  We study the combined effects of periodically varying carrying capacity and
survival rates on the fish population in the ocean (sea). We introduce the Getz
type delay differential equation model with a control parameter which describes
how fish are harvested. We will modify and extend harvesting model of an
exploited fish population to include periodic and rotational harvesting rates.
We study the existence of global solutions for the initial value problem,
extinction and persistence conditions, and the existence of periodic solutions.
