Behavioral analysis of a nonlinear three-staged population model with age-size-structure

We present and analyze a nonlinear egg-juvenile-adult model, in which eggs and juveniles are structured by age, while adults by size. The model consists of three first-order partial differential equations with initial and boundary conditions. Existence and uniqueness of nonnegative solutions to the state system are investigated by the method of characteristics and a comparison principle. Extinction or persistence conditions of the population are obtained via the upper-lower solution technique. Our investigation extends some results in the literature.