Are your shoals big enough?

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What's your thing? A glint of goldfish? A fever of stingrays? If you're a public aquarist, you might even have an army of herring. Draft, nest, school or shoal, I'll wager at some point in your hobby you've owned a multiplicity of fish, says Nathan Hill.


Most of us love a decent shoal. Retailers know this, which is one of the reasons they love to have their tanks brimming with fish. Nobody gets inspired by three timid Neon tetras huddled like Victorian-era beggars in a corner, but we've all been suckered in at some point by a majestic throng of movement and colour.

Even the 'simple' fish like danios or platies take on a different lustre when housed in their hundreds. Some of my favourite ever step-by-step features have been triggered this way. I go window shopping for one thing and come back besotted with the first heaving tank I see.

I recently caught the tail end of a discussion on social media, where a well known aquarist was asking traders genuinely (with I felt, more than a tinge of resigned sadness) whether they struggled to sell fish in realistically sized groups. The responses were, for the best part, less than encouraging (including my own) with the general consensus being that it was difficult to do, in part due to customer inclinations, as well as the prohibitive cost of buying large numbers of fish.

Despite adding my tuppence worth to the debate (I was, and still ever so slightly am involved in the trade, after all) I couldn't get the topic out of my head. Most contentiously, I couldn't pin down exactly what could be classed as a 'realistically sized group'.

Let's not be mistaken that many fish are highly social animals. And why wouldn't they be? The benefits of shoals, both to nervous prey fish, as well as stalking predatory packs, are vast. As we increasingly learn about social learning, organisation and recognition in fish, so we continue to be surprised at phenomena more usually associated with higher animals like herding mammals.

It may be that shoals have leaders, for example. For many a lay aquarist, it might seem that a shoal is little more than a terrified mass of dumb, unthinking individuals, blindly lunging from one state of existence to the next. Any migration is brought about by clumsy chance, or maybe even a collective will to feed and explore. But gradually we are learning the roles that many different fish play within a shoal, to the extent that swimming cohesion may be pointed by key members. Dominant individuals might lead the way, though the complexities of some large shoals might also mean that those up in front are the hungriest, and most willing to take risks, while those in the middle might be content to just waddle along with the masses, letting those around them run the gauntlet of being picked off.

In some panic situations, those fish willing to make a snap decision can encourage others to do the same. In some experiments, where a handful of guppies have been trained to swim through a small aperture when frightened, other naïve guppies that are unaware of the escape route will quickly follow the 'leader' fish through to safety. In the same situation without the experienced guides, naïve fish will just bluster about in terror.

We know that shoaling fish learn from visual clues given by each other. There's an old saying about panic spreading like wildfire, but it holds true with fish as well as humans. When a fish becomes frightened, this can manifest visually as bars, clamped or extended fins, paling or deepening of colour, excited and exaggerated movements and catatonia. There are chemical cues often given, too, in the form of what's known as 'schreckstoff' roughly translated simply as 'bad stuff'.

On smelling the bad stuff, other fish will assume predators are nearby. But schreckstoff isn't always needed for fish to freak. Certain shoaling species, upon seeing a stressed or panicking fish in a different tank, in different water, will automatically go into flight mode, trying to get away from perceived danger. Both schreckstoff and visual cues are reasons I've been on the fence about certain recirculating systems, but that's a blog for another time.

Researchers are well aware of this phenomenon, to the point that fish for experimentation need to be kept out of visual range of other fish in tanks being used.

What does this tell us so far? Well, for one, those who are struggling to keep a happy shoal at home might be automatically on to a non-starter. Fish personalities are usually divided between 'bold' and 'shy' fish, which is pretty self explanatory. Before you scoff at that, it's the well understood a phenomenon that it needs to be carefully factored in to any experiments on fish behaviour or welfare. Anyway, if you happen to have bought a small shoal made up exclusively of shy fish without a 'leader', then it stands to reason that they'll all be wary about exposing themselves to the open in your aquarium. Throw in the wildcard of one excessively nervous fish and it'd be amazing if your they ever appeared at all.

From a reassurance point of view, most studies point to shoalers expressing heightened nervousness  when kept in low numbers. Even the humble goldfish (and when was the last time you saw a shoal of goldfish in a tank?) differs its behaviour when kept in small quantities. One or two fish on their own will spend considerably less time foraging about than if there are five or more.

Why is this? Well, predation seems to be the key driver, as you might expect. There's a 'many eyes' theory to shoaling, where the collective optics of a hundred fish will spot danger far more quickly than those of just one. So in a crowd, it's safe to shove your snout into the mud and have a right royal grub about for a juicy worm. Do that solo, and you shouldn't be surprised if some cheeky Pike saunters up behind you to take advantage of your distraction.

In the aquarium

But I've yet to hit on to the core point of this blog: the numbers. Just what constitutes a shoal in the tank sense?

Hands up, I honestly do not know. The data is simply too scant. I can tell you, for example, that increasing numbers of Corydoras catfish are more content (assuming that increased foraging is a sign of contentment) than just two or three.

I want to draw on one Cory-confusion that seems to be rife (and frequently touted by traders, much to my aghast spluttering), and that's the 'mixed shoal' approach. Frequently I hear of hobbyists who have been told that any cory is fine with any other cory, and that they have free reign to toss Pandas with Bronze, loretoensis with similis, and any other of the million combinations you could dream up. I'd personally go on the record and say that this isn't so. All shoals have a homogeneity about them, for good reason. It's safe to blend in with others and not stand out. If I'm swimming in a shoal of herring wearing a high-vis jacket and flashing deely boppers, I should not at all express surprise when a Barracuda slams headlong into the throng and singles me out. I'd be a lot less stressed if I happened to not stand out. I'm happy to extend that same sentiment to Corydoras cats, and wouldn't be surprised if experiments eventually show it to be the case.

For tetra, I'll wager that the same is taking place. Yes, wild shoals of species like Neons are often loose, and not tightly packed clusters such as we imagine when we think of fish like Herring. If anything, these looser aggregations are more a convenient way of staying in touch (fishy Facebook, if you will) than a hard solution to the problem of rampant predation, but they still serve a social and reassurance purpose.

But then we've all experienced shoals that drift apart in our tanks, which kind of tosses any theories of 'needing' to be in a shoal out of the window. A group of ten that initially stayed tight together soon start to loosen, some sauntering in to the plants, one playing with the filter outlet — I'm sure you can relate to this.They simply stop behaving as shoals do, which I've always assumed is in part due to lack of predators and other sources of concern.

Is this a bad thing? It may be unnatural, so to speak, but the fish themselves usually appear pretty stress free. They don't show weakened immune systems or compromised appetites, for example.

In print articles, I have repeatedly pleaded (sometimes tacitly, sometimes blatantly) for aquarists to consider the needs of their fish and purchase numbers above the standard three- and six-deal bundles that are frequently offered. But at the same count, to have a true shoal might be unrealistic. There's always a painful contradiction (I'm guilty of writing it in many features) to the line 'go for a shoal of six' or something of that ilk. Simply put, six isn't a shoal. At the top end of the definition, the world's largest shoals number hundreds of millions, over regions covering several square kilometres. At the lower ends, a few dozen fish congregating might be enough to keep some species content. But six is really just sales pitch.

There again, the reality is that a lot of fishkeepers don't have space for vast numbers. Most starters begin with tanks of 60cm/24in or so, though typically the market is steering ever more towards affordable nanos. Even when space is available, many fishkeepers don't want to limit themselves to a single species and fill a 120cm/48in tank with dozens of Glowlights. And for the larger shoalers, fish like Piranha, a tank of monumental size would be required to keep a 'sensible' number.

So how should the trade respond? It'd be nice to see someone campaign for increased shoal awareness, but beyond becoming a sales ploy, I doubt it'd take hold. At least not in the mind of a new aquarist who wants one Lemon tetra, one Black neon, one Cardinal and one Cherry barb in their 30 l set up. And the retailer who says 'no' to that customer just committed corporate suicide for the sake of ethical integrity. That customer will happily go elsewhere and buy those fish. But for that casual aquarist, there will be dozens more who want to do the right thing, and genuinely express an interest in knowing the 'correct' way to keep a shoal. To keep saying 'six is fine' might be the retailer (unintentional) lie that comes back to haunt, over and over.

Through the course of the social media discussion that spurred this blog, one comment stood out above all others, and I hope I do the author no disservice by quoting him anonymously. What he wrote was: "I think that we firstly need to educate people into not thinking that because a fish is cheap it's expendable. The animal welfare act is very clear that you must 'maintain optimum conditions at all time'".

Wise words, indeed, though should this wording of optimum conditions also include shoaling animals living in minimum group numbers? I think it should, but I've always been a bit purist about keeping livestock as a pet.

With that in mind, I'd like to invite open and public dialogue on this very neglected aspect of hobby ethics. Is it wrong to keep fish in small shoals? Do we have an obligation to replicate naturally occurring numbers where possible? What are your own experiences with shoals and shoal behaviour?

I want to write a follow up piece at some stage, once I've found what the views of hobbyists are, so all and any information that you can share at this time is incredibly valuable.