Following an investigation into the Oak Bluffs Monster Shark Tournament in July that included undercover surveillance and videotaping, the Humane Society of the United States announced yesterday that it had evidence of high-stakes illegal gambling activities occurring during the controversial fishing tournament.

The society, which opposes the tournament, has sent a letter to Massachusetts Attorney General Martha Coakley calling for an investigation into the alleged illegal gambling.

“The addition of high-stakes gambling on top of cash prizes only heightens the incentive to kill large numbers of these ecologically important animals, some of which are threatened with extinction,” said John Grandy, senior vice president for the humane society.

In a five-page letter to the state attorney general, legal counsel for the humane society Jon Lovvorn said an undercover investigation showed possible illegal bets taking place during the three-day tournament.

“Our investigation has revealed evidence that the tournament, sponsored by Sharks Unlimited, Inc., is largely a platform for illegal gambling activities incoming bets totalling bets in excess of $1 million . . . we believe that these activities are crimes under Massachusetts law,” Mr. Lovvorn wrote.

The letter claims the investigation revealed several different activities allegedly violating state gambling laws. One involved betting pools known as added entry divisions, where participants vie for the chance to win additional prizes. The added entry division consists of four separate divisions; teams can participate in any or all of the divisions by paying entry fees ranging from $600 to $5,000.

The team that selected the winner in any given division won all the money pooled in that division, the letter states; reportedly over $366,000 in entry fees were collected, recorded and pooled by Sharks Unlimited, also known as the Boston Big Game Fishing Club.

The letter claims the bets were illegal because they were written down and recorded on the tournament’s entry form.

“Under Massachusetts law, ‘registering’ a bet means committing the event or transaction to writing . . . the added entry division is textbook pool betting because the participant gamblers pay a fixed price into a pool, and then make a selection on the outcome of the tournament, with the winner’s payoff depending solely on the number of gamblers and the number of winners,” the letter says.

The letter asks the attorney general to investigate the matter further.