How do you count octopuses? Very carefully At left. volunteer diver Kevin Tower readies to enter the waters of the Puget Sound near Seattle (AP photo / Thinkstock)
How do you count octopuses? Very carefully
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To check on the health of the giant Pacific octopus population in Washington's Puget Sound, an unusual census takes place every year. Volunteer divers, enlisted by the Seattle Aquarium, take to inland waters to look for their eight-tentacle neighbors.

Weighing as much as 150 pounds with tentacles that can span up to 20 feet, the giant Pacific octopus lives up to its name. It's the biggest octopus in the world. It calls the waters off Seattle home, though this is only part of its vast range over the Pacific Ocean.

"The Puget Sound offers good habitat, water temperature and an abundant food source for them," said Kathryn Kegel. She is a Seattle Aquarium biologist.

Known as one of the smartest creatures in the sea, the giant Pacific octopus leads a relatively short life, between three and five years. They are terminal maters. That means once they mate, they die soon after.

"They are big hunters of crab, clams, scallops, things like that," Kegel said.

Because the giant Pacific octopus is not on federal endangered- or threatened-species lists, there are no current studies on the Puget Sound population. In fact, it's unknown how many live in the area, Kegel said.

That's where the Seattle Aquarium and its troops of volunteer divers step in.

From the waters off Seattle to the maritime border with Canada, 27 divers looked for the giant Pacific octopus, or G.P.O. as it's called. They dove at 11 sites around Puget Sound last month. The aquarium asked the divers to count how many octopuses they saw, note the depth of their finding and the type of hiding spot.

This year, the census counted 28 octopuses, while divers found 17 last year.

"We've been watching the numbers go up, then kind of go down, then kind of go back up," Kegel said. "That could be having to do with population and mating. As they all peak and mate, they slowly die off, then they start to grow back up again."

The volunteer nature of the census means the count is not rigidly scientific, she said.

Two years ago, after a diver killed an octopus, state wildlife officials changed the rules to carve out protected habitat for octopuses. They used the data from the census as well as information from the dive community.

Puget Sound hosts a healthy scuba diving community, and the giant Pacific octopus is one of the main attractions, even though the water is cold and dark.

Octopuses can be challenging to spot. They are nocturnal and hide in their dens during the day. The divers use flashlights and dive in areas historically known for being octopus homes.

"They were hiding in their holes sleeping," volunteer diver Kathryn Arant said. "They had been eating because there were shells all around them."

Critical thinking challenge: What sort of restrictions would you expect to apply within protected habitats for octopuses?

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