A Complete Analysis of Pond Aquaculture Water Quality Requirements and Management Measures

With the development of high-density, intensive aquaculture production, the frequency of water quality changes in aquaculture water bodies has accelerated, causing eutrophication of pond water quality, and coupled with the increase in pond water pollution sources today, water quality management is more important than ever. Water quality is closely related to aquaculture yield, diseases and economic benefits. Water quality management should adopt comprehensive management to make water quality conditions meet the requirements of aquaculture species.

1. Pond Water Quality Requirements

To raise a pond of fish well, you need to raise a pond of water well. Fish farming water quality requirements are summarized in four words: "fertile, lively, refreshing, tender."

1.1 Fertile

The concept of "fertile water" is mostly still based on the total amount of plankton or on the quantity of digestible phytoplankton. "Fertile" mostly refers to dense water color and high plankton biomass. When there is little plankton in the water, the water color is light and transparency is high; when there is much plankton, the water color is dense and transparency is low, so in production, water transparency is still used to indicate the fertility of the water. When measuring, in order to avoid interference from non-biological suspended matter in the water, one should stand on the pond embankment at the upwind end. Generally, the transparency of fertile pond water should be about 20-30cm. This is equivalent to a total phytoplankton content in the water of more than 20mg/L.

1.2 Lively

"Lively water" refers to water color and transparency that constantly change; liveliness is caused by changes in aquatic organisms. According to observations, the most typical lively water is the change in water color caused by massive reproduction of Uroglena. Uroglena belongs to the phylum Xanthophyta and is a flagellate algae; it swims fast and has pronounced phototaxis. During the day, it often produces vertical and horizontal movements with changes in light intensity. At dawn, it is evenly distributed in the upper and lower water layers; after sunrise, it gradually concentrates toward the surface layer; around noon, most of it concentrates in the surface layer; afterward, it gradually sinks and disperses. The transparency at 9 o'clock and 13 o'clock can differ by 7cm. When this algae gathers on one side or in one corner of a fish pond, the phenomenon of half the pond red and half the pond green appears. We often see in ponds water masses with varying shades of color, like patches of dark clouds, which are formed by massive reproduction of flagellates. All flagellate algae have similar phenomena; generally, the quantity in the surface layer in the afternoon is greater than in the morning.

The biological meaning of water quality being "lively" is that plankton reproduce quickly, energy circulation in the pond is fast, and every link in the entire food chain operates normally; it also means the pond is in a virtuous cycle. This kind of water is good water.

1.3 Refreshing

"Refreshing" means the water quality is clear, the water color is not too dense, and transparency is not less than 25cm. The reasons for excessively low transparency may be extremely high plankton, or cyanobacteria dominance (cyanobacteria generally concentrate in the surface layer), or excessive mud, sand and other suspended matter. Excessive suspended matter and algae that fish cannot use in the water are both factors affecting filter feeding. In the current aquaculture model, because fish filter-feed in large amounts, plankton is not easily maintained at very high concentrations; excessively high biomass is often due to natural feed not being fully utilized and poor material circulation in the water. In well-growing ponds, phytoplankton biomass is generally within 100 mg/L; exceeding 100 mg/L is roughly the dividing line between "fertile water" and "old water" in flagellate ponds. However, in cyanobacteria ponds, the phytoplankton biomass of "fertile water" often exceeds 200mg/L.

1.4 Tender

"Tender" means the water quality is required to be fertile but not old, meaning the algae cells in the water have not aged and there are not many cyanobacteria. There are mainly two kinds of so-called "old" water: one is water color turning yellow or brown, and the other is water color turning white. Water color turning yellow or brown is a manifestation of aging algae cells; the composition of algae in the water has not changed, but lutein and carotene within the cells increase. In appearance, the water color is yellow or brown. Fish farmers call this water old tea water (yellow-brown) and yellow wax water (withered yellow with green). Water color faintly turning white is mainly a manifestation of the growth of cyanobacteria, especially small cyanobacteria such as powdery Microcystis and Coelosphaerium. The pH value of this water is very high, often above 9-10, and transparency is very low, usually below 25-20cm. During the day, as the intensity of turbid white increases, alkalinity rapidly decreases. It can thus be seen that water color turning white is the result of carbon dioxide deficiency causing bicarbonates to form carbonate crystals. At the same time, the rise in pH promotes the growth of cyanobacteria.

2 Measures for Regulating Water Quality

2.1 Remove silt and improve the bottom substrate

Excessive silt easily deteriorates water quality and induces fish diseases. For healthy aquatic product aquaculture, silt should be kept at about 5cm, and at the same time the pond should be exposed to the sun until the pond bottom cracks or the pond bottom frozen for several days, and 100-150kg of quicklime should be applied per 667m2.

2.2 Reasonable stocking density

The quantity of fingerlings stocked should be determined according to pond conditions, species and size of fingerlings, feed supply, and feeding management techniques.

2.3 Control the amount of bait fed

According to the total annual bait amount and the size of the fish, determine the monthly bait amount; according to season, weather, water color, fish activity and feeding conditions, determine the daily bait amount. Generally, it is enough to let the fish eat until they are 80% full.

2.4 Scientific use of fishery drugs

For aquaculture ponds producing healthy aquatic products, drug use should strictly follow the relevant regulations for pollution-free aquatic products, using fishery drugs with the "three effects" (high efficiency, quick effect, long-lasting effect) and the "three smalls" (small toxicity, small side effects, small dosage). Accurately calculate the dosage and apply it reasonably. Prevent abuse of fishery drugs, blindly increasing dosage or increasing the number of medication times, or prolonging medication time.

2.5 Regularly add and change new water

Water should be injected and changed at appropriate times to improve pond water quality, keep water quality fresh, and supplement dissolved oxygen in the water body. Water can be changed once every 15d. From June to September, the peak growth season for fish, water should be changed at least once a week. Each water change should be 30cm; pond water should first be drained before adding new water.

2.6 Adopt microbial methods

Cloud fermentation EM probiotics, nitrifying bacteria, Thiobacillus, lactic acid bacteria, yeast, Gram-positive actinomycetes, etc. can be used to improve the water body by biological methods, so that the water quality reaches the standard of "fertile, lively, tender, refreshing," improves the immune function of fish, and prevents the occurrence of bacterial fish diseases.

2.7 Turn on the aerator at appropriate times

Turn on the aerator at appropriate times to increase dissolved oxygen in the water body and maintain an excellent environment in the pond water body. The general rules for turning on the aerator are: turn it on at noon on sunny days (11 o'clock to 14 o'clock), turn it on in the morning on cloudy days, turn it on at midnight during continuous light rain, and do not turn it on in the evening; turn it on early when fish surface for air, and turn it on every day during the season when fish surface for air; turn it on promptly after partial harvesting, and turn it on more when wind is weak. Operating times are: when turned on at midnight, the time is long; when turned on at noon, the time is short; after fertilizing, in stuffy and hot weather, and in ponds with large area and large fish carrying capacity, the operating time should be long; without fertilizing, in cool weather, and in ponds with small area and small fish carrying capacity, the operating time should be short.

2.8 Regulate the pH value of pond water

The suitable pH range for freshwater aquaculture water bodies is 6.5-8.5, and the optimal range is 7.2-8.0. During the aquaculture period, when the pH of the pond water body is less than 7.0, an appropriate amount of quicklime can be applied according to the situation; if the pH is greater than 8.5, an appropriate amount of gypsum or acid (acetic acid, hydrochloric acid) can be applied.