Shoaling fish and behaviour

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    Over the years I have seen many threads and posts on here asking for suggestions for fish which will shoal in our systems.
    So what do we specifically mean by shoaling behaviour- we have all seen videos and TV programs showing huge groups (note avoiding the ‘shoal’ word:laugh:) of mackerel, etc. seemingly swimming as one organism but can we achieve that is our little (whatever size tank you have) aquariums. Are we kidding ourselves that keeping a few fish of the same species together constitutes a shoal or merely a group that tolerate and don’t kill each other. What is your definition of shoaling and have you seen this behaviour in your systems.

    Why do fish shoal in the wild, safety from predators is often sited- will that change if there are no predators present. Have you seen any changes when a group have been kept for a while with no predator/food pressure. What other factors do you feel affects shoaling activity and have you seen that behaviour in a tank

    Many fish are said to shoal in our systems, cardinals, anthias and some tangs to name a few, can you suggest any more. But, as I said earlier, are we just kidding ourselves, are they shoaling or are they tolerating each other- we have all read threads about Squamapinnis anthias killing off the weakest in a group. Many cardinal fish are nocturnal, are they grouping together as part of their normal daytime behaviour and do they disperse at night (it would be interesting to hear night time observations from people who have kept them).

    How many fish constitute a shoal- can sessile fish i.e. clownfish hosting an nem be considered a shoal or is it just free swimming species.

    Lots of areas for discussion here- enjoy:D


    Great choice of subject, Haydn.

    From a scientific basis, any in-depth discussion should consider the differences between shoaling and schooling. In broad terms, shoaling can be applied to fish that group together in a social context, whilst schooling can be considered as a synchronised/structured shoaling behaviour, primarily by the same species of fishes.

    However, there are pitfalls in this broad terminology, and anyone that has studied in-depth fish biology in tertiary education will know that these terms are used interchangeably, not just in lectures, but widely in scientific literature. Nevertheless, these are fascinating subjects, and it’s well worth reading up on papers, and comparing the research to your own experiences. Yes, it can apply to what you can observe in aquaria, but especially so with what you can see within seconds of dipping your head under the surface when on holiday. I’d also add that these behaviours can be readily seen in some public aquaria exhibits – anywhere with a school of mackerel in a very large exhibit will provide an obvious example.

    It’s also interesting that anyone who indulges in birding/ornithology will be able to draw parallels with shoaling/schooling and flocking behaviours.

    Back to aquaria – for our purposes, shoaling is the term that is more applicable (though not exclusively so).

    There are many theories on why fish should shoal or school, and some seem to have more weight in the literature than others, and some apply more readily to aquaria. Below is a list of some of the associated behaviours, along with examples to give some context:

    Predator avoidance: an obvious behaviour, and by shoaling, it may be difficult for any predator to target and isolate a single individual. Shoaling provides more pairs of eyes, and so a proportionally greater probability of detecting danger. Shoaling also results in a reduced probability for an individual to be predated, by dilution in numerical terms.

    Applying an aquaria-based context is difficult for obvious reasons, but those foolhardy aquarist who stock lionfish and groupings of small fishes eg. may be able to directly view avoidance strategy. Sardines, herrings, silversides are all examples of the stuff we see on TV, and on the reef, it is perhaps most obvious with fusilierfish being hunted by jacks, by lionfish crepuscularly hunting mullets, or juvenile reef sharks/needlefish hunting baitfish in the shallows.

    Optimal foraging: By shoaling, aggregating fish can be a signal to others that a given site is of high value in nutritional terms. This also relates to travelling shoals that search for sites of optimal foraging, and therefore facilitate recruitment to the shoal. Shoaling also may facilitate opportunism, by overwhelming the foraging/breeding territory of territorial species.

    Within aquaria, there is an obvious example, with fish congregating in the area of food dispensers. Fish may be using less energy by spending less time foraging over the whole aquarium, and more time below the feeding port. Such fish will also be first to gain the nutrition. Another example of OF, is the reaction of fish towards others that have encountered food eg. placing food on the surface results in feeding activity that immediately attracts others, long before any olfactory stimulus would have applied.

    On the reef, examples are legion. Shoals of surgeonfish aggregate, convicts being an excellent example, that forage over large areas of the reef, and in doing so overwhelm the territories of other fish, allowing them to gain access to resources that may be difficult to obtain in reduced numbers (see photo below re convicts and PBTs). Wrasses and butterflies also exhibit the same behaviour on occasion, and can overwhelm the territory (and devour the eggs) of other fishes such as breeding triggers eg. Yet another obvious example of OF, and one that can readily be observed anywhere on the reef, is the behaviour where fish form a hunting shoal, usually around another predator (moray, octopus etc.) – the primary predator is followed whilst foraging, and following predators benefit from prey being flushed. The same broad principle applies to when your feet stir the substrate as you wade, and other fish take advantage. Shoals may also develop in proximity to other breeding shoals in order to feed on eggs eg., or may aggregate at cleaning stations.


    Spawning: shoaling allows the aggregation of individuals of both sexes to release gametes in close proximity. Again, the roving of a shoal to a spawning ground, facilitates recruitment and so the shoal size grows, and therefore facilitates for greater genetic diversity.

    Difficult to apply to examples in domestic aquaria. Wild examples are obvious – too many to list.

    Community socialisation: a shoal may aggregate on a temporary or permanent basis, and aids the formation of a communal social structure, and/or provides information to fish on which members constitute the population of a given area.

    The hierarchal communities of anthias and damselfish are two pretty obvious examples, where a shoal constitutes a community with a permanent/semi-permanent, and ordered social structure – be it a shoal of several thousand anthias, or a shoal of clowns and/or damsels hosting an anemone.

    Shoals may aggregate on a temporary basis, those individuals leaving their home ranges to join together and exchange information on the individuals that occupy a given area. This is documented for butterfly species eg. and may apply to a vast range of species such as shoaling PBTs eg., where individuals seem neither to aggregate for feeding or reproductive purposes.

    Hydro-dynamics: individuals of a shoal benefit from the reduced energy demand when travelling over distance, aided by a reduction in drag provided by an individual ahead.

    Difficult to apply to aquaria, but the anecdotal reports of a correlation between increasing shoaling activity/increasing water velocity in certain species may be a function of this behaviour. I’ve crudely tested the theory with squamipinnis anthias, and my own observations did give the theory some credence. However, there may be other reasons why these fish engage in this manner.

    As I’ve mentioned above, the theoretical benefits of this aspect to a shoal can be tangibly contemplated when viewing a shoal of pelagic fish in a large public aquarium display. On the reef, you can readily observe shoals moving in long streams, often in single file. Very seldom do you observe shoals moving in a broad flank. Are fish moving in this way to reduce drag, or is the reason to present less of a visual target to predators….? Perhaps a bit of both.

    Please feel free to comment or add to the examples above – considering the size of the membership, I’m sure there is a vast amount of knowledge out there for reefers to share. 🙂

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