At least a dozen hammerhead shark pups that were found dead close to the shore along Kane'ohe Bay in Kahalu'u on Wednesday probably were tossed aside by a fisherman after being unintentionally caught in his "lay net" and dying, a fisheries expert said yesterday.

John Naughton, Pacific islands environmental coordinator for the National Oceanic and Atmospheric Administration Fisheries Service, said more tests are needed to determine conclusively what killed the newborn sharks.

The prevailing theory is that they became what is known as "bycatch" in a fisherman's net, Naughton said.

"It's not unusual to find dead pups in Kane'ohe Bay, especially at this time of year, but it is somewhat unusual to find them in these numbers," Naughton said.

The summer months are the birthing season for hammerhead sharks and Kane'ohe Bay is a favored birthing location, Naughton said.

There were differing reports as to how many of the dead hammerheads were found, ranging from a low of 10 to a high of nearly 40.

Naughton said mother sharks typically have litters of up to 30 pups. He said all of the dead sharks were about the same size, and he estimated their age at about one week.

Small sharks like the ones that were found typically get their wide heads stuck in lay or "gill" nets and are unable to extract themselves.

Hammerheads are one of the shark species that have to keep moving to have seawater flow over their gills to extract oxygen.

The sharks that were found dead were so young and their breathing systems so fragile it probably wouldn't have taken very long for them to expire after becoming stuck in a net, Naughton said.

As long as the "eye" is of the required size, gill nets are legal in Hawai'i if they are not left in place for more than four hours and are checked at least once every two hours.

"We ask that people who find live hammerheads in their nets, or catch them with a pole and a hook, release them. They are not a dangerous type of shark and the ones that were found dead belong to the most common species of hammerheads in the world," Naughton said.

The dead baby sharks could have grown to about eight feet in length. "But even at eight feet, their mouths are pretty small," Naughton said.

Newborn hammerheads prefer to feed in the soft sediment along the shoreline of large bays looking for small fish or invertebrates to munch on, Naughton said.

"They have been in Kane'ohe Bay for eons and will continue to be there for eons," he said.

Reach David Waite at dwaite@honoluluadvertiser.com.