Boning and fillet knives

They look similar on a shelf: both are narrow, both curve up to a point, and both are sized around six to nine inches. The difference is in the blade's thickness and flex. A boning knife has to lever around joints and through gristle, so it is stiff. A fillet knife has to follow a fish's ribcage, so it bends.

Using a boning knife on fish works, roughly, and wastes meat along the ribs. Using a fillet knife on a chicken thigh is a good way to bend or snap a thin blade.

If you only buy one and you fish, buy the fillet knife.

Electric fillet knives are a third category and they are worth considering if you clean numbers of panfish. They cut through ribs rather than around them and turn a bucket of crappie from an hour's work into fifteen minutes, at the cost of a little more waste.