Logical Reasoning

PT148 · S1 · Q17 In deep temperate lakes

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In deep temperate lakes, water temperatures vary according to depth.

Conclusion (So)

If you're looking for lake trout in deep temperate lakes while they lakes are still partially iced over in late winter, you should fish in a shallow bay or close to the surface or off a rocky point.

Evidence

In deep temperate lakes, lake trout are found in the coldest water.

In winter, the coldest water is at the top. In summer, the coldest water is at the bottom. The change in temperature distribution (the "turnover") occurs in fall and late winter.

Evaluate

The conclusion is trying to give trout fishing advice for a particular scenario, one that's kind of ambiguous.

We know during winter we'd fish close to the surface, because that's where the coldest water and thus the trout would be. During summer, we should fish close to the bottom, because that's where the coldest water and thus the trout will be.

But what if it's on the borderline between those seasons? If it's late winter and partially iced over, we're right around the "turnover" time, when the cold water switches from being on top to being on bottom.

Has it already turned over? Should we still be going by winter rules (fish on top) or by summer rule (fish on bottom)? The author's conclusion is saying that we should fish near the top of the lake, so apparently the author thinks the cold water is still on top of the lake.

note: tons of us are confused by what the conclusion is even saying because of two vocab words. "Eschew" means "to avoid / to reject". And "summer haunts" means "places you hang out in the summer". So the conclusion is conversationally saying, .

Goal

Let's look for something like, or .

17.

Which one of the following is an assumption on which the argument depends?

The ease with which lake
Out of Scope: ease of catching
Nothing in the argument is talking about the ease / difficulty of catching trout. We might think to ourselves, "Can't we infer this? If the trout are closer to the top in the winter and closer to the bottom in the summer, then isn't it easier to catch trout at certain times / temperatures than at others?' 1) we don't know if it's easier or harder to catch trout at the top vs. bottom of the lake 2) we don't know if the water temperature of the lake actually changes. The argument was never talking about the overall temperature of the water. It was only talking about the distribution of temperatures within the water. Overall, we want to look out for this type of trap answer on Necessary Assumption -- it's baiting us into picking it because it sounds like a "true" idea we could derive from somewhere in the paragraph. But the correct answer shouldn't feel like "an Inference we can derive from the premises". It should feel like "a thought the author must have been having in order to arrive at her conclusion".
19% picked this
Cold water is denser, and
Out of Scope: dense / heavy
Nothing in this paragraph deals with the density or weight of water. If we negated this and said that cold water is not denser than warmer water, it would change nothing in the argument. The seasonal change in distribution of cold water occurs every year, regardless of which type of water is denser or heavier.
1% picked this
Lake trout are found exclusively
Too Strong: exclusively
This luring us with their classic bait of, "Only Thing Mentioned = Only Thing". Just because this paragraph only mentions lake trout in deep temperate lakes doesn't mean we get to say that lake trout are only found in deep temperate lakes. If we negate this and say that, "Lake trout are also found in other types of lakes", that wouldn't hurt this argument at all.
4% picked this
Lake trout do not alter
Out of Scope: feeding habits
Given that the author is telling us that trout live at the top of the lake for part of the year and at the bottom of the lake for the other part of the year, it seems very likely that the author might believe that lake trout do alter their feeding habits. There are probably different things to eat at the top of the lake than at the bottom. But not only is this answer contradicting how we might guess the author would be thinking, it's also just not important to the logic. This argument is just about instructing anglers whether to aim for the top of the lake or the bottom of the lake. Whether trout's feeding habits stay the same or not, it's still the case that they're living near the top for half the year and near the bottom for half.
8% picked this
Correct
In deep temperate lakes that
Why this is right
This is saying, "if the lake is still partially iced over in the late-winter, then the water hasn't switched yet (turnover has not yet occurred). The coldest water is still on top of the lake, so that's where the trout are." That's definitely what our author seemed to believe, since he's recommending that anglers aim for the top of the lake. If we negate this, it would say, "in deep temperate lakes with ice on the surface, turnover has occurred." That would mean that the cold water has now shifted to the bottom of the lake, and since the trout go where the coldest water is, the trout would have shifted to their summer haunts. Since negating this would badly weaken the argument (in this case, it would almost flat-out contradict the conclusion), it's the correct answer.
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.
68% picked this

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