Question

Biologists studied the migration patterns of a specific species of salmon in a coastal river system. Their observations indicated that large migrations consistently occurred during periods of heavy rainfall, which dramatically increased the river's flow and lowered its temperature. They noted that even on sunny days with otherwise stable environmental conditions, migration numbers were low unless a rain event had occurred upstream. The team hypothesized that changes in water flow rate and temperature—not daily light cycles or lunar phases—trigger these mass migration events: the increased flow provides a faster route and reduces predation risk, while cooler temperatures signal optimal physiological conditions for the journey. Because migration strength tracked these specific environmental cues, the researchers predicted that artificially maintaining stable flow and temperature conditions would suppress migration, whereas rapidly altering these conditions would enhance it.
Question:
Which finding, if true, would most directly support the researchers' hypothesis?
A. When researchers installed flow regulators and refrigeration units to maintain the river section at a constant flow and temperature, the number of migrating salmon remained low for the entire duration of the trial, even during periods of full moons and stable light cycles.
B. In separate laboratory tests using artificial river tanks, salmon showed strong migratory behavior when exposed to rapid changes in light exposure simulating twilight and dawn, even if the water flow and temperature were kept constant.
C. Surveys of several nearby river systems found that the timing and success of salmon migration varied widely across different tributaries, independent of local rainfall patterns or measured water flow rates across the study region.
D. When researchers introduced a chemical pheromone known to induce stress responses into a section of the river, salmon migration rates increased dramatically, even under stable flow and temperature conditions.

Answer 1

A

Explanation: A shows that artificially keeping flow and temperature constant suppressed migration (even when lunar/light cues were present), which directly supports the researchers' claim that changes in flow and temperature are the key triggers. The other options either implicate different cues (B, D) or show no relationship with flow/rainfall (C), which would contradict the hypothesis.