Dipping a camera underwater while on board a jetski wouldn't be most people's first response after spotting "hundreds" of sharks - but that's exactly what Louis Giachetti did.

The Auckland fisherman captured what he says was hundreds of sharks hunting for food directly beneath him yesterday afternoon at Flat Rock fishing spot near Kawau Island.

"I didn't freak out, it was just annoying they kept eating all the fish I was trying to catch," Giachetti told the Herald.

Despite losing multiple lures to the sharks, Giachetti said the experience made it all worth it.

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"It was incredible to see, there were just so many. I was quite alarmed at one stage because they were so hungry.

"A few were jumping out of the water to catch the fish."

He said it was about 5km to 10km from shore and there were about four fishing boats nearby whose crews also couldn't believe how many sharks they were seeing.

"For the last four years, I've been fishing in the area and I've never seen so many. Last summer there were quite a few but so many more this year," Giachetti said.

The fisherman didn't leave empty handed, saying he managed to catch three snapper before the sharks got to them.

Louis Giachetti, 48, was happy to get at least three snapper the sharks did not. Photo / Supplied
Louis Giachetti, 48, was happy to get at least three snapper the sharks did not. Photo / Supplied

Department of Conservation (DoC) marine scientist Clinton Duffy, who viewed the video, said he counted at least 11 sharks in frame at once.

"Five to six were visible most of the time, so there were obviously a lot there."

Duffy confirmed they were bronze whaler sharks which commonly form loose schools like this one, sometimes numbering hundreds of individuals.

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"These sharks don't look very large, I suspect they're juveniles – they don't mature until they're over 2m length. Its more common to see juveniles in large schools than adults."

He said if Giachetti was catching fish at the time or using barley to attract fish, that would explain why they appeared hungry.

"The struggles of the fish or scent of minced-up fish would have triggered a feeding response ... most sharks feed at night but can be stimulated to feed at any time.

"With so many other sharks around, the level of competition between individuals in a school is high and they tend to attack potential prey faster than if they were alone in order to get it before another shark does ... a classic case of if you snooze you lose."

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Duffy said as a general rule unless you were familiar with sharks and shark behaviour you should treat any shark 1.8m or larger as potentially dangerous and leave the water with minimal commotion.

"Avoid swimming in areas where there are large concentrations of fish that may be potential prey for sharks, any where people are fishing or cleaning fish, and swimming at night or dawn and dusk as these are peak feeding times."

More about bronze whaler sharks:

• They feed mainly on fish, small sharks and rays.

• They reach a reported maximum size of about 3m long and are generally not considered dangerous, although they can become aggressive towards people in the water if fish blood or struggling fish are present. This generally appears to be a competitive behaviour, they are attempting to drive you away from the food. This can of course involve biting.

• Swimming in a large school like this does have potential advantages for juveniles such as protection for predation by larger sharks and it possibly make corralling schooling fishes easier.

• Mature female sharks often school up in large numbers during pregnancy and before giving birth.

• Schooling in warm shallow water is thought to help speed gestation of the pups and giving birth en masse can increase the chances of new borns surviving their first few hours when they're most vulnerable to predators.