A recent spate of aggressive sharks seen in near-shore waters at a popular spot on Oahu’s North Shore is likely due to the spawning season of juvenile fish, according to a shark expert.

The sightings are no surprise to Carl Meyer of the University of Hawaii’s Institute of Marine Biology. He has been tracking shark movement patterns and habitat use around the Hawaiian Islands for more than two decades.

“The schools of fish are probably attracting sharks close to the shoreline where they are easier to spot,” he said. “The shark activity will taper off as the schools dwindle or disperse.”

Large schools of juvenile akule (halalu) have been observed at Waimea Bay and other coastal areas in recent weeks, according to Meyer.

On Monday, Waimea Bay lifeguards reported multiple aggressive sharks likely feeding on fish about five yards offshore. Warning signs were posted and beachgoers were urged to stay out of the water.

Though no sharks were spotted Wednesday, signs at Waimea remained on the beach as a precautionary measure because of frequent sightings in the past several days. Last Thursday, an aggressive 6- to 8-foot shark was observed feeding on fish about 30 yards offshore.

“Juvenile akule (halalu) form schools in many sheltered bays and harbors at this time of the year,” Meyer said in an email, adding schools of fish at Waimea typically start to disperse in mid- to late September.

Fishing for halalu is a popular pastime among fishermen in Hawaii.

He noted sharks routinely visit Waimea Bay but are not seen by beachgoers. In 2007, Meyer and fellow researchers installed an underwater receiver at Waimea as part of their ongoing research on shark movements across the Hawaiian chain.

The receiver, about the size of a large shampoo bottle, records movement of sharks tagged with ultrasonic transmitters when they swim within a few hundred yards of the device. The receiver that functions 24 hours a day throughout the year records the time and date of the sharks’ movement.

Each tagged shark has a unique identification code that provides information to researchers on exactly which tagged shark was detected by the receiver.

Species detected at Waimea include tiger sharks, oceanic blacktip sharks, sandbar sharks, Galapagos sharks and scalloped hammerhead sharks. Meyer pointed out the receiver can detect only tagged sharks “so actual total shark presence in the bay will always be higher than the frequency of detection of tagged individuals.”

It’s unclear at this time what species were observed thrashing in near-shore waters Monday as researchers have yet to retrieve the latest data from the receiver. “Our last download was in August,” Meyer said.

The device, at the bottom of the mouth of the bay, is part of an array of shark-tracking equipment throughout the Hawaiian Islands.

For almost 25 years, Meyer and fellow researchers have been tracking and tagging sharks in Hawaii’s waters. Currently, they are monitoring over 100 sharks.

“Some individual sharks (including some of those detected in Waimea Bay) have been tracked for over a decade,” he said. “We have learned a lot about shark movements and habitat use over this period, but still have important, unanswered questions. Hence, research is ongoing.”