@Lilliwyt
If you don't mind tinfoil hatting in your symbolism, here's a bit of possible symbolism:
They're all ocean fish, and as she goes along the fish are from deeper and deeper depths. Deep & dark waters tend to symbolize loneliness and isolation, so it *could* represent her feelings of such, which isn't too much of a stretch when you consider how young she was and how far Fuyu moved away. For a child, someone moving even a town or two over night as well be a different planet of they don't have contact info.
Or the artist might have just wanted to draw their latest acid trip; both are possible.