How do fish know if they're swimming in the right school?

From ABC Science:  How do fish know if they're swimming in the right school?

ish can't see themselves, or use a mirror! How do they know they are in the right school of fish?— Jim
Almost 80 per cent of the more than 20,000 known fish species school at some point in the life cycle.
Schooling helps reduce the risk of being attacked by predators, and also makes swimming easier because the fish position themselves so they are able to slipstream in their neighbours' wake.
Some species school only when they are vulnerable juveniles, others when they are older. They begin by swimming in pairs and then in larger and larger groups of the same species.
While fish have big eyes to help them find prey and keep track of each other up close, they rely on their chemosensory system to track other fish of the same species in the vastness of the ocean, says Dr Ashley Ward, a fish biologist at the University of Sydney.
"A fish can smell itself, and recognises others with the same smell," says Ward, who studies the social behaviour of fish.
Fish use smell to sniff out a partner with a strong immune system.
The smell of an individual fish is genetically programmed by major histocompatibility complex (MHC) molecules, which are crucial to vertebrate animals' immunity to disease.
"As a by-product, MHC affects the way we and other vertebrates smell," explains Ward.
"Even though all the fish in a school may look alike, when it comes to choosing a mate, picking one that smells different, that is, not related, will ensure the resulting offspring will have the best range of immune responses."
But when it comes to choosing who to hang out with, the smell of where a fish lives and what it eats is even more important.
"Like humans, who take on smells from our diet such as garlic or asparagus, fish clearly smell which individuals live in their microhabitat and preferentially associate with them," says Ward.
"We think the reason that they can do this is that another individual which carries that 'local' smell might have important local knowledge," he says.
But the way they smell, and their ability to smell each other can be affected by chemicals introduced into the aquatic environment.
"Water pollution might mean they have trouble recognising their school or potential mates," says Ward.

Fishy frenemies

Fish are pretty picky when it comes to choosing their school friends.
"Shoaling (schooling) fish have a strong preference for associating with familiar over unfamiliar fish.
"If territorial or solitary, they recognise their neighbours and are less aggressive to them than outsiders. And when it comes to mating, promiscuous fish recognise and avoid the ones they have mated with previously," says Ward.
Less well-proven, he says, is the way fish choose foraging companions. Recognising their foraging companions' strengths and weaknesses may be the difference between finding a meal, and ending up as one.
Some recognise poorer competitors and prefer to associate with them, he says, presumably because they know they will end up with a bigger share of the day's catch, while others show a preference for good foraging companions from the past.
"This might be those that forage better, are better navigators or are skilled at avoiding predators," Ward says.
"It has also been shown that fish have personalities, and exhibit certain types of behaviour, where they may be either bold or shy.
"While they may look alike, groups comprising a diversity of personality types, that is a mix of bold and shy fish, have been shown to outperform those composed of one type."