#13 - When so many oysters died off the coast of Britain

[Dave Killoran]

The infamous oyster question from this test This one was the topic of much discussion afterward, and it's certainly a tricky answer. Let's take a closer look!

The stimulus presents the following paradoxical situation:

• The fact that water temperature had recently risen was originally thought to have been the cause of an oyster die-off that was so bad that certain native species became endangered, but it was later determined to have been caused by TBT. TBT was then banned and consequently almost eliminated from British waters, but the endangered oyster populations have not grown.

The paradox is why didn't the removal of TBT—a known causal agent in killing oysters—then allow the endangered oysters to flourish and grow? The question stem asks you to help explain that outcome.

Note first that the increase in water temperature is treated as a fact, as is that TBT caused the endangered oysters to die off, and that TBT is largely eliminated from British waters. As is usual in Resolve/Explain questions, you aren't going to be asked to dispute these facts, but to instead come up with a solution that allows all of them to co-exist.

Answer choice (A): Although the temperature increase may have slowed, remember that water temperature had already been ruled out as the cause and so this answer doesn't help us understand what is happening.

If you were thinking that this answer showed a further increase in temperature which was then the reason the oyster populations did not grow, the two problems with that view are that temperature wasn't the initial cause and that this still does not explain why the removal of TBT didn't lead to growth.

Answer choice (B): While oyster food sources could be useful to explaining this situation, the information in this answer does not help since the barnacles are stated to have different sources of food than the oysters. We have no information about those food sources, and thus no way to construct an explanation of the situation here.

Answer choice (C): This is the correct answer. At first glance, the idea of imported oysters may seem irrelevant, but note that the imported and native species are in a battle: the imported oysters "flourish" at the expense of the native oysters. This is key because when TBT was used, it also killed the imported oysters, but now that TBT is eliminated these imported oysters can flourish again at the expense of the native oysters. Thus, we have an explanation for why the native/endangered oysters are not recovering: now that the TBT is gone, the competitor/imported species are keeping them in check, which is especially the case since they thrive in warmer waters (which are known to be present).

Answer choice (D): This is great news on an environmental front, but it isn't helpful to our paradox because we want to know why the oysters aren't coming back now that TBT is gone. This could have been helpful if they said the other chemicals used to remove barnacles are harmful to the native oysters, but that's the opposite of what is stated here.

Answer choice (E): We know that TBT has been nearly eliminated from British waters, and so this answer does not help explain the situation (and, even if it were relevant, it would slightly cloud matters since the waters are known to be warmer).

[prometheus1992]

Yes, thank you for this explanation! I was also admittedly snagged by this problem on test day.

I think part of what makes this one particularly confusing is that it introduces extraneous information that is ultimately irrelevant to the stimulus and argument structure that Dave laid out, namely the fact that water temperature was considered and then rejected as a possible cause of the native oyster population decline. A number of the answer choices reference water temperature, and I think anyone who read carefully and was aware that the water temperature was definitively eliminated as an explanation would be able to rule out any of these answers.

It's also highly difficult to predict what the answer will be to these types of discrepancy questions, and they give the test-makers a lot of room to be creative with the explanation they come up with. I don't think anyone would have pre-phrased, "Well, it must be those darn imported oysters!", but now that I'm reviewing it, C is clearly the correct answer because it's the only one that provides a valid explanation.