What is the only registered piscicide for controlling fish populations mentioned in the text?

Rotenone is the only registered piscicide specifically mentioned for controlling fish populations. It is a natural compound derived from the roots of various tropical plants, primarily used for its effectiveness in eradicating unwanted fish species in aquatic environments. The use of rotenone is regulated due to its toxicity to fish and other aquatic organisms, making it a targeted tool for managing fish populations when necessary.

In contrast, the other substances listed are not designated primarily as piscicides. 2, 4-D and fluridone are herbicides used for controlling aquatic vegetation rather than fish, while endothall is also typically employed to manage aquatic plants rather than fish populations directly. This distinction emphasizes the specific nature of rotenone as an effective and registered option for piscicidal purposes in aquatic pest control.