Due to the manner in which the shark nets operate, it is impossible for them to target the shark species that are considered as potential threats to swimmers. There is no way to discriminate against which marine animals they trap. As a result, hundreds of non target animals die through entanglement and eventually drown, including dolphins, turtles and whales.
Many shark species (including some of the target species) are endangered and shark numbers worldwide are decreasing at an alarming rate. The majority of animals that are caught and eventually die in shark nets are endangered and considered harmless to humans.
Shark nets do not reach all the way to the bottom of the ocean, and have large panels missing, so it's a little like putting up a fence in a paddock that only goes three quarters of the way around - you may deter some of the cows from coming in but it is certainly not an effective or efficient way to operate. In addition, shark nets give people a false sense of security.