Shape of Fish, Shrimp Shoals Examined

CAMBRIDGE, Mass., Sept. 16 (UPI) — Schools of fish and swarms of ocean krill get together in almost identically-shaped shoals, seeming to follow an unknown “rule,” Scottish researchers say.

A study published online in Current Biology says shoals of fish and swarms of krill hang out in groups that take on the same overall shape, not a simple sphere, cylinder or ovoid but something more akin to an irregular crystal, the researchers say.


“The fact that several species of fish and krill that live in very different locations — from the tropics to polar oceans — form shoals that are the same shape suggests that the same forces are at play in diverse ecosystems; there is a common ‘rule’ for shoal shape,” Andrew Brierley of the University of St. Andrews in Scotland, said.

Sonar used to record the three-dimensional shape of Antarctic krill swarms found they had the same ratio of surface area to volume even as the overall size and density of the group varied. Studies of fish like sardines and anchovies from diverse locations turned up the very same pattern, Brierley said.

Using computer modeling, researchers came up with an apparently simple explanation — individual fish and krill juggle their access to oxygen-rich water at the outer boundaries of the shoal or swarm against the risk of being eaten by predators.

If oxygen availability is a major driver of shoal shape, Brierley says, then changes are in store. Oxygen concentrations are declining as the world’s oceans warm so shoals will have to adapt accordingly, becoming smaller or less densely packed.

“The ease (or difficulty) with which fishermen can catch pelagic fish and crustaceans — catchability — can vary as a function of shoal size, so understanding the response of shoals to changing oxygen concentration will be of commercial as well as ecological importance,” the researchers wrote.

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Categorized | Fish, Other
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