Researchers in South Australia estimate changes in shark populations inhabiting local waters by monitoring what is te...

shunhe on July 24, 2020

Hi @Anna2020,

Thanks for the question! So let’s walk through the stimulus first. I wouldn’t categorize this as a correlation/causation mix-up, since the argument isn’t really arguing that something caused the shark population to stay the same. Basically, we’re told here that we measure changes in shark population with the CPUE, which is basically how many sharks commercial shark-fishing boats catch per hour per kilometer gill net set out in the water. The CPUE has stayed about the same since 1973, so the argument concludes that the population of shark has to be about the same as well.

We can see almost immediately that there’s a problem with this logic, and here’s an example. I ate two mushroom pizzas last year, and I ate two mushroom pizzas this year. Therefore, the total number of pizzas I ate both years must be about the same. But I could’ve just eaten more pizzas one year compared to the other and had a different proportion of mushroom pizzas to total pizzas! We’re assuming that because the numerator is the same, the denominator has to be the same as well; we’re assuming that the rate stays the same.

Now take a look at (E), which tells us that we can weaken the argument by pointing out that since 1980, commercial shark-fishing boats are using better technology that lets them find sharks with greater accuracy. Well, if this is true, then the boats should be finding more sharks if the shark population stays the same. But they’re not, which actually implies the shark population went down! So (E) shows us this weakness, and that’s why it’s the right answer.

(C) is wrong because it just tells us that some sharks die when they get caught in nets intended for other fish, but that doesn’t really affect this argument. Maybe they died because of incidental mortality back in 1973 too, so we can’t conclude that there’s a difference there.

(A) is wrong because we only care about the waters around South Australia anyway, so knowing this doesn’t weaken the argument.

(B) is wrong because we don’t care about if sharks are the most profitable or not, we just care about the total shark numbers. So knowing about this subgroup doesn’t weaken the argument.

(D) is wrong because we don’t care about these quotas or about sharks measured in tons, we just care about if the number of sharks in the waters has stayed the same or not.

Hope this helps! Feel free to ask any other questions that you might have.