Consider the following statements and choose the correct answer:
1. The waters of estuaries are among most naturally fertile in the world
2. The high productivity in estuaries is brought about by the action of tides, which promote a rapid circulation of nutrients and aid in the removal of waste products
3. "Mariculture" is done in the oyster farms of Japan where oysters are grown suspended on rafts hanging from floats
4. Estuaries and marshes are among the ecologic regions of the world that are most seriously threatened by human activities