The two latest shark attacks in Hawaii are part of an annual fall spike of shark bites in October and November that has included nearly a third of all such incidents in recent decades, a shark expert said Monday.

Carl Meyer, a shark researcher with the Hawaii Institute of Marine Biology, said the spike may be driven by a combination of factors, including an annual population boom of tiger sharks in coastal waters and a jump in feeding activity among pregnant females.

Saturday’s attack on a 44-year-old swimmer off Lanikai came just eight days after a 25-year-old man was bitten by a shark at the North Shore surf spot known as Leftovers.

Meanwhile, an eel has been tentatively fingered in a third incident, initially reported as a shark attack off Waikiki on Saturday night.

The state Department of Land and Natural Resources said Monday it is working to confirm the cause of the injury to the Waikiki surfer.

DLNR officers were told by medical personnel that it was likely an eel bite, said department spokeswoman Deborah Ward, but the investigation is ongoing.

Meyer, who has studied sharks for more than 20 years, said Hawaiian oral tradition has clearly linked the fall months to a risk of shark bites. Additionally, Hawaiians have long known that fall is pupping season for tiger sharks, he said.

Scientists have added to the traditional body of knowledge of sharks in Hawaii using modern tools such as electronic tracking.

Meyer, working with University of Hawaii colleague Kim Holland, has shown via remote tracking that adult female tiger sharks captured at remote atolls in the Hawaiian archipelago are most likely to migrate to the main Hawaiian Islands during the fall months when pupping occurs.

“However, we don’t yet know how much this increases the overall number of large tiger sharks in the main Hawaiian Islands,” Meyer said in a statement.

Meyer said it takes a great deal of energy to nourish each litter of shark pups during pregnancy, and previous studies have shown that pregnant females of other shark species are in poor condition prior to giving birth and just afterward because they are essentially starving themselves to nourish their pups.

“We do not yet know for sure whether this loss of condition also occurs in pregnant female tiger sharks, but we have no reason to believe that it does not,” he said. “It is possible that pregnant and postpartum female tiger sharks are feeding more frequently than other individuals, as these hungry females try to replenish their diminished energy reserves. However, this is just an unproven hypothesis at this point.”

While there are indeed more large female tiger sharks in Hawaii’s coastal waters at this time of year and there are more feeding sharks, it’s important to know that shark bites occur in all months of the year here and that the number of bites at any time of the year is extremely low compared with the number of people in the water, the researcher said.

Meyer said the most likely reason for a higher number of shark bites in recent decades is the fact that Hawaii’s number of ocean users has increased substantially.

Historically, Hawaii averaged between two and three shark attacks a year in the 1980s. During the past two decades, the annual average edged up to between three and four shark attacks.

“Surfing and swimming with other people and staying out of murky water are simple actions we can take to avoid being injured by sharks,” he said.

Regarding the eel incident, Waikiki Aquarium biologist Alan Nelson speculated that a 33-year-old man surfing at night was bitten by an undulated moray eel, a species found in the shallow reefs that comes out to the surface at night to feed.

The surfer may have surprised the eel, prompting a defensive attack.

“Usually a human is way too large of a prey item,” he said.

Nelson said most eel bites cause relatively little damage, but there are exceptions. Moray eels, he said, have needle-sharp teeth that can inflict injury ranging from the superficial to the severe, with wounds that range from punctures to deep cuts.

The undulated moray eel, with a brown body and a yellow or green head, grows to 3-1/2 feet in length.

Nelson said Saturday’s victim probably didn’t encounter the larger and more dangerous giant moray eel or yellow margin moray eel because they occur in ocean waters deeper than 30 feet.

When approached by a swimmer or diver, most eels either disappear into their hole or open the mouth wide in a defensive posture toward the intruder.

While unprovoked attacks have been reported, Nelson said, most eel bites occur when swimmers or divers place their hands or feet too close to the eel’s shelter.