Optimal management of nursery ponds is the principal concern in an aquaculture operation because fry-to-fingerling represents the period of development where the highest level of mortality occurs. In well managed nurseries, survival rates can run as high as 90%. The rates depend upon food quality and availability, water culture and other ecological parameters, stocking rates, and the degree to which culture practices are tailored to the needs of the particular species.
Pre-stocking management is directed toward optimizing the pond environment to eliminate conditions that lead to reduced growth and survivability. It also focuses on establishing conditions where natural food is readily available.
Controlling Algae and Aquatic Weeds
Overgrowth of certain types of algae and other aquatic weeds is detrimental because they directly interfere with the availability of planktonic algae by consuming large amounts of available nutrients, interfering with free movement of young fry. They also create stress on the fry by creating large differentials in dissolved oxygen from daytime to nighttime, which is exacerbated by decomposition of the additional organics. Fouling of machinery like pumps and filters can quickly cause a big hit to survival rates.
Prevention should be the primary focus for controlling the growth of aquatic weeds and algae. Manual efforts include dewatering and desilting of established ponds, removing floating or rooted masses of established weeds, and trimming or extending pond margins.
Chemical control of unwanted plants is usually faster (although more expensive) than manual methods and, if treatment includes full decomposition of existing weeds, can ensure that nutrients are returned to the pond to support further food production. However, no single chemical treatment can guarantee 100% eradication of all weed types, and their use can introduce concerns about environmental effects. Any type of significant weed control is best handled in the pre-stocking stage, since the effects of weed decomposition can produce an environment harmful to fry.
Eradicating Unwanted Fish
Fish that prey on vulnerable fry and small juveniles are an obvious problem. Other fish can also harm your nursery population by directly competing for space, oxygen, and food. Manual removal of these types of fish isn’t practical, so the best solutions are thorough dewatering, in the manner of seasonal ponds, or the use of poisons when dewatering isn’t possible.
Many different types of fish toxicants are effective and available, but particularly if your goal is to produce a product suitable for human consumption, the selection is critical. Important criteria for selection include:
- Fish grown in a poisoned pond should still be suitable for human consumption
- Adverse effects on pond biota
- Duration of toxicity
- Residual effects
- Ease of application
- Cost and availability
Eradicating Predatory Insects
Aquatic insects that prey upon fry or compete with them for food should be eradicated whenever possible, however they present a different type of challenge since they cannot be entirely eliminated from the environment. A relatively easy treatment that can be repeated as needed, throughout the growing season, is to apply a thin film of oil or soap emulsion over the pond surface. This should not be heavy enough to cause harm to the fry or planktonic algae, but chokes the respiratory tubes of problematic insects.
Direct application of poisons is also an option, but care should be taken to avoid anything that can affect insects beyond the pond itself. Wholesale eradication of insects in any natural ecosystem will result in significant harm or even death, resulting in larger problems for the aquaculture operation in the form of fines and costly remediation efforts.
Fertilization
Ponds should be fertilized well before fry are introduced, to give water chemistry a chance to stabilize and for natural food organisms like planktonic algae to get established. Ideally, if a layer of sludge is allowed to remain at the bottom of the pond, fertilizer and minerals are absorbed upon application and then slowly released over time, maintaining an optimal level of nutrients to support the food supply.
The efficiency of biologic processes such as decomposition are strongly regulated by pH. If oxygen levels are low, anaerobic processes create an acidic environment, while fish ponds perform better in slightly alkaline conditions - typically around 6.6 - 7.5.
Liming is a critical step in fertilizing a nursery pond. In practice, pH values can cycle daily, so the addition of lime not only increases alkalinity but hardens the water and reduces variations. Lime also increases nutrient availability by helping organic matter in pond sediment mineralize faster.
Natural and chemical fertilizers are typically added periodically throughout the season, and should be preceded by an application of lime, as needed. Care should be taken in the choice of chemical fertilizers since those can trigger algae blooms which may harm young fry.
Assessment of Food
Very young fry feed voraciously on planktonic algae, so it’s important to ensure that plankton levels are high enough to support healthy growth in relation to the stocking density. As the fry grow, they’ll quickly be able to consume a wider variety of food, including supplemental food appropriate for the particular species.
