To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project' s completion, only 300 per day made the journey. Clearly, the fish pass is defective. Which of the following, if true, most seriously weakens the argument? ['On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.', 'During spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream.', 'Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.', 'The construction of the dam stirred up potentially toxic river sediments that were carried downstream.']