Official Explanation

B, F, G

The first blank describes something that researchers would track in order to make decisions about “river release points” that will lead to changes in the salmon population. The GRE does not expect you to be an expert on wildlife biology, so if you're not sure what “river release points” are, don't worry about it. Just focus on the context clues you do understand. In the second sentence, the distance that salmon travel determines how healthy they are and, by extension, how healthy the ecosystem is. Thus, predict that the research studies under discussion look at the fish's movement. Two of the choices for the first blank are related to movement, but only one is not specific to humans. (B) migration means “to move from one place to another” and is correct. (A) immigration can be eliminated because it means “moving to another country to live there,” and fish don't think about which country they're in. The (C) evolution of the salmon is something that could be under study, but the context clues do not suggest that this research is about that.

The third blank relates salmon varieties that “have greater growth rates” and “tend to be healthier” to “larger populations.” The ideas can be connected with the prediction result in, which works well with (G) yield, meaning “to produce something.” There is no supporting evidence in the sentence that the hardy salmon species are going to either (H) supplant, meaning “take the place of,” or (I) undermine, meaning “make something weaker,” any other species.

Finally, you can use the clues at the end of the second sentence to make a prediction for the second blank, which describes what kind of population the researchers want to achieve. The goal for researchers is “to introduce more of the hardy salmon species.” Based on this information, predict that the researchers want to create a more diverse population. A population that is (F) heterogeneous is one that is “made up of diverse parts,” which matches the prediction. Both (D) homogeneous and (E) kindred mean “similar” and can be eliminated.