Fish can be cultured in one of four culture systems—ponds, raceways, recirculating systems or cages. A cage or net pen is a system that confines the fish or shellfish in a mesh enclosure. By strict definition, a cage and a net pen differ based on their construction.
A cage has a completely rigid frame (on all sides) and a net pen has a rigid frame only around the top. However, the terms “cage” and “net pen” are often used interchangeably. Marine cages are often called net pens, even though they have completely rigid frames, and vice versa.
Structural differences have little effect on production practices or the environmental impact of these systems. This fact sheet will use the term “cage” collectively. Cage culture uses existing water resources (ponds, rivers, estuaries, open ocean, etc.) but confines the fish inside some type of mesh enclosure.
The mesh retains the fish, making it easier to feed, observe and harvest them. The mesh also allows the water to pass freely between the fish and surrounding water resource, thus maintaining good water quality and removing wastes.
Cage culture probably originated with fishermen who used cages to accumulate fish for market. Over time, they learned to feed the fish in these cages to increase their size and improve their overall health.
The first cages used for culturing rather than just holding fish were probably developed in Southeast Asia about the end of the 18th century. These cages were constructed of wood or bamboo and the confined fish were fed trash fish and food scraps.
Modern cage culture in the U.S. began in the 1950s with the advent of synthetic materials suitable for cage construction. Universities in the U.S. began conducting cage culture research in the 1960s. Freshwater cage research in the U.S. has been limited because large-scale, open-pond culture was more economically viable and received most of the research focus.
There has been little research on marine cage systems because of regulatory issues, a limited number of good quality sites, and the high cost of research. Currently, however, there is renewed interest in marine cage or net pen culture in the U.S. Federal and state regulations are being reexamined in an effort to streamline permitting processes and define rules, oversight and liabilities. These issues have not been resolved at the time of this writing.
Therefore, this series of fact sheets will be devoted to freshwater cage culture and not address unresolved marine cage culture issues. Today, freshwater cage culture is practiced to a limited extent in the southern U.S., usually as an alternative type of agriculture for small and limited- resource farmers.
Freshwater cage culture also allows farmers to use existing water resources that may or may not be used for other purposes. The fish produced are usually sold to local niche markets.
As wild-capture fisheries have declined and aquaculture has expanded, these niche markets have also grown. The result may be opportunities to increase cage production.
Freshwater cage culture is not foolproof or simple. On the contrary, it is more intensive in many ways than pond culture and probably should be considered as an alternative commercial enterprise only where open-pond culture is not practical because of excessive depth, obstructions that prevent harvest, or predator problems.
Advantages of Cage Culture:
Resource use flexibility
Cage culture can be established in any suitable body of water, including lakes, ponds, mining pits, streams or rivers with proper water quality, access and legal authority. This flexibility makes it possible to exploit underused water resources to produce fish. (Specific state laws may restrict the use of public waters for private fish production. Contact your state Extension service or natural resource agency for regulatory information.)
Low initial investment:
Relative to the cost of pond construction and its associated infrastructure (electricity, roads, water wells, etc.), cage culture in an existing body of water can be inexpensive. At low densities (relative to pond surface acreage) cages often do not require aeration or any electrical source.
Cage materials are not especially expensive and many kinds of cages can be constructed with little experience.
Author:
Michael Masser
