Nursery ponds are used for many different types of fish who need an open environment with a natural food supply and high water quality. While these ponds aren’t ideal for actually hatching the eggs, they are easily set up for handling fry from the earliest stages of life. During this growth stage, the fish must inflate their swim bladder, learn to eat a wide variety of nutritious foods, and avoid being eaten by any siblings growing slightly faster. Each species of fish has its own requirements for water quality, and staying within the parameters recommended for the fish will result in faster and more profitable growth.
Koi at all life stages prefer water with a pH balance between 7 and 8. This is a higher pH than is required by many edible fish, and maintaining it can take a lot of adjustment. Aim for a settled pH with plenty of buffering compounds in the water before any fry are added since the swings from adjustment can cause slow growth or mass die-offs. They need as little ammonia and nitrites in the water as possible. 6 parts per million (PPM) of dissolved oxygen is the minimum amount necessary for koi fry growth, with 7-8 PPM of DO recommended when possible.
With some large goldfish species offering nearly as much profit as koi in a hardier package, these ornamental fish are easy to raise in outdoor nursery ponds. A pH balance anywhere from 6.0 to 8.0 will work well for goldfish fry, with a higher number offering more protection against illness and stress for the fish. Safe water temperatures range from 60 degrees Fahrenheit to 75 degrees F. Ranging goldfish fry from the hatching stage onward requires a pond with very low turbulence and a large bottom surface for feeding. Unlike koi, goldfish prefer to wait until their food sinks to the bottom to eat, especially when in the early stages in growth.
As an edible fish, tilapia must only be treated with chemicals and medications explicitly listed as safe for food use. Tilapia prefer a highly alkaline environment with a pH balance as close to 8 as possible. This is one of the biggest reasons to choose a pond environment rather than an open cage on an existing body of water. The fish can handle lower levels of dissolved oxygen than other species, with just 3 to 4 PPM required at all times for growth. They also want higher temperatures than ornamental fish with a preferred range between 72 degrees F and 85 degrees F. This means that most hatcheries can only produce one or two tilapia batches in the height of summer when temperatures are right or must rely on costly heaters for rapid growth.
On the opposite end of the water temperature spectrum, rainbow and brown trout require relatively cold water temperatures and a strong flow with a specific amount of current. Trout in the wild lay eggs in gravel beds at the base of small waterfalls and minor rapids, so it’s not surprising their fry need turbulence when raised in contained areas like ponds. Spray bars and vortex generators are used to keep the fry moving and to add oxygen to the water. The fastest growth happens in water between 48 and 58 degrees F, making it easiest to raise trout outdoors in the spring and fall. Trout also need a high amount of dissolved oxygen, with a minimum of 7 PPM and a recommended rate of 9 PPM for rapid growth. No matter the water quality, trout can take 2 months or longer to reach the fingerling stage.
Bass are quite tolerant of wide temperature ranges and water quality parameters once they’re mature, but the fry are still somewhat susceptible to fungal infections and damage under certain conditions. Unlike other fish, bass grow faster with a limited amount of dissolved oxygen that remains under 4 PPM. 3 to 3.5 PPM is an ideal DO rate, but only a small amount of fry will be lost even if it drops as low as 2 PPM. Adult fish have a surprising resistance to damage from ammonia or nitrates, but fry prefer ammonia levels below .8 PPM and less than 100 PPM of nitrites.
Despite their reputation as extremely tough fish, channel catfish require a tight range of water quality parameters during the fry stage. Optimal growth happens in water temperatures between 78 and 82 degrees F, while dissolved oxygen must stay above 5 PPM. They can handle a wider range of pH measurements than some other edible fish, with fry thriving anywhere from 7.0 to 8.5. Highly sensitive to ammonia, catfish fry require levels of dissolved waste to stay below .05 PPM at all times. Nitrite levels should also remain as low as possible, with adult fish only handling a maximum exposure of 25 PPM before 50% of the population dies off. Fry losses can begin with nitrite levels as low as 10 PPM.
Other Common Species
Walleye, grass carp, bluegill, sunear, and many imported tropical fish can all be raised in nursery ponds as well. Each of these fish has their own requirements, but they tend to all fall between 6.0 and 9.0 on the pH balance scale, desire water temperatures between 60 and 75 degrees F, and need high levels of dissolved oxygen. If you plan to combine multiple species of fry to take advantage of different levels of the pond, make sure they share common water quality requirements so there’s no one species that grows much slower than the rest.
If you’re unsure about the specific water quality requirements for your preferred fish species, contact the existing hatcheries in the industry or the supplier for your eggs or fry. Most of these fish require slightly different water quality measurements for the hatching process, so make sure you understand the variations between tanks and outdoor nursery ponds before setting up a single-source water system.