Like humans, fish display much diversity in terms of social behaviour. Some fish prefer to spend time solo.
The Painted Comber likes to hide alone under rocks whilst giving 'side-eye' towards anyone approaching.
Some fish like to hang out with small, diverse groups of compadres.
Are they feeding together or posing for an album cover?
And some fish like to roam the ocean with hundreds or thousands of others just like them.
Homogeneous heroes of the sea.
There's something very magical about watching a gigantic group of fish zoom past. But what makes fish decide to gather together in shoals or schools, you ask? Even if you didn't ask, I'm still going to tell you. What do you think this post is, a democracy?
Shoals vs Schools of Fish
In order to help you win pointless pub quiz arguments about the differences between shoals and schools, I have provided some very important information about the definitions.
Shoaling Fish
Shoaling fish swim together but they do so in directions independent from each other. Being a part of the group doesn't restrict an individual's expression of their identity (ya know what I mean, like). In human terms, they're like one of your most fun social groups - not always the most organised, but you tend to feel energised after spending time with them.
Damselfish swim together but... slightly apart.
Schooling Fish
Schooling fish swim together in a coordinated, organised way. It's as if they are all of one mind, one body, but made up of hundreds and thousands of individuals. They're like the hobby social group in which you can achieve impressive things with, but after a while you need some time alone from them. You know, so that you can remember who you actually are and what you really think after detaching from the 'collective'.
School of European Sprats. Research has found that they don't need no education nor do they need thought control (since the latter comes naturally to them).
However, to keep things simple I will be referring to schools, shoals or any other gathering of fish as an aggregation.
The Benefits of Aggregation
Strength in Numbers
A large swarm of homogenous fish all swimming close together can seem like a much larger creature if you're a predator fish (or if your swimming goggles have the wrong prescription). In that way, small fish can group together to intimidate the larger ones.
Left: What I see when I convert a fish aggregation pic to grayscale.
Right: What a predator fish may see (artists impression).
Aggregation can also reduce aggression of individual fish towards their own species, even when competing for the same resource. The longer the time spent bonding, the less aggressive fish can be with each other. This phenomena will probably vary across different species, however, and will be dependent on how high-maintenance certain fish friendships can be.
Foodie Friends
You've heard the phrase 'many hands make light work'. Well, when it comes to aggregating fish, 'many eyes make it easier to locate food'. Admittedly, that could have been catchier.
You're more likely to be aware of 'that cool new place to eat' when out with your friends than when wandering the streets alone.
Just like when your friend shouts out 'I've found cake!' and the entirety of your squad then descends upon said cake, when one fish in a aggregation finds food it is communicated to the others via 'feeding movement'. Additionally, you can often find transient groups of fish from different species', co-located, all sharing the same food niche.
Surmullet fish can form small cliques with other species, united by an appreciation for benthic dishes (e.g. small crustaceans). Can you see the camouflaged turbot dining with them?
There it is.
However, individual fish in smaller feeding groups or rebellious ones who have broken away from their aggregation can sometimes gain access to food faster than if they'd kept with their crowd.
A solo pompano seems to be reflecting on whether it was a good decision to 'go it alone'. More likely though, they're working out how to eat a drowned fly without having to share it with anyone.
It's all dependent on those cost:benefit calculations constantly churning away in their tiny aquatic brains. But sometimes I like to think that fish have complex ethical debates with themselves about whether to be selfish or form selfish herds.
Stress Reduction
Some fish are just born sociable - they have no choice but to aggregate, otherwise they'll feel FOMO. Like humans when they bored or miss their friends, fish can indeed feel depression.
Damselfish in distress.
Research has shown that if a damselfish is removed from its friends it shows an increase in metabolism and a reduction in physical health. It's always something to consider when building aquariums. If you're displaying fish that naturally aggregate, make sure they have plenty of friends (besides yourself of course).
Leaders and Followers
What I find so impressive about aggregations of fish, especially the coordinated schooling species, is that they all seem to 'agree' on which direction to go. Even if there's more than a hundred of them - they all reach a consensus with apparently no bickering or passive aggression. However, in a relatively small group of humans it can seem like an impossible task when deciding where to go for dinner. Schooling fish just 'move' without debate.
'To McZooplankton!' they all said. At once. No complaints.
There is some debate about how aggregating fish make decisions; is there a leader or is there a 'collective will'? Research suggests that a small group of 'elites' run the show and that body size can affect whether or not fish will 'lead' or 'follow' in shoals of golden shiners. There are both benefits and risks associated with being a leader. Fish at the front often have earliest access to food. In fact, there is a relationship between being a hungry fish and being a leading fish.
This hungry striped seabream may as well be the President.
However, being at the front makes you more at risk of being gobbled up by a predator. In fact, when aggregating fish become frightened, their first instinct is to bury themselves as centrally as possible within their crowd of friends. Interestingly, if a fish becomes injured or shaken it releases a 'fear pheromone', which triggers an escape response in neighbouring individuals. The very Austrian-German biologist Karl Ritter von Frisch named the pheromone 'Schreckstoff' (literally meaning 'fright-stuff').
Pictured: Schreckstoff in action (Artist's Impression). Drawing by Rakel Stensmyr from Stensmyr & Maderspacher, 2012.
'Schreckstoff' itself is a mucus mixture of glycosaminoglycan (GAG) chondroitins (specifically chondroitin-4 and -6 sulphate), and the purified components have been shown to be responsible for the 'fear response' in zebrafish. To be honest, I would be a bit frightened if someone threw mucus at me.
Final Thought: Crowds vs Individuals
Many different species of animals group together for the same reasons that fish aggregate. And yes, that definitely includes humans. As 'groups' or 'crowds' of similarly dressed or similarly behaving individuals, humans can accomplish many fantastic things including beautiful traditional dances, large-scale comedic moments and of course, protest marches that can influence the media, current politics and the course of history. Just like fish aggregating, human 'shoals' are that of people gathering together to work towards a goal that is 'bigger' than that of the individual. However, akin to the fish that break away from the collective, it's just as important for humans to recognise when it's better to be an independent thinker or to simply go with the flow.
That said, I thought I'd make a little video about crowds both on dry-land and underwater. Hope you enjoy..
Video features the choral masterpiece 'I Follow Rivers' by the legendary Lykke Li. Do follow her.
Thank you to everyone who follows my blog!