(1) According to the knowledge analysis of energy flow and interspecific relationship, in order to increase the yield of fish in ponds, fertilization can increase the number of phytoplankton by reducing the number of competitors and predators or increasing their food sources; The food and living space of wild miscellaneous fish are the same as those of fish A and fish B. Fishing wild miscellaneous fish can save living space for fish A and fish B. In order to stabilize the yield, the number of fish should be kept at K2.

(2) Because perch is in the next trophic level of fish A and B, and each trophic level consumes a part of energy, the output of perch cannot make up for the decrease of fish A and B. 。

(3) If the number of zooplankton in the pond is greatly reduced due to pollution, the number of fish B mainly feeding on zooplankton will decrease, but the number of fish A will not change much.

So the answer is:

(1) The competition of phytoplankton increases the living space of fish A and B and food a+b2.

(2) Reduce

(3) Reduce food sources.