A state-funded study discovered that the reason for the shark invasion lie in the construction of a new port to the south of the city during which two freshwater estuaries were sealed off from the ocean. As bull sharks used to travel up these streams to reproduce (unlike most species of shark, the bull shark can survive in both fresh and salt water, making it the scourge of the rivers of Central and South America and Lake Nicaragua's most famous denizen - check out Edward Marriott's Wild Shore: Life and Death with Nicaragua's Last Shark Hunters for an excellent story about the bull shark and those who stalk it for a living), the building of the port forced the spawning females to find an alternate estuary which just happened to skirt past the Praisa de Boa Viagem.
Right now various solutions to the shark problem are being debated, but in the meantime the only surefire way not to end up staring down the jaws of an angry bull shark is to heed the signs on the beach and stay the hell out of the water.
Weird library synergy bonus: while typing this post someone came to the desk to check out three books about the movie Jaws. Freak-ay!