I loved how the tale started but the twist after Ain and his mother gets left behind is just... meh. He's suddenly a (crown) prince? His Gramps' empire spanned a whole continent and has an egalitarian-cosmopolitan feel to it? His aunt (a high-percentage furry) just happen to know the particulars...
This would've been more tragic if I don't know that fishes can pretty much construct a whole food chain with themselves and phytoplankton...
And that a majority of sea mammals (mermaids has mammary glands) are piscivorous...
Not quite. Human excrement has been used as fertilizer for quite a long time (at least since 17th century pre-isolationism Japan). It does need further processing before it can be an optimal fertilizer, true.