Larval fish culture is one of the riskiest phases of freshwater fish culture, but it can be one of the most profitable. Special planning is required to overcome the risk of high mortality during fry culture.
Producers must have a dependable larvae supply, a facility appropriate for fry and fingerlings, the right size fry, the right kinds and quantity of food, and fry weaned from natural to prepared foods. They must also take special care in handling fish and preparing the pond.
This fact sheet concentrates on the relationship between fry size and the types and sizes of zooplankton found in culture ponds. Zooplankton is required as a first food for many cultured fish; for others it contributes to faster growth and higher survival.
Pond size:
Ponds for fry culture and small fingerling production should be smaller than grow-out ponds. Ponds from 0.1 to 3 acres are ideal because they are easier to harvest and will produce more natural food per unit area.
There is a higher ratio of pond bottom area to water volume in small ponds than in large ponds, which increases the availability of fertilizing nutrients and resting zooplankton eggs. Increased shoreline to water volume increases the number of small insects being blown into the pond, and they may be a significant source of food for fingerlings.
However, ponds with lots of shoreline may have more problems with predaceous wading birds. Using many small ponds rather than a few large ponds may ensure that at least some fingerlings get to market. Smaller ponds allow the farmer to more easily control the size of the fish by manipulating nutrient (either fertilizer or feed) input.
Small ponds also allow the farmer to more easily determine fish size and estimate survival rates because it is easier to locate the fish. With many small ponds instead of a few large ones, farmers can grow fingerlings of different sizes for various markets.
Spreading different sizes of fingerlings among different ponds also helps minimize cannibalism. Farmers can rotate the harvest among many small ponds rather than harvesting the same pond over and over; this reduces stress.
Of course, the benefits of smaller ponds must be balanced against the increased costs and decreased pond area per acre of land that result when small ponds are used.
Fry size:
Tiny fry eat only tiny prey, but tiny fry are preyed upon by many creatures bigger than they are. It is important to know the size of the fry you are stocking and to make sure that the pond you are putting them into contains plankton of the size that will be their prey and is also void of creatures that will prey on the fry.
The total length of cultured fish fry (Table 1) when they hatch varies from 2 mm for sunshine and white bass to more than 15 mm for muskellunge. In most cases, fry are a few millimeters longer than the values in Table 1 when they are stocked into ponds.
Suggested stocking times in the table are based upon the size of the fry and the sizes and types of zooplankton that show up at different times in ponds that have been filled with well water and fertilized. It is safer to stock earlier than the time listed than to stock later. Stocking later increases the chance that predaceous zooplankton or insects will be present.
Plankton types and sizes:
Pond plankton is composed of tiny plants called phytoplankton and animals called zooplankton, as well as organisms that are not easily classified into those two groups (such as protozoans and bacteria).
Planktonic organisms are suspended in the water and are so small that even slight currents move them about. Fish fry eat zooplankton, phytoplankton, and tiny plants and animals attached to objects on the pond bottom.
Most fish fry eat three main types of zooplankton—rotifers, copepods and cladocerans. For the tiniest fish fry, such as the newly hatched fry of sunshine bass or white bass, small rotifers may be the only zooplankton small enough to eat.
For larger fry, the smallest rotifers may not provide enough nutrients to make chasing and ingesting them worth the effort. Copepod nauplii, which are just-hatched copepods, are important first foods for larval fish, too.
Protozoans may also be eaten, but little is known about their contribution to fry diets. In general, the smallest of the main zooplankton groups are rotifers (Fig. 1). Body lengths of rotifer species vary from 0.04 to 2.5 mm.
This diverse group of animals obtained their name from their “wheel organ,” a ring of cilia that appears to rotate around the mouth. They are often the earliest visible zooplankton to appear in ponds, hatching almost immediately after ponds are filled.
Rotifers reach maturity 2 to 8 days after hatching and some species can increase in number very rapidly. Rotifers that show up later in ponds, when larger zooplankton are present, are usually much larger than the first that appeared. The first rotifers hatch from “resting eggs” that survived on the pond bottom during inclement weather or while the pond bottom was dry.
Most of them hatch into females that reproduce asexually until pond conditions become harsh. Then sexual reproduction occurs and resting eggs are again produced. Copepod nauplii are the next largest zooplankton to appear after ponds are filled and fertilized (Fig. 1).
They hatch from resting eggs that were dormant on the pond bottom. Resting eggs are produced in the fall by sexual reproduction. At other times reproduction may be sexual or asexual. After hatching, the young copepods grow by shedding their exoskeletons up to 12 times in stages called instars.
They reach maturity about 18 days after ponds are filled. As they grow they provide larger and larger food for larval fish. The largest freshwater copepods may reach 2 to 3 mm. Only large fry, such as those of channel catfish, have mouths big enough to eat adult copepods initially.
Although copepods may be prey for larger fish fry, sometimes the roles are reversed. Introducing small fish fry into a pond full of large copepods can be disastrous. One group of copepods, the cyclopoids, is predaceous. They feed on smaller zooplankton and even on fish fry. When cyclopoid copepods are prevalent, they may eat all the fish fry stocked.
Cladocerans, often called water fleas because of their shape and “hop-sink” type of locomotion, are the third major group of zooplankton found in freshwater ponds (Fig. 1). Larger fry and even adults of some fish species often selectively prey on these crustaceans.
Cladocerans 2 to 3 mm long are commonly found in culture ponds several weeks after the ponds are filled. Often only female cladocerans are found, except in early spring and late fall. Like some other zooplankton groups, cladocerans hatch from resting eggs when ponds are filled. Later, eggs are held in the females before hatching.
Cladocerans compete with rotifers and calanoid copepods for phytoplankton.
Author:
Gerald M. Ludwig
