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. The sharks that are the most profitable to catch are those that tend to remain in the same area of ocean year after year and not migrate far from where they were born. The waters around South Australia are the only area in the world where that particular species of shark is found. Since 1980 commercial shark-fishing boats have used sophisticated electronic equipment that enables them to locate sharks with greater accuracy.
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.
Which one of the following, if true, most seriously weakens the argument?