All whales that visit the estuary are wild animals; many, like the humpback, may be mothers with calves. The water from their blowholes will usually be visible long before the rest of the body is. They will defend themselves vigorously and violently if they believe they or their babies are in danger. A recommended distance of at least 100 yards must be maintained between a vessel and the whale, and pursuing one when it does not seek contact is illegal. It is not unknown for some species to be inquisitive and should the whale approach willingly, the boatmen must turn off the engine and give the beast a way out so that if the whale feels uneasy, it can leave.