That's a tough one to answer. Often I end up redrawing in my cad system using the converted image as a guide. There are some things you can try.
1. Scan in black and white, not grey scale.
2. Scale your scanned image up and then reduce it in your cad program. Some of the broken chains will be reduced enough to be OK.
3. After your image is scanned take a close look to see if you can fill in the blank areas before you convert to vector.
4. Try to draw with dark even width lines and avoid thick or filled in areas.
No matter what you do, you will most likely have some clean up work to do on a raster conversion. A simple circle may convert to hundreds of lines, arcs, or splines.
Your whale looks really good. I would redraw over top of the conversion in a different color and then delete the original. You could use mostly 3 point arcs and you may want to recreate arcs to make them tangent. With practice that could be redrawn in minutes to give you an acceptable cad file.
Do a Google search for raster to vector tutorial and you will find several more suggestions.
Good luck and have fun