Silly thoughts on why fish shoal

A quarter of the known fish species shoal throughout their life, quoted a scientist working on fish shoaling behavior. I wondered how these numbers compare with terrestrial animals. Among invertebrates only  few taxa live in groups, the celebrated ones being bees, ants, wasps and termites. Agreed that many higher vertebrates especially mammals live in groups such as elephants, hyenas, primates, meercats and wolves (I am sure you can think of at least 5 more). However it seems a small proportion of terrestrial animals group compared to fish. For argument  sake, let us agree that fish group more than terrestrial animals. Here is the main point I want to make: why do fish group more than terrestrial animals? Here is a brilliant and silly idea as to why:  I think it is because in water as opposed to on land, there is a third axis to group/aggregate. This extra dimension (basically depth) adds to the space available for grouping. It would be really cool if one could construct a mathematical model to study grouping tendency in 2-D space and then add more dimensions to the model to see if and how grouping tendencies increase. Vishu? Jaideep? Any thoughts?

But seriously, fish shoal for small talk and gossip and some discuss the meaning of life.




  1. vishuguttal

    @Manvi, Very nice post. I was also intrigued to hear that number on the proportion of fish species that shoal/school — staggering! Apart from numbers, of course, schools are so amazing and that is why I love to understand them. Coming to your point on why aquatic species may show more grouping behaviour in comparison to terrestrial species — I think that is a fantastic question (& not at all silly)) if factually correct. Your thought on how dimensionality to move might play a role is also incredibly cool and brilliant (& not silly again). There are two things (among many others) that a third dimension does: (i) It changes encounter rates among individuals (ii) It changes available space for grouping.

    You are entirely correct on point (ii) that third dimension offers more space to formation of group (individuals and populations!). But I am not sure if that is required for grouping. I would rather think that a individuals might want to group and move synchronously if they are constrained in space (hence, might collide more frequently if they all to move on their own instead of synchronously). So naively speaking, if space is an issue I may argue that two dimensional terrestrial world (ignoring flying birds for now), with lesser space compared to a three dimensional aquatic world, should be grouping more!

    That brings me to point (i) I mentioned above. All else being equal, two randomly moving animals are more likely to encounter in two dimensions than in three dimensions (this can be proved using probability theory and also makes sense). Hence, if formation of a group is contingent uopn individuals encountering others, it should be more likely in terrestrial species than in aquatic :-).

    So, in a sense your question remains unresolved if it is indeed true that aquatic species shoal more frequenty than terrestrial ones. It could be because of many other factors, ranging from predation pressure, availability of food, etc. Is there a single dominant factor?

    • Manvi

      Thanks for a different perspective of encounter rates. I think encounter rates would be crucial for group formation at low densities of animals and not as much at high densities of animals(please correct me if wrong here). This and given what you said I’d ask a different question – Should fish shoal sizes be larger compared to animal group sizes because they have a third dimension to move about. (Theoretically assuming really high densities of both)
      Also adding to your list of 2 points, synchronicity increases/decreases with dimensions. So would it be easier/harder to synchronize movement in 3-D space than in 2-D space because you have neighbors both above and below you in 3-D space?

  2. torsey

    Nice thought. By extending your argument, so should birds. Any idea whether many more species of birds flock too compared to terrestrials animals?
    Also, do you think the medium might have something to do with shoaling? Water is better at conducting sound, and most probably olfactory cues too. So predators might be aided by more cues underwater leading to an aroused anti-predator behaviour.

    • vishuguttal

      @torsey, No wonder you are from a communication lab :-). My gut (now full of junk food) feeling is that for fish the visual information is the most important one. Binoy also mentioned the same yesterday for climbing perch. Certainly some cues are required, but communication is an optional recipe for group behaviour and may be species specific (also, do you distinguish cues vs communication?).

      Your comment “So predators might be aided by more cues underwater leading to an aroused anti-predator behaviour.” does not explain why prey should group. In fact, if the cost of predators finding prey through sensing anti-predatory chemicals is more than the benefits of dilution and other anti-predatory benefits of grouping, grouping should not occur.

      • torsey

        There seems to be a whole line of research showing how chemical cues are used by prey for detecting presence of predators, and visual cues for checking their size and other information. e.g.
        So do you think cues used by freshwater fish might be different compared to marine fish, due to the sheer size of the water body and visual clarity, etc.? Anne, Bharti, any suggestions?
        The ‘cues vs. signals’ argument for my understanding is that signals are intended for a receiver, and cues have no such thing.

        My simplistic argument “So predators might be aided by more cues underwater leading to an aroused anti-predator behaviour.” was based on grouping as an anti-predator behaviour.

  3. vishuguttal

    Reblogged this on Vishwesha Guttal and commented:

    Here is an interesting post by Manvi (graduate student working with my colleague Kavita Isvaran at CES) on why so many fish species shoal in comparison to terrestrial species whether it has to do with being aquatic vs terrestrial species.

  4. Sumithra

    So, of the many different explanations that exist and are commonly used to explain shoaling…schooling in fish, could someone tell me which may be used to explain grouping and coordinated movement in predators? The only one I know of is coordinated hunting as one might see in lions and orcas. Apparently some sharks, particularly hammerheads, also school. Are these also for the increased efficiency of hunting in packs and is that the only explanation?

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