Because the native salmon in Lake Clearwater had nearly disappeared, sockeye salmon were introduced in 1940.
Hypothesis
The two populations of sockeye salmon differ genetically because they have each adapted genetically to their distinct habitat.
Evidence
In 1940 a genetically uniform group of sockeyes was introduced into the lake. Now there's a group in the deep part of the lake and a group in shallow part, who don't interbreed and now differ genetically.
Evaluate
As soon as we see the word hypothesis (a causal explanation for a phenomenon) in the question stem, we know we can use a Curious Fact / Explanation template.
Curious Fact: Why are the deep water sockeye now genetically different from the shallow sockeye? Author's Story: They have each adapted genetically to their distinct habitat
We then evaluate this via two lenses: 1. is there some OTHER WAY to explain the curious fact? 2. how plausible is the AUTHOR'S WAY of explaining it?
Usually Strengthen questions deal with plausibility of the author's way. Is it plausible that each sockeye population has adapted genetically to its distinct habitat?
Can evolution actually work that quickly? Normally the genetic drift of a species takes thousands or millions of years, not the 80 years since 1940. The only time that evolution works super quickly is with a very isolated population and a very distinct selection pressure (like, all of a sudden, there's sulfur in the water and only the fish that mutate a gene that tolerates sulfur will survive and pass on their genes, so within a few generations, the sulfur-tolerant fish dominate and effectively the "genes" of that species have been changed).
To strengthens the AUTHOR'S STORY, we would want to know that the deep lake and the shallow lake have very different and very important selection pressures, in order for us to think that over the course of 80 years we would have had a mutation so strongly selected for that the populations of sockeye are now genetically different.
Is there some OTHER WAY to explain how these once genetically identical populations of sockeye are now different? Hmmm, how can you change genetic makeup, other than by mutation / natural selection? The only other way that genes get affected is through mating. When two animals have sex, their offspring is a genetic blend of each of the component animals. So maybe the deep lake sockeye and shallow lake sockeye now differ from each other because at least one of them is mating with some other animals, thereby changing the gene pool.
Goal
We either need something that strengthens the idea that the two different parts of the lake have such different survival needs that it's plausible that evolutionary selection pressure could have changed the genetic makeup of the two populations over just 80 years, or we need something that rules out an alternate explanation for why the two populations now have different genetic makeup.