In deep temperate lakes, water temperatures vary according to depth. In winter, the coldest water is at the top; in s...

Ravi on December 19, 2018

@GLEE,

The stimulus tells us that in winter, the coldest water is at the top and during summer, it's at the bottom. We're also given info that the changes in temperature distribution ("turnover") occur during the fall and the late winter. Based on the question stem, we know that this is a strengthen with a necessary premise question.

This is what the "turnover" cycle looks like:

winter (turnover) - -spring - -summer - -fall (turnover) - -winter (turnover) - -spring - -summer...

What this means is that for most of the year, the cold water is at the bottom or moving toward the bottom. and from the fall to the late winter, the cold water is at the top or moving toward the top.

The stimulus then tells us that if anglers seek lake trout in deep temperate lakes while these lakes are partially iced over in late winter, they will do best to eschew the lake trout's summer haunts and fish instead in a shallow bay or close to the surface of a rocky point.

See the problem with this language? It's ambiguous. We don't know when in late winter these anglers are trying to catch trout. Is it before or after the turnover has occurred? If it's before the turnover has occurred, then they would be best fishing close to the surface. However, if it's in late winter after the turnover has happened, then they would be best off fishing in deeper water.

In looking at the answer choices, we can use the negation test since this is a strengthen with a necessary premise question. The negation test effectively takes out that premise from the argument. If the argument falls apart with the negation, then we know that the premise is essential for the argument.

(A) is tricky because it uses 'time of year' and 'water temperature' in its answer, but when looking at it closely, it is not essential, as its negation does not make the argument fall apart. The negation of (A) is that the ease with which lake trout can be caught by anglers does not necessarily vary with the timer of year and water temperature. If this were true, the argument could still stand, so this is not our answer.

(B) is not necessary. The negation is cold water is not denser and heavier than relatively warmer water. O.K, Does this make our argument fall apart? Nope.

(C) isn't essential. If lake trout are found in shallow lakes, this would not wreck the argument.

(D) isn't necessary. If lake trout did alter their feeding habits from one part of the year to another, the argument could still stand. I think you may have gotten hung up on (D) because the stimulus said that lake trout will be found, as a rule, in the coldest water. However, where lake trout are found does not mean necessarily have to do with their feeding habits. Their feeding habits could remain the same throughout the year even if for part of the year the trout are at the bottom of the lake and for the other part of the year they're at the top of the lake.

(E) is what we're looking for. in fact, it's what we predicted after we scrutinized the stimulus. The negation of (E) is in deep temperate lakes that have ice residues on the surface, late-winter turnover has occurred. If this were true, then the argument would fall apart, as the anglers would be best off fishing in deeper waters, not shallow waters.

Hope this helps. Let us know if you have more questions!