Enclosures
Cages and Pens
Nursery ponds are usually built to be independent of either hatching or grow ponds. Some aquaculture operations set up cages or pens to take advantage of the natural movement of water in a river or the ocean, for example. Pens and cages designed for fry require a fine mesh that prevents tiny fish from escaping while still allowing water to flow freely through the enclosure. Usually they’re designed to allow expansion as the fry grow. Overall, either system involves a relatively low capital investment.
Cage and pen aquaculture makes fish easy to harvest once they’ve reached target size, and since the size and number of mesh cages are easily changed, expanding capacity (or reducing it) is a simple matter. However, since the enclosures are designed to integrate with natural water sources, the water quality is difficult to control and parasites or diseases are always a risk. When conditions are crowded, low dissolved oxygen in and around cages can cause significant stress, inhibit growth and reduce survival rates. Natural food sources may not be able to adequately supply a crowded pen, so complete nutritional supplementation is necessary.
Ponds
Man-made ponds are always more expensive to construct from scratch, compared to tossing out a few mesh enclosures, but nursery ponds offer significantly more control over your fry’s environment and health. A nursery pond exclusively devoted to rearing fry can be optimized precisely to support the highest survival rate and rapid growth. If you have separate species with the same requirements, you could combine the strategies and use a single nursery pond but separate them using net cages.
A well designed and managed nursery pond can rapidly mature fry to the fingerling stage in as little as three weeks. Open ponds (as opposed to closed-off or indoor ponds) require relatively little maintenance and allow the growth of natural food for robust fish.
Other Considerations
Size/volume/orientation
It may not be immediately apparent, but the shape and orientation of your nursery ponds matters. If you’re working with a rectangular pond, you may choose to orient it so the longest side is parallel to the prevailing wind. This encourages water movement from wind energy, and increases oxygenation. In warm months, it can also minimize temperature extremes.
If, on the other hand, you’re working in a windy climate and you’re worried about excessive wave action in your ponds, you may choose to orient them with the long side perpendicular to the prevailing wind. This limits the distance that wind can build up momentum in the pond, and will significantly reduce waves and splashes.
Since nursery ponds hold such tiny fish and can be maintained in high density populations, they rarely need to be more than a few percent of the total production area of an aquaculture operation. Even if you have the luxury of plenty of space, it’s much easier to make adjustments to water chemistry or deal with an invasive plant species when the pond is small.
Even though fry are tiny and you’re able to devote surprisingly little space for it, keep in mind that your fish will stay there until they’ve reached fingerling size. Consider the population of your pond with 3-4” fish, and make sure that the pond is large enough to accommodate them until you transfer them to lakes or growing ponds. Keep your stocking rate in line with your pond volume as well. Even if you’re growing carp which are remarkably resilient in the face of poor water quality, the fry stage is delicate and needs the best conditions you can supply. Avoid losses by keeping the water quality as high as possible, with backup equipment to deal with any sudden changes.
Stress-free environments
In urban aquaculture setups there may be fewer options, but outdoor nursery ponds are usually designed to make the fry life stress-free. Confusion induced by light reflecting from light-colored fiberglass and plastic tanks, for example, can stress the fry and reduce their feeding efficiency. Walleye and catfish are particularly vulnerable to this. Dark colored liners, used in outdoor ponds, or inside in tanks or other containers, can eliminate this issue. Sloped walls, a natural sludge layer, and other natural elements contribute to a relaxing environment.
Specialized Features
Even more specialized features may be available for the specific fish you’re raising. For example, some species of shrimp prefer to gather along tank walls, and you can improve your stocking density by building additional surface areas where the shrimp fry can congregate.
