Nursery ponds can come in a variety of shapes and sizes, from excavated reservoirs, to concrete, tarpaulin, or rubber ponds, or even by simply enclosing fish via a mesh cage. An earthen reservoir is dug out from the ground itself, and is usually surrounded by earthen barriers. These typically provide open air access, improving algae growth and aeration, but can also increase risks of pests or flooding. This is similar for other man-made reservoirs, like compounded ponds or tanks. These methods are generally somewhat more expensive than pens or cages, but provides much closer control of your fishes' conditions and health.
Some aquaculture designs use mesh pens to separate a section of a larger, sometimes natural body of water. This is significantly cheaper than other nursery designs. This provides some protection and isolation for the small fish, and gaurauntees less competition for food. However, the makeup of the water and the conditions of the fry's environment is much harder to control and adjust if anything goes wrong.