Why during spear fishing hunter aims the tail of fish rather then head of fish?

The answer frank gave is probably fairly close. the visual phenomenon he's referring to is "refraction." I wouldnt normally bother with correcting someone, but you did ask for scientific so I thought it best.

Also, ever played space invaders? Well, if you shoot at where the last ship is, you'll miss. You have to shoot at where its going to be.