The month is only a week old, and already shark sightings have shut down a handful of Hawaii beaches, “aggressive” sharks have been spotted in waters off Oahu and Maui, and an 8-footer actually bumped into a snorkeler off Kihei, leaving her with minor scrapes on her arm and leg.

Welcome to Sharktober, the month in Hawaii when statistics indicate you are most likely to be the victim of a shark attack.

Add November, the month with the second most shark bites historically, and you’ve got an annual fall spike accounting for nearly a third of all such incidents in recent decades, state records show.

While Hawaiian oral tradition has linked the fall months to a risk of shark bites, and Hawaiians have long known that fall is pupping season for tiger sharks, there is no rock-solid scientific explanation for the phenomenon, said University of Hawaii shark researcher Kim Holland.

In fact, Holland’s shark-tagging data indicate that more sharks are found in Hawaii’s nearshore waters during the winter months than in the autumn.

“Sheer numbers don’t explain it,” Holland said.

But there is at least one possible explanation.

Holland and UH Hawaii Institute of Marine Biology colleague Carl Meyer have shown from remote tracking that adult female tiger sharks in the Northwestern Hawaiian Islands are most likely to migrate to the main islands during the fall months when pupping occurs.

One theory suggests that it takes a lot of energy to nourish each litter of shark pups during pregnancy. Studies have shown that the pregnant females of other shark species are in poor condition prior to giving birth and just afterward, since they are essentially starving themselves to nourish their pups.

If the same holds true for tiger sharks, this might explain why those females would become more aggressive feeders as they try to replenish their diminished energy reserves. The result: more shark bites in the fall.

“But that’s pure speculation,” Holland said. “The truth is we have no smoking gun, no direct line of cause and effect.”

The veteran shark researcher described the October phenomenon as a mere statistical bump.

“Even with the bump it’s a low number. It’s just a statistic,” he said.

With nearly 40 years of data going back to 1980, the numbers indicate there have been fewer than 30 shark bites in October. That indicates that there are years when there were no shark bites during that month, Holland said.

What’s more, there has never been a fatal shark attack in October.

Meyer said it’s important to remember that shark bites occur in all months of the year and that the number of bites at any time of the year is low compared with the number of people in the water.

Still, Meyer said his team is planning future scientific studies to determine whether changes in tiger shark condition and behavior associated with pupping might cause the fall spike in shark bites.

“Even though we don’t yet know the cause, the higher numbers of shark bites seen during October and November warrant extra vigilance by ocean users at this time of year,” he said.

The state Department of Land and Natural Resources offers sound advice for how to reduce the risk of a shark bite. Oceangoers should surf and swim with other people and stay out of murky water to avoid being injured by sharks.

Other tips:

>> Do not go in the water if you have open wounds or are bleeding in any way.

>> Avoid harbor entrances, areas near stream mouths, channels and steep drop-offs, especially after heavy rain.

>> Do not wear high-contrast clothing or shiny jewelry, as sharks see contrast well.

>> Get out of the water if fish or turtles start to behave erratically. Be aware that dolphins are prey for some large sharks.

>> Remove speared fish from the water or tow them at a distance behind you. Avoid swimming near people fishing or spearfishing, and stay away from dead animals in the water.

>> Swim or surf at beaches with lifeguards, and listen to their advice.