This is a cause and effect argument. The observed effect the author is trying to explain is that since 1973, the CPUE for a particular species of shark has remained fairly constant. The author's proposed cause for this observed effect is that "the population of that species in the waters around South Australia must be at approximately its 1973 level."
(A) is incorrect because it is irrelevant to the stimulus. We are focusing on whether or not the population of that species of shark IN THE WATERS AROUND SOUTH AUSTRALIA is at the same level it was in 1973. Whether or not there are more of this species somewhere else has no bearing on our argument.
(B) is incorrect because sharks being profitable or not has no bearing on whether due to the same CPUEs from 1973 and today for a particular species of shark, the population of that shark has remained at the same level it was in 1973.
(C) is incorrect because we are trying to find the answer choice that shows that though the CPUE for 1973 and the present are the same, that does not necessarily mean the population of that species in the waters around South Australia must be at approximately its 1973 level. This answer choice does nothing in regards to weakening the stimulus. "Incidental mortality" being a significant threat to shark populations is completely irrelevant.
(D) is incorrect because we are not touching upon how the premise, the fact that the CPUEs from 1973 and the present are the same, does not support the conclusion. Whether or not the quotas designed to protect shark populations limit tonnage or individual number has no direct bearing on whether due to the CPUE we know that the shark population is roughly the same as it was in 1973.
(E) is CORRECT because it points out an alternate cause for our observed effect (i.e. it is not that the population is the same, but rather that commercial fishing boats are using better technology). If commercial shark-fishing boats have used sophisticated electronic equipment that enables them to locate sharks with greater accuracy, then they should be locating many more sharks than they had previously. However, if the CPUE from 1973 is still the same as the present CPUE, then that means there are less of this specific shark species than in 1973.
Hope that helps! Let me know if you have any more questions!
CaseyAugust 19, 2015
Wouldn't the incidental mortality being a threat to the population offer an alternative cause that the population is not the same but the CPUE does not account for a potential high rate of incidental mortality? Whereas better technology and accuracy does not necessarily mean they're locating more sharks, but instead they're meeting their quotas faster and stopping in order to stabilize the population, thus having a consistent CPUE.
NazSeptember 3, 2015
You are assuming that they have a quota of shark. Remember that the CPUE for any species of shark is the number of those sharks that commercial shark-fishing boats catch per hour. Instilling a quota would go against the point of the CPUE. We wants to see how many sharks care caught per hour because that will give us a good gauge any changes in the shark populations inhabiting those waters.
Even if "incidental mortality" is a significant threat to shark populations, that has no effect on our argument.
Remember, we are looking for an answer choice that explains why even though we are still using CPUE to estimate changes in shark populations inhabiting local waters, the fact that the CPUE from 1973 has remained fairly constant to the present doesn't necessarily mean that the population of that species in the waters around South Australia is not at its 1973 level.
Answer choice (C) merely tells us about another significant threat to shark populations. We do not know what the level of this threat was in 1973 or how this threat has progressed to the present. We simply aren't given enough information about this threat to be able to sway the argument in any way.
Hope that helps! Please let us know if you have any other questions.
awashingDecember 26, 2016
It's interesting that the reasoning provided for the correct answer here is precisely the reason why I did not select it; the answer choice seemed like a "got ya" in that because the sophisticated equipment would have surely led to more sharks being located than before, the CPUE remained the same thus not offering an acceptable alternative cause for the effect: the constant CPUE level for the sharks. (E) almost seems like a chicken or the egg selection.
