The gathering and migration of a gigantic fish shoal has been observed for the first time by US scientists. Hundreds of millions of herring were watched using an acoustic observation system developed by Nicholas Makris of MIT and Purnima Ratilal of Northeastern University.

“As far as we know, this is the first time we’ve quantified this behaviour in nature and over such a huge ecosystem,” says Makris (MIT press release).

Iain Couzin, a researcher at Princeton University who was not involved in the research, told New Scientist, “I don’t know anything that’s close to this scale.”

The image right shows the sequence of shoal growth from ‘there’s nothing there’ to an ‘oh my god that’s huge’ group kilometres long. This happens in the space of an hour.

“It’s not gradual; it’s – BANG,” says Makris (Science News).

The giant shoal off the coast of America formed when spawning Atlantic herring (Clupea harengus) around the Georges Bank in the Gulf of Maine reached a critical density of 0.2 fish per square metre. This led to a rapid transition into synchronized movement of fish.

These findings confirm previous theories about the behaviour of large groups of animals, but are the first time that these theories have been shown in wild populations of this scale.

The researchers also found the shoal ‘commuted’ from deeper water to the bank to spawn in darkness, before returning the next day to the safety of the deep.

“After analyzing the data carefully during the initial days at sea, I noticed what seemed to be a daily pattern of fish shoal formation,” says Ratilal (Northeastern press release). “When I predicted what would happen the following day, and it turned out to be right, we knew we had discovered something really important.”

Makris adds, via the BBC, “This is truly a commute. And there are truly cities of fish down there.”

Critical Population Density Triggers Rapid Formation of Vast Oceanic Fish Shoals is published this week in Science.

Image: Nicholas Makris