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.
Since 1980 commercial shark-fishing boats have used sophisticated electronic equipment that enables them to locate sharks with greater accuracy most seriously weakens the argument.