Chapter 9: Extensive Production: Pond Management
Successful pond management begins with good pond preparation and obtaining good quality larvae for stocking. Pond preparation may begin before even eggs are available as it can take 10 to 20 days or more to have pond conditions suitable to receive fish larvae. In the meantime, broodfish must be spawned, eggs incubated and hatched, and the larvae held until at least mouthparts are formed before they are stocked into nursery ponds. Each hatchery should develop a general schedule as to when larvae of a given species should be available, and in anticipation of those dates pond preparation procedures should begin. Management schedules need to be adjusted according to the time required to obtain adequate zooplankton blooms. For fish larvae that rotifers are the desired first food, target densities of rotifers should be >500/L in an expanding population of predominately egg bearing females. Where cladocerans are the target food organisms, an initial target density should be >200/L in an expanding population with no females holding resting eggs.
Stocking rates for primary nursery ponds range from 100,000 to 5,000,000 or more fish/ha depending on the species, pond fertility, and the length of the culture period. The higher stocking rates would be for first feeding fish larvae of species that need small organisms such as rotifers as a first feed. The lower stocking rates would be used for fish larvae that accept cladocerans as a first food or larvae that had been held and fed in the hatchery until they were large enough to consume cladocerans or larger food items that are found in the nursery pond. Keep in mind that growth rate in a primary nursery pond is a function of the availability of an appropriate size zooplankton and the quantity of zooplankton is finite. Lower fish stocking rates mean more zooplankton/ individual fish and in turn a faster growth rate/fish. A higher stocking rate means less zooplankton/fish, a slower growth rate, and if the stocking rate is too high, will result in reduced survival. In primary nursery ponds the goal is to produce as many individual fish as possible that are large enough to be safely harvested and are acceptable in the market. It is best to stock individual lots of fish into separate nursery ponds. This allows tracking specific genetic groups, their performance, and disease susceptibility. If there is not enough fish to stock a nursery pond at the desired density on a given day, it is best to stock what is available if you are close to the target density. As the fish will grow faster and reach the target size sooner you should be prepared to harvest the pond sooner than planned. Stocking fry into the same pond in separate lots on different days is not recommended as it results in greater size variation at harvest, possible cannibalism, and creates more management problems. Each lot of fry held in the hatchery should be monitored as to mortality rates and percent deformities. Marginal quality fry in the hatchery should be discarded rather than be stocked into a nursery pond where their chance of survival is low.