Thanks to a novel imaging technique, scientists are now able to study how gigantic shoals of fish, consisting of hundreds of millions of individuals, form in real life.
In a paper published in a recent issue of the journal Science, Nicholas Makris and coauthors utilized a technique Makris helped invent known as Ocean Acoustic Waveguide Remote Sensing (OAWRS) to study how Atlantic herring (Clupea harengula) form vast shoals to mate off the coast of the northeastern United States (Gulf of Maine).
This imaging technique involves blasting low-frequency sounds from underwater speakers mounted on the back of one boat and detecting them with another boat towing an array of microphones and located many kilometres away.
OAWRS allows the team to take images of an area about 100 kilometres in diameter every 75 seconds.
The authors found that shoaling is triggered by a reduction in light level and begins when herring reaches a critical population density (0.2 fish per square metre).
Under the influence of a small group of individuals, a chain reaction begins whereby the fish clump more closely together and attract other fish.
This behaviour propagates to other nearby fish as a wave that moves tens of kilometers in tens of minutes; this is a speed about 15"30 times faster than the fish swim.
The result is the synchronized movement of hundreds of millions of individuals over a large area (up to 40 kilometres) towards shallower spawning grounds, where the fish spend the night mating.
The study confirms theories about the behaviour of large groups of animals in general, which have only been predicted through theoretical investigations, computer simulations and laboratory experiments but not observed in situ.
The authors also believe that their results provide information essential to the conservation of marine ecosystems that vast oceanic fish shoals inhabit.
For more information, see the paper: Makris, NC, P Ratilal, S Jagannathan, Z Gong, M Andrews, I Bertsatos, OR God, RW Nero and JM Jech (2009) Critical population density triggers rapid formation of vast oceanic fish shoals. Science 323, pp. 1734"1737.