How to assess the quality of fish ponds is a key concern for most fish farmers. One of the primary indicators of water quality is its color. The color of the water is closely related to the amount of organic matter, the population of plankton, and their diversity. Here's a detailed analysis:
First, understanding the origin of water color is essential. After filling the pond, the color emerges due to dissolved substances, suspended particles, and especially phytoplankton. The type and quantity of these microorganisms are the main factors that determine the water’s appearance. Since different species of phytoplankton contain various pigments, the color of the pond can vary depending on which species dominate. The length of time a certain color persists also depends on the life cycle and generation turnover of the plankton.
Second, the relationship between fertilization and water color plays a significant role. Fish ponds require proper fertilization, particularly in major breeding areas. Initially, the water may appear yellowish-brown, gradually shifting to yellow-green, then to a soft green, and finally to a blue-green hue. This change reflects the dominance of different types of algae: diatoms and green algae cause yellow-green coloration, while flagellates lead to green water. Eventually, when blue-green algae take over, the water turns blue-green.
With manure-based fertilization, the water color changes from dark brown to yellowish-brown, then to tea brown, and finally to reddish-brown. This occurs because golden algae and diatoms dominate initially, creating a yellow-brown tone. As these decline, cryptophytes, diatoms, and dinoflagellates become more prevalent, resulting in a dark brown color. Later, when diatoms drop significantly, the water turns red-brown.
Different animal manures also influence the color: cow manure leads to a light brown hue, pig manure produces a reddish tint, chicken manure results in a yellow-green color, and duck manure tends to make the water dark green.
Third, identifying beneficial water colors is crucial for successful fish farming. Two ideal colors are green (such as yellow-green, brown-green, or oil-green) and brown (like yellow-brown, red-brown, or green-brown). These indicate a high concentration of plankton, which is easily digestible for fish. However, if the water appears pale green, dark green, or bluish-gray, it may only show the presence of phytoplankton but not their quality. Such water is often considered "thin" and less suitable for fish culture. If the water turns black, brown, or green with a foul smell, it's a sign of poor water quality—old or deteriorated water can lead to fish deaths.
The occurrence of algal blooms has both positive and negative aspects. On one hand, it indicates good fertility and a diverse plankton population, which supports fish growth. On the other hand, it is hard to maintain for long periods, and inexperienced farmers may struggle to manage it properly. When weather conditions change, algae can die off rapidly due to oxygen depletion, leading to a sudden deterioration in water quality, turning it black and smelly, which can result in mass fish mortality.
Lastly, understanding the patterns of water color changes helps in managing pond quality. The goal of observing water color is to determine whether it is healthy or not. For optimal fish pond conditions, the water should be well-fertilized, lively, and cool, with fertilization being the key factor. However, being "fatty" doesn't always mean good quality. Many ponds have non-digestible algae, indicating old water. Visually, such water remains unchanged for days. In contrast, well-fertilized and rich water shows daily changes: it appears lighter in the morning and thicker in the afternoon, reflecting the phototactic behavior of algae. Additionally, wind direction affects the color, with the upper wind area appearing lighter and the lower wind area darker. This pattern often leads to algal blooms, which signal high-quality water, while thin or old water lacks this dynamic change.
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