(1) Fish pond food chain
When external water is introduced into each fish pond, some inorganic nutrients, organic debris, phytoplankton, zooplankton, bacteria and other aquatic organisms are also brought in. The nutrients on the soil surface of the fish pond are also dissolved and released into the water. The introduced aquatic organisms become "seeds" and form two food chains in the fish pond together with the stocked fish; Autotrophic food chain and heterotrophic food chain.
Autotrophic food chain:
The introduced phytoplankton, autotrophic bacteria and some aquatic plants use inorganic nutrients in water and sunlight for photosynthesis. On the premise that inorganic salts are satisfied, they will continue to grow and reproduce and become producers of organic matter, thus forming the natural primary productivity of fish ponds. Zooplankton, as a first-class consumer, feeds on phytoplankton, autotrophic bacteria and some organic debris. Other aquatic animals and fish choose to eat the above phytoplankton, zooplankton, bacteria and organic debris, as well as aquatic plants according to their different feeding habits. This is an "autotrophic food chain" starting from phytoplankton and autotrophic bacteria.
Heterotrophic food chain:
Start with heterotrophic bacteria. With the introduction of water into the fish pond, the organic debris, the remains of aquatic organisms and some living organisms that have lost their resistance to bacteria provide nutrient substrates for heterotrophic bacteria in the fish pond. Heterotrophic bacteria decompose these organic substances into inorganic substances, so heterotrophic bacteria are called reducing agents in the material cycle. In the process of decomposing organic matter, heterotrophic bacteria gain material energy, grow and reproduce, and form the productivity of heterotrophic bacteria itself. As long as the above nutrient substrates can be met, the productivity of heterotrophic bacteria will continue to form. After bacteria multiply in large numbers, due to their special physiological functions, bacteria form many bacterial micelles (also known as aggregates), and bacteria and their bacterial micelles become food for lower aquatic animals such as zooplankton, and fish feed on these aquatic animals. Because some bacterial micelles gather a large number of bacteria, and their shapes increase, some larger aquatic animals, including fish, can also eat. This forms a so-called "heterotrophic food chain" starting from heterotrophic bacteria.
In a fish pond, these two food chains do not exist in isolation, but organically cross and depend on each other to form a network, which is called a food chain network, as shown in figure 12.
For example, inorganic salts produced by heterotrophic bacteria decomposing organic matter provide a nutritional basis for autotrophic food chain. Heterotrophic bacteria can also decompose biological corpses in the autotrophic food chain.
These two food chains are interdependent. In this food web, farmed fish as consumers participate in two food chains, both of which are in the last link. They need to eat a large number of producers or low-level consumers in the previous link before they become fish pond products. Therefore, in order to obtain fish products, especially filter-feeding omnivorous fish, it is necessary to cultivate two producers and low-level consumers at the front of the food chain, and make them grow and reproduce continuously to obtain a certain yield. That is, as the fishing proverb in China says, "If you raise a pool of fish, you should raise a pool of water first." In order to make the producers and consumers in the previous links grow and reproduce in large numbers, we must first meet the basic nutritional needs of the first link of the two food chains, namely inorganic nutrients and organic debris. Under normal circumstances, these two nutritional bases are far from being satisfied simply by introducing water and releasing the excreta and excreta of soil and aquatic organisms (mainly fish) in fish ponds. This will inevitably limit the growth and reproduction of each food organism in the two food chains. Those filter-feeding and omnivorous fish whose main food is plankton, bacterial micelle and organic debris are bound to starve to death. We tried to introduce nutrient-rich Wuli Lake in Wuxi into fish ponds and raise omnivorous fish without adding any fertilizer. Although there was no fish disease, the survival rate was only 82.5% and the net yield was 94.5 kg/ha. However, the survival rate of fish ponds using livestock manure is 92% ~ 98%, and the net yield is 1660.5 ~ 2564.5 kg/ha.
Figure 12 natural food chain network in fish ponds