osprey
鶚 refers specifically to the osprey, a fish-eating bird of prey.
鶚 combines 鳥 (bird) with 咢, which likely contributes the sound. The character was created to name the osprey.
The bird radical 鳥 on the right tells you it's a bird. The left part 咢 looks like a beak and sharp eye, fitting a fish-hunting osprey.
For ガク, imagine an osprey perched on a gak (invented sound like 'gawk') branch, staring at the water: gak -> ガク.
osprey (Pandion haliaetus); fish hawk