Why do salmon look different when the're babies and grown ups?

                                                                   

Before we went on winter break our salmon had yolk sacks. They were small and their yolk sacks were big. They looked like about an inch long.
When we got back the salmon grew a lot! They have a brownish color, and they have little black spots on them. Some fish can swim at the bottom. One fish can go all the way up to the surface! Some fish can go half  way up. The salmon are about 13 centimeters. Mrs Miller said that when more salmon lose their yolk sacks, we will start feeding them. We call that buttoned up, which is when they absorb their yolk sack.

This is what they look like now: