Diadromous fish migrate between freshwater and marine biomes during their life cycle. The migration's obligate nature is why diadromous fish...

Step 1: Decode and Map the Passage

Part A: Passage Analysis Table

Note about vocab 'obligate'.

If you do not know the meaning of the word 'obligate', then also you can solve this question. There are multiple paths that can be taken:

1. If you can link this word with another more popular word 'obligatory' or 'obligation'. These words have the sense of 'requirement' built into them. Thus, this word 'obligate' will also have that sense of "requirement".
2. If you are unable make that linkage, then also here is the thought process.
   1. Replace 'obligate' with 'some kind of' - with an understanding that this is actually an important point.
   2. Now read - some kind of nature of this migration is the reason behind something…
   3. And you read further…
   4. As you analyze the rest of the sentence, you will realize that in the grand scheme of things knowing exactly what obligate means is not required to solve the question.
3. But you should use this opportunity to build your vocabulary - add one more cool word to your vocabulary knowledge.

Part B: Passage Architecture & Core Elements

Visual Structure Map:

DIADROMOUS FISH (migrate between biomes - required)
↓
[BLANK relationship word]
↓
EURYHALINE FISH (tolerate salinity - optional)
├── Example: Blackchin tilapia (survives salt, no relocation)
└── Contrast: Wild salmon (does relocate)

Main Point: Diadromous fish are fundamentally different from euryhaline fish because their migration between biomes is obligatory, not optional.

Argument Flow: The passage defines diadromous fish as biome migrators, then sets up a comparison with euryhaline fish that can tolerate salinity but don't require biome changes, using specific examples to reinforce this key distinction.

Step 2: Interpret the Question Precisely

This is a fill-in-the-blank question asking us to choose the best logical connector. The answer must create the right relationship between what comes before and after the blank.

Step 3: Prethink the Answer

• From our analysis, we see that diadromous fish have an obligate (required) migration between biomes, while euryhaline fish merely have the ability to tolerate high salinity but don't actually have to relocate
• The word "merely" is a big clue here - it suggests euryhaline fish are somehow less than diadromous fish in terms of their relationship to different environments
• The blank needs to show how these two types can be distinguished or separated because:
  • Diadromous fish MUST migrate (it's obligate)
  • Euryhaline fish CAN tolerate salinity (it's optional)
• So the right answer should express how diadromous fish can be clearly distinguished or set apart from euryhaline fish based on this fundamental difference in their life cycles