Visual Inspections
On a daily basis, basic visual checks should be conducted on the nursery ponds. Early detection of emerging problems allows you to respond quickly and minimize injury or death. If a visual inspection reveals concerns about fish health, start with a water change while you determine the cause of the problem. Nothing kills young fish faster than dirty or contaminated water and any delay may result in significant fish loss.
- Evidence of unusual mortality is an obvious distress signal and should trigger a thorough investigation.
- Color and transparency of pond water can also alert managers to potential problems.
- Ideally, water should be greenish-brown in color and transparency should be in the range of 8 - 20 inches as measured by a Secchi disc. Readings outside these ranges, coupled with color changes are signs that justify further investigation.
- Reduced water depth can indicate leaks that require immediate response.
- In outdoor ponds, if there’s been recent heavy rainfall and the depth has increased appreciably, it’s a good idea to do a water quality test using a field kit, paying particular attention to changes in pH and dissolved oxygen. Typically, a pH range of 7.0 to 8.5 and oxygen levels of 6 to 9 ppm indicate optimal conditions.
- Pre-dawn oxygen levels are an important measurement of the balance of the whole pond ecosystem. If oxygen levels are very low prior to sunrise, it indicates significant risk of fish kill due to oxygen depletion.
- The presence of aquatic weeds or an excess of accumulated waste at the bottom of the pond can result in wide ranges of dissolved oxygen. If removal is not an adequate response, a water change is probably called for.
Regular Maintenance
In addition to daily inspections, certain types of maintenance activities should be scheduled. For example, in order to maintain optimal quality, water changes should be performed regularly. The frequency depends on your stocking density, the type of feed you’re providing, and the species of fish you’re raising.
Water Changes are a MUST!
- High stocking density requires more frequent changes since the pond will get dirty faster from higher levels of fish waste and leftover food. Generally every two weeks is the maximum amount of time you should go between changes; weekly is a better schedule in most cases.
- If the dissolved oxygen level in the pond begins to fall, a water change should be performed immediately.
- Water changes are essential to the health of your pond ecosystem and to the health and growth of your fish. Fish waste, uneaten food and other organics quickly begin to break down, releasing nitrates, nitrites, ammonia and other toxins. These natural byproducts of decomposition are unavoidable, but regular water changes allow you to remove a sizable percentage of those compounds, keeping the overall levels as low as possible.
- Even if you have world-class filtration installed in your aquaponics facility, water changes are still a must. The filtration system is only designed to keep water in good shape between water changes. Still, high quality filtration is a must. Without it, you would likely need to do 10% water changes daily instead of weekly.
- Scheduled water changes are not optional, and not secondary to your workload elsewhere. The nursery pond hosts your stock at its most vulnerable stage, and a significant die off here will be felt all the way to harvest.
- Aim to change 10% to 15% of your total water volume every week; larger ponds or those with a lower bio load may be able to tolerate a smaller change, but always plan on performing the changes at least weekly.
- There is one possible situation where you won’t actually need to do water changes, so much as water replacement. If you’re working with a shallow pond with a large surface area, you may find that you’re losing up to 10% of your pond’s volume each week simply due to evaporation. Compare the volume you’ve lost to the required volume for a water change - make sure you’re adding the correct amount of fresh water as indicated, and you’re good to go.
How to Change Water
- Start by preparing the correct amount of fresh water for your pond. Store it in a temporary pool or container, add water conditioners to dissipate chlorine and chloramines (which don’t evaporate on their own), and let it stand for 12-24 hours.
- Once your fresh water is fully prepared, use a pump or siphon to remove an equivalent amount of water. Make sure you don’t accidentally draw up any of your fry - you can use a filter or fine mesh cover for your uptake hose instead.
- Gently add (or siphon) the fresh water into the nursery pond - it should be added immediately after you complete the siphon-out step.
A longer term solution to the chore of water changes is to set up a permanent trickle system that constantly brings in fresh water and allows an equivalent amount to cycle out. This is an excellent choice for large ponds, which can be pretty labor intensive for manual water changes, but it does represent a significant capital outlay. Measure that against never having to devote time and energy to weekly water changes, and you may conclude it’s worth the investment.
Conclusion
In any kind of aquaculture operation, whether you’re producing catfish fingerlings and juveniles to stock private fishing ponds, or preparing to jump into a new organic tilapia market for local restaurants, the nursery pond is the backbone of your operation. It deserves the highest level of care, from design and construction to seasonal and daily management. Scrimping on your nursery pond could spell disaster for the whole operation, so it’s worth doing your research so you can select the highest quality materials, from supplemental foods to quality pumps to durable liners.
