Why do fish school?
I’m Jonathan Bird and welcome to Blue World Academy!
Everyone knows that fish often gather in schools, but the biology of fish schools is actually
pretty complicated, and not that well understood.
Schooling has been extensively studied, and a number of theories have evolved, trying
to explain why fish get together.
First of all, what is a “school” of fish?
Not every gathering of fish is technically a school.
Biologists have specific names for different kinds of gatherings of fish.
A loose and unorganized gathering of fish, often containing different species, is called
an aggregation.
Sometimes fish aggregate because they are all feeding on the same thing, or making use
of the same section of reef for protection.
A single species of fish in an aggregation is called a shoal.
In a shoal, the fish are all pointed in different directions, milling around, doing their own
thing.
But once the shoal becomes polarized—which means they are all pointing in the same direction
and working together, then it’s known as a school.
Fish form schools for several reasons, the principal reason being safety in numbers.
Schools containing thousands (or even just hundreds) of nearly identical fish will confuse
predators and make it difficult to single out and attack any one individual fish.
Another advantage of schooling: it makes it easier for some fish to find food.
With many more eyes on the lookout, a school can find potential prey more effectively.
Also, schooling brings the sexes together and increases the odds of successful reproduction.
Many fish species form schools when it comes time to mate.
This school of trevally jacks in the Galapagos has gathered to spawn.
Pairs break off the main aggregation, with one of the fish changing to a dark color.
The color warns other fish to stay away.
But if they don’t, the darker fish will chase them away.
When the pair has finished spawning, the dark one will quickly return to the normal silver
coloration.
But schooling doesn’t just promote mating—it can also do the opposite.
Scalloped Hammerhead sharks hunt individually at night.
During the day, they often gather into huge schools of hundreds of animals.
But these schools normally consist almost entirely of females.
One theory suggests that the females aggregate into schools to protect themselves from males
trying to mate with them.
Schooling doesn’t just provide proximity for the individuals, it also provides efficiency,
allowing a school of fish to move with less effort than they would if each of the fish
were swimming alone.
Essentially, drafting in the wake of another fish reduces drag.
So how do fish accomplish this complicated act of synchronized swimming, with a carefully
maintained spacing between fish?
Primary, it has to do with vision— each member of a school follows some key feature
of the fish around it, such as a stripe or spot on their bodies, fins or tails.
The vibration-detecting lateral line, a row of sensory cells that runs along the sides
of the fish’s body, also provides information about a neighbor’s movements.
But the lateral line is second in importance behind vision.
Because of this dependence on vision, schools usually break up or at least become far less
tightly integrated at night.
On night dives, we very rarely see schools of fish like we do during the day.
And I’ll end this video with a riddle.
Why are fish so smart?
Because they spend all day in schools!
Ha!
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