Abstract: This paper describes the logistic growth equation of population and explains the mathematical nature of the equation. Based on the observation of the cohort biomass of file fish population, the equation of the cohort biomass of file fish population is thus obtained as follows: N_i =700000/(1+e^{-1.0012i+2.3614}). The difference value of the population biomass between time interval T₁ and T₂ is called the increment equilibrium yield in fisheries. The author deems that it is much suitable to estimate the equilibrium yield by using the equation of Y_E=N_i+1-N_i+C_i under the nonequilibrium condition. Finally, the paper makes practical calculation of the biomass of the file fish population.

Key words: Swamp wetland, Vegetable organs, N content dynamics