorized that a school may fool a predator into thinking it is a large entity, not one merely comprised of smaller individuals. A school makes finding a mate(s) easier. Swimming is more efficient because individuals can use the draft created by those ahead of them, consuming less oxygen. This also means a school can travel farther and longer than the individuals comprising it could.
How fish school is more difficult. Scientists don’t believe this is a learned skill, they think it’s genetic. According to the North Carolina Aquariums website (www.ncaquariums.com): “Visual markers play a big role — each member of a school follows some key feature of the fish around it, usually a stripe or spot on their bodies, fins or tails. Because of this dependence on vision, schools break up or at least lose their internal structure at night. The vibration-detecting lateral line, a row of sensory cells that runs along the sides of the body, also provides information about neighbors’ movements.”
Blacksmith (Chromis punctipinnis): There are 28 genera and 350 species of damselfish worldwide. Most live in the tropics but five are found off the SoCal coast. Blacksmith are members of the damselfish family, Pomacentridae, and are close relatives of garibaldi. Shoals of garibaldi are usually small but those of blacksmith can number in the hundreds. Females lay eggs in nests groomed by the males and guarded by one or both parents. They are very protective and will charge predators much larger than themselves, including humans. While garibaldi nests are usually out in the open, those of blacksmith are less conspicuously found under ledges and in caves. I have never seen one (or realized I was seeing one). The larvae that hatch from the eggs are planktonic. Blacksmith are zooplankton eaters, found from Monterey Bay to central Baja, in shallow nearshore waters to depths of 160 feet. They can reach a maximum length of 12 inches but are usually 4 to 8 inches long.
Jack mackerel (Trachurus symmetricus): There are about 32 genera and 140 species of the Carangidae family and 15 of them live in SoCal waters. Only half of jack mackerel’s name correctly identifies it. Though it is a jack, it is not a mackerel. Like other members of its family, T. symmetricus are strong swimmers who hunt in open water. Adults can reach lengths of 32 inches. While you are unlikely to see schools of fish that big in a California kelp forest, you will see schools of juveniles, which can also include Pacific sardines, Pacific mackerel and blacksmith. Identifying the fish can be difficult as they share a similar shape and often the same silvery color. These fish prefer cooler waters and will move north when SoCal waters warm up. I have encountered schools of juvies off San Nicolas Island.
Pacific barracuda (Sphyraena argentea): Barracuda are found worldwide, mostly in tropical and temperate seas. There is only one genus and 21 species, two of which can be found in SoCal waters. I have seen just two schools of Pacific barracuda, one off Santa Cruz Island, the other off Santa Catalina. None of the fish was happy to see me, and quickly fled the scene. From the photo you would be hard pressed to tell if these fish are shoaling or schooling, though they were most definitely schooling when I first glimpsed them. Pacific barracuda can reach lengths of four feet, though they are usually about half that size. They are found in small schools in waters from 3 to 120 feet deep, from Alaska to the Revillagigedo Islands. They are easy to identify because of their distinctive shape and color and mouths full of sharp, pointed teeth.
Salema (Xenistius californiensis): Salema are members of the grunt family, Haemulidae. There are 17 genera and at least 145 species of grunts. They are found worldwide, including in Pacific and Caribbean seas. Salema range from Monterey Bay to Peru and are said to be abundant off Catalina, which is where the school seen here was photographed. They are adept at schooling and provide a textbook example of what it looks like. Note that the fish are about the same size, which is another factor in schooling. Salema can be 12 inches long. They live in shallow nearshore waters down to depths of 60 feet.
The author wishes to thank Dr. Milton Love, author of Certainly More than You Want to Know About the Fishes of the Pacific Coast, for his help in the preparation of this article.
Fish Stats
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Gnathostomata
Superclass: Pisces
Class: Actinopterygii
Order: Perciformes
Family:
Blacksmith: Pomacentridae
Jack mackerel: Carangidae
Pacific barracuda: Sphyraenidae
Salema: Haemulidae
Fishy Talk
According to grammarist.com, “The plural of fish is usually fish, but fishes has a few uses. In biology, for instance, fishes is used to refer to multiple species of fish. For example, if you say you saw four fish when scuba diving, that means you saw four individual fish, but if you say you saw four fishes, we might infer that you saw an undetermined number of fish of four different species.”
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