What does the term "anadromous" refer to in fish behavior?

The term "anadromous" describes a specific migratory behavior in certain fish species, particularly those that spawn in freshwater rivers after living in the sea. This life cycle involves fish that begin their life in freshwater, migrate to the ocean for feeding and growth, and then return to freshwater environments to reproduce.

By choosing the option that defines anadromous fish as those that spawn in freshwater after living in the sea, it clearly aligns with the biological characteristics and behaviors observed in well-known anadromous species, such as salmon. These fish make significant migrations from the ocean back to their natal freshwater streams to ensure the continuation of their species, highlighting their unique reproductive strategies and adaptability to different aquatic environments.

The other options describe different migratory or lifestyle patterns that do not accurately represent the anadromous behavior specifically. For instance, the first option implies a reverse movement from rivers to the sea, which is not characteristic of anadromous fish, as they generally move in the opposite direction for spawning. The last two options involve either exclusive habitats or simultaneous living conditions that do not capture the essence of the anadromous life cycle.