Wild herring prove fast organizers

Watching fish shoal reveals a turning point in density

Eat your hearts out, people struggling to walk three dogs (or one cat).

SHOAL TIME A wide herring shoal coalesces briskly on the northern flank of Georges Bank around sunset (at 18:08) on Oct. 3, 2006. Starting as dispersed fish (A), a few gatherings of fish (B) start a chain reaction of forming an increasingly dense shoal within tens of minutes (B-F). Hourly images (G-J) as the shoal moves show increasing thickening into a mass of roughly 250 million herring (J). courtesy of Nicholas Makris

WIDE ANGLE This schematic shows OAWRS (Ocean Acoustic Waveguide Remote Sensing) detecting fish shoals as one ship emits signals (blue arcs) that bounce off fish to indicate their presence (red arcs) to a second ship towing a receiver. Older sonar just directed a beam downward and searched a much smaller area as the ship cruised along a transect. courtesy of Nicholas Makris

GROUND TRUTH Herring that researchers pulled up in a trawl came from large shoals that OAWRS technology had detected. Fish lengths in the shoal of breeding adults didn’t vary much from a mean of 26.7 centimeters. courtesy of Nicholas Makris

The first broadscale analysis of wild fish forming shoals finds that order can break out fast. Within half an hour, a bunch of herring that have reached a certain density can coalesce and move together in a coordinated mass more than 20 kilometers wide, according to a study in the March 27 Science.

 “It’s not gradual; it’s — BANG,” says first author Nicholas Makris of MIT.

Makris, Purnima Ratilal of Northeastern University in Boston, and others reported in 2006 that they had developed a low-frequency acoustic system that greatly expands the underwater range that fish monitors can watch at one time. OAWRS, for Ocean Acoustic Waveguide Remote Sensing, can collect data from an area 100 kilometers in diameter about every 75 seconds, much broader than older systems.

“It’s the difference between a pup tent and a skyscraper,” says Julia K. Parrish, a fish schooling specialist at the University of Washington in Seattle. She says the new study, based on data collected using OAWRS, “is very cool.”

The study analyzes shoals that formed in fall 2006 as herring gathered at night to spawn in the shallow waters of Georges Bank, off the coast of Massachussetts. During the day those shallow waters make dining easy for birds and the many other herring predators, so the herring retreat to deeper, darker waters where they’re less convenient to catch. At dusk, though, fish coalesce into an orderly shoal and swim together back to the shallow water.

Clustering in a shoal helps fish coordinate their spawning and reduces the risk that any one fish will get nabbed by a predator.

Monitoring the formation of the evening shoals revealed a pattern, report Makris and his colleagues. At first herring mill around in deeper water without much order, but around sunset some little clumps form that seem to set off a chain reaction of organization. When fish drawing closer to each other reached a critical density — at least 0.2 fish per square meter — herring began to close ranks more tightly to form an orderly mass swimming together.

Once the mass starts to form, a wave of shoal-joining behavior spreads through water at a rate faster than fish can swim. Makris compares the phenomenon to sports fans doing a wave in a stadium. The wave behavior shows up in far seats long before the first waver could walk to those seats.

Lab studies in locusts and some computer models have predicted that there might be a critical density that sets off this sudden burst of self-organization. Or, as Makris calls the more general event, the phase transition in a system of self-driven particles.

Looking at the findings, Parrish says she now wonders if herring need a far bigger population than previously imagined in order to form a shoal properly. That’s a question conservationists need to think about, she says.

Parrish and Makris both use the term “shoal” instead of “school.” The fish schools Parrish studies are even denser crowds, with fish only a body length apart, so she welcomes the new work as catching what may be the precursor to schools.

Learning something about this stage raises new questions for modelers. “I expect people to be running to their computers,” she says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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