Researchers in South Australia estimate changes in shark populations inhabiting local waters by monitoring what is termed the catch per unit effort (CPUE). The CPUE for any species of shark is the number of those sharks that commercial sharkfishing boats catch per hour for each kilometer of gill net set out in the water. Since 1973 the CPUE for a particular species of shark has remained fairly constant. Ttherefore, the population of that species in the waters around South Australia must be at approximately its 1973 level.
A significant threat to shark populations, in addition to commercial shark fishing, is incidental mortality that results from catching sharks in nets intended for other fish most seriously weakens the argument.