What makes 250,000,000 fish gather in the same place?

By Ed Yong | March 26, 2009 2:00 pm

Blogging on Peer-Reviewed ResearchOn the 3rd of October, 2006, Nicolas Makris watched a quarter of a billion fish gather in the same place. They were Atlantic herring, one of the most abundant fishes in the ocean and one prone to gathering in massive schools. This was the first time that anyone had watched the full scope of the event, much less capture it on video.

The first signs of the amassing herring appeared around 5pm and by sunset, the gathering had begun in earnest. Once a critical level of fish was reached, the shoal expanded at a breakneck pace, suddenly growing to cover tens of kilometres within the hour. By midnight, the shoal contained about 250,000,000 individuals – 50,000 tonnes of fish gathered in one place.

The ability of fish to congregate in gigantic schools may be familiar but until now, we’ve known remarkably little about the things that set off these gatherings. Without Facebook as a coordinator, what causes small groups of herring to take sociability to an extreme? Scientists have tried to follow gathering fish aboard research vessels but these can usually only see a small fraction of the massive schools are any one time.

Makris wasn’t so hampered. He used a new technique called Ocean Acoustic Waveguide Remote Sensing (OAWRS) that can visualise fish populations over vast distances in real-time. It needs two ships, one to send out sound waves in all directions and a second to pick up their echoes as they bounce off fish and floor alike.

In an instant, it can scan an area of ocean 100km in diameter, and it can update its images every 75 seconds, providing an unprecedented view of the genesis of herring shoals. The location was Georges Bank off the coast of Maine, where herring migrate to spawn in early autumn. Makris pointed his instruments at an area where herring historically gather, and waited.

The get-togethers always started as the light of the afternoon starts to fade. At first, the herring are scattered and solitary, spread out in a thin layer close to the ocean floor. But just before sunset, they start to get organised. Small, vertical clusters form that rise above the seafloor and merge horizontally.

The school expands in a series of waves spreading out over tens of kilometres from several starting points. These waves move at more than ten times the swimming speed of a herring, which means that they’re the result of fish converging on multiple points rather than rushing towards a single centre.

Makris found that the get-together only really kicks off once the fish reach a critical threshold of one fish every five square metres. Below this level, fish would gather at a rate of less than 0.1 individuals per square metre per hour. Above this critical mass, the pace increased by more than 50 times. By midnight, the shoal had grown to 40km at its widest point, and it stayed that way until the returning light of day caused the herring to dissipate once more.

Once formed, the shoal migrated as one at a slow speed of 0.2m/s. This pace matches that of an average swimming herring and suggests that the millions of fish were swimming synchronously. They headed south towards shallower spawning grounds, presumably to shoot out eggs and sperm en masse.

It’s unlikely that the herring form large shoals in order to feed. As he was watching the fish, he also took samples of them using trawlers. He confirmed that over 99% of them were indeed Atlantic herring but also found that virtually all had empty stomachs. Indeed, past studies have found that herring don’t feed when they spawn.

Makris thinks that a synchronised orgy is the main purpose of the gathering horde. He suggests that the shoals form in deeper waters because there, they are less likely to be attacked by predators like whales, dolphins and tuna that prefer to hunt in shallower seas. Only when they have formed a massive defensive swarm do they head for inland to breed and even then, only under cover of darkness.

Reference: Science 10.1126/science.1169441

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CATEGORIZED UNDER: Animal behaviour, Fish, Fishing
MORE ABOUT: Fish, herring, school, shoal

Comments (16)

  1. Drekab

    250,000 is not a quarter of a billion. Cool post though.

  2. Ed Yong
  3. Nice summary. Very interesting. Good writing. Exact numbers aren’t to be had, anyway.

  4. Stu

    Holy mackerel, those herrings know how to party!

  5. Chris

    Drekab: “contained about 250,000,000 individuals” does, in fact, match the phrase “a quarter of a billion fish”

  6. Ed Yong

    I actually added three extra zeroes after Drekab’s comment. Mathematical FAIL.

  7. MattK

    I wonder what will happen once commercial fishermen get their hands on this technology. Sage restraint and respect for the ecosystem no doubt…

  8. They will devastate the fish population if the fishermen ever find out how to use this tech. It is amazing though how all those fish can congregate like that.

  9. I’ve always wondered how fish manage that. Birds too; you’ll see them all gathered together and then suddenly, without any warning, they all do a left turn at the same time. Fish seem to have even less prior signalling; as they don’t even make any noise.

  10. Craftierthanyou

    What if they had facebook all along?

  11. Ian

    “What makes 250,000,000 fish gather in the same place?”
    Are you fishing for answers, Ed?

  12. Ed Yong

    MattK and Brett – I disagree; in terms of actually finding fish, this doesn’t really have any advantages over existing tech. It’s advantage is in visualising the entirety of a mega-shoal as it forms. I can’t think that would be of any interest to fishermen who could much more easily find out where some big shoals are.
    Lab Rat – I seem to remember a couple of papers which show that complex shoaling behaviour can emerge if individuals follow some very very simple rules of movement. Anyone else remember something similar or can provide links?
    Ian – You’re a man after my own pun-loving heart.

  13. The video is “no longer available” :(

  14. Fish food, I’d guess. Or a mating ball. They might start out looking for food where cool waters well up and progress to spawning when the densities are great enough that sperm has a good chance of meeting egg. The herring feed the cod or tuna, the cod or tuna feed the dolphins and the sharks. Something small eats the eggs and sperm. They are there so that they overwhelm the feeding capacities of the predators, as cicadas do, and have some fertilized eggs left over. That’s my guess.
    Then we come along and say, “you could practically walk on the back of those cod” (500 years ago). “They won’t miss it if we take 10% or 20 or 30.”

  15. waterboy

    The British Columbia herring fishery occurs in late February or Early March each year. This year, the biomass,in ONE area, was estimated at almost 60,000 tonnes. AND, the Dept. of Fisheries and Oceans allows a 20% harvest of this almost bottom rung of the food chain! Then, they wonder why the salmon stocks are down, then they wonder why the killer whale population is shrinking.
    The spawn turns the water a fantastic light blue and the eagle, seal, gull and sea lion population skyrockets. People complain about the barking, there are so many pinnepeds! Ah- nature, the wonderful mystery that we try to unravel…

  16. makinde adekunle

    this is a nice research work. I recommend it to all limnologists and fisheries guru.

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