Drawing foreshortened fish is challenging. Your brain knows the real proportions when viewed from the side. It is tough to force your brain to shorten one dimension and not the other. Trust what you see. Use your pencil to make proportional measurements rather than relying on your intuitive sense of how the fish “should” look.
Click on the first  image to start a step-by-step slide show.
Draw the posture line, or central axis of the fish.
Even though the pike’s body is long, block it in as a foreshortened oval.
Add a line for the base of the tail.
Use a set of parallel guides to block in the angles of the eyes and face.
Simplify the pike’s face to a simple box.
Observe the negative shapes around the tail.
With the negative shapes as a guide, draw in the shape of the tail, paying close attention to the angles of the tail and body.
Block in the locations of the posterior fins.
Draw two dots to indicate the origin of the pectoral and pelvic fins.
Draw lines from the dots indicating the length and angle of the fins.
There is space between the fins. Add another set of parallel guides (using the same angle you used on the face) as spacers for the fins.
Add another set of parallel guides from the bottom of the fins.
The final set of parallel guides shows you how long the far set of fins should be if the fins are symmetric.
Use the guide lines as a platform for drawing the details of the fish.
If you are going to add shading, light, details to the body keep in mind that the fish is a three dimensional shape. Visualize a set of ovals through the body.
Another way to visualize the structure of the fish and help you with shading is to think of the fish as a set of flat planes. Where would the shadows fall as you move the light source around?