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Mystery of Florida's Giant Jumping Sturgeon Solved?

Stefan Lovgren
for National Geographic News
October 30, 2006
 
It's a quiet Friday morning on the Suwannee River in northwestern Florida, when a giant fish suddenly leaps six feet (two meters) out of the water and crashes back into the river.

The stunt is performed by a Gulf sturgeon, a giant fish that traces its roots back to the days of the dinosaurs and can grow up to 8 feet (2.5 meters) long and weigh up to 200 pounds (91 kilograms).

Every few minutes over the next hour, more sturgeons reproduce the feat.

Why the sturgeons jump has been the topic of a long-standing debate. Some scientists have suggested they do it to avoid predators; others have proposed that they do it simply for fun.

(Read related story: "Giant Jumping Sturgeon Stir Up Mystery in Florida River" [August 17, 2006].)

Ken Sulak, a biologist at the U.S. Geological Survey in Gainesville, Florida, thinks he has found the answer.

The sounds jumping sturgeons make are distinct from the sounds of other jumping fish, Sulak says. He believes the jumping is a form of communication that sturgeons use to connect with larger groups and maintain community cohesion.

"I think of sturgeon-jumping sounds as being equivalent to cows mooing—announcing to the larger group the presence and position of individuals," Sulak said.

"Still Get a Thrill"

A subspecies of the Atlantic sturgeon, Gulf sturgeons are found in the coastal rivers of the Gulf of Mexico. The Suwannee River, which runs from southern Georgia through northern Florida, contains the largest population of Gulf sturgeons.

(See an interactive map of the Suwannee River.)

According to one 2001 estimate, between 5,500 and 7,650 adult sturgeons live in the Suwannee.

The sturgeons spend the winter months in the Gulf and migrate into the river around March, when they begin to spawn.

After spawning, most sturgeons spend the summer and fall in the middle or lower reaches of the Suwannee, where they congregate in deep channels or holes.

The jumping activity peaks in late June through late August, when observers say the fish pop out of the water almost constantly, particularly in the mornings and the evenings.

"I've worked on the river for 30 years, and I still get a thrill out of it every time I see a sturgeon jump," said Jerry Krummrich, a fishery biologist with the Florida Wildlife Commission in Lake City.

Myths Busted

Sulak says the jumping must be important to the sturgeons, considering how much exertion the flamboyant displays take.

"Jumping expends energy, thus the tradeoff in behavioral importance must be worth the expenditure," he said.

But while there have been at least a dozen explanations as to why the sturgeons jump, none of them satisfies Sulak.

Since the sturgeons only feed when they're in the Gulf of Mexico, he says the jumping in the river can't be related to feeding.

Nor can it be related to spawning, because the fish don't jump during the spawning season in March and April.

Large sturgeons don't have any real predators, either, so the jumping is not an attempt to escape danger, Sulak says.

He also rules out the theory that the fish leap to rid themselves of parasites.

"The old myth of various types of fishes jumping to shed parasites is indeed a myth," he said.

"Any self-respecting parasite, equipped with hooks and suction devices, would not detach from its host simply because the fish jumps. Parasites only jump ship when the host dies."

Staying Together

The sturgeons jump mainly within the same eight half-mile (0.8-kilometer) stretches of the Suwannee River where the fish tend to gather, he says. Biologists refer to these places as holding areas—regions with deep, calm waters where the fish congregate to conserve their energy.

Sulak has recorded and analyzed jumping sounds at one of these holding areas.

A single sturgeon produces a series of sounds just before and just after jumping, Sulak says, with a loud crash in the middle as its body slams against the surface of the water.

The smack is amplified by the posture of the falling fish, which typically flops sideways so that its flank hits the water. This sound travels long distances in the air and probably even farther underwater, Sulak says.

He believes the sounds amount to a sort of calling card, announcing to other nearby sturgeons that some in the local community have found a prime holding area.

"It may be important for members of the population to find these holding areas, where conditions facilitate conservation of energy stores from intense marine feeding," Sulak said.

"I theorize that Gulf sturgeon use this characteristic sequence of sounds to announce and affirm to the group the location of the aggregation in the holding area," he added.

Sulak says this theory makes biological sense, because several species of sturgeons are known to produce sounds to communicate during spawning season and at other times.

"This suggests that communication by sound is typical and important among sturgeons," he said. "Since they typically live in dark tannic or muddy waters, vision is of very limited use in terms of group cohesion."

Sulak adds that his theory is further supported by the fact that the Gulf sturgeon also jump when among feeding groups that gather off shore in early winter.

This suggests that the fish may make their jumping announcements to declare when good feeding areas have been found.

"After fasting for six to nine months in fresh water, it is critical to sturgeon to locate and stay on the best feeding grounds as soon as they emigrate to the Gulf," he said.

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