In hybrid zones where reinforcement is occurring, you should see a decline in

gene flow between distinct gene pools

Explanation:

In ecological studies, a hybrid zone can be described as an area in which mating occurs between organisms of two closely related species. As a result, viable offspring are produced. When reinforcement occurs, we will see that a decline in gene flow between organisms of these species will be seen. The organisms which are only closely related will be able to reproduce to produce viable offsprings. In a stable hybrid zone, continued production of hybrids will take place.

for a different answer set the answer would be because the oxygen in the water had depleted (apex)