Fore-section
The fore-section of the fish brain, also known as the "forebrain," is composed of two sections. The first section is the olfactory bulbs, which serve as smell detectors to the outside environment. The second section is the telencephalon, which receives signals from the olfactory bulbs. The telencephalon deciphers the signals and allows the fish to understand smells within the environment. The fore-section of the fish brain is also thought to have links to memory and learning.
Hind-section
The hind-section of the fish brain, also known as the hindbrain, tends to control more automated responses within the fish brain. It controls things such as swimming as well as the swim bladder. The hindbrain connects to the muscle function and the direction that the fish is going. Usually the hind-section of the brain is the largest part of the fish brain, but in some species---especially species that don't move a lot---it is smaller than other sections. Other processes that are thought to occur in the hindbrain include chemical detection.
Mid-section
The mid-section, also known as the midbrain, sits in the middle portion of the fish brain. Its main purpose circles around visual function. It processes information sent from the eyes and controls sight. Many species of fish are known to hunt primarily using sight; within these species, the midbrains are much larger due to an increase in the dependence upon this section of the brain. Functions that the mid-section of the brain is thought to control include sound direction detection, some learning processes and certain muscular control.