Every evening in early fall, Atlantic herring gather in shallow water to spawn. These groups of fish, known as shoals, can number in the hundreds of millions. Now, by training acoustic sensors on a swath of sea off the northeastern United States, researchers have documented how these shoals form in real time.
For the past few years, Massachusetts Institute of Technology engineering professor Nicholas Makris and colleagues have been developing ways to use low-frequency sound to measure things such as undersea mountains and hurricane intensity (ScienceNOW, 15 April 2008). They blast sound waves from underwater speakers on the back of one boat and detect them many kilometers away with a second boat towing an array of microphones. Three years ago, they reported that the data could be used to image huge populations of fish (Science, 3 February 2006, p. 660).
Now the researchers have used their acoustic tools to track how the fish swarm, a behavior that is shared by bees, birds, and other animals probably to help them avoid predators, find food, and mate more efficiently. For the research, Makris's team probed a circle of ocean 100 square kilometers across--the size of Massachusetts--every 75 seconds. The group found that shoaling begins in deep water when clusters of a few individual herring reach a certain density--0.2 fish per square meter. These individuals then clump more closely together and attract other fish. This behavior spreads to other fish in a wave that moves tens of kilometers in tens of minutes--about 10 times faster than the fish swim. Shoals of fish stretching up to 40 kilometers then head to the shallower spawning grounds, where the animals spend the night mating. The study appears in this week's issue of Science.
Until now, the only experimental data on swarming came from lab experiments with locusts. The sequence of events seen in herring shoaling is exactly what scientists who study swarming in other animals would expect, notes Iain Couzin, an animal behavior researcher at Princeton University who did the locust studies. "It's nice to see this type of pattern play out in a natural system," he says.
For fisheries scientists, who until now have only observed shoal formation on a much smaller scale, the study "is an important validation of several key hypotheses about what drives fish schools," such as how decisions that individuals make about predators and food are copied by other fish, says fisheries scientist Tony Pitcher of the University of Vancouver. He worries, however, that Makris's acoustic methods could contribute to overfishing if fishing boats use them to find shoals of fish. Makris hopes that won't happen. For one thing, companies would need permission from a country to deploy the acoustic sensors in its waters, he says.
)
)
)
)
)
)
)
)
