Edge angle is always a trade off between durability and sharpness. A wider angle protects deterioration of the edge against harder obstacles like bone. A thinner, razor like edge is sharper but tends to dull quicker. In a perfect world, I would have two knives set up, one for filleting and one for skinning. For one knife, if you don't plan on cutting bones, 15 degrees is a good point. If you like to slip the whole side off of a perch and then fillet out the ribs, go thicker, like 25 degrees.