They Sound Less Magical That Way

Am I the only one to notice that mermaids aren’t fish at all?  They’re half whale

Mermaids aren't half anything.  They're entirely fictitious.  

Tell that to Mermaid Melissa.  She will likely beg to differ.  

Anyway, what's important here...

For certain definitions of "Important"...

...is that common lore about mermaids is entirely bogus.  I have proof!  

See, a common defining characteristic of see life is the orientation of the tail, or "caudal fin."  Caudal is from the Latin meaning "Tail."  That is the fin that is commonly found on the back of a fish and or marine mammal.  In every case of "Fish," the tail is positioned in a vertical orientation, meaning that propulsion is created by the fish moving the tail from side to side.  In every case of marina mammal, (whales, porpoises) the tail is positioned horizontally, so the fish moves by moving the tail up and down.  

Here is a visual description of my point.  

Fish:

Whale:  

 And when you compare this to the standard mermaid:

You can plainly see that the mermaid's tail is positioned horizontally, which makes sense.  A mermaid's propulsion method is commonly depicted as an up and down motion, which would correspond to a standard set of human knees and hips, and the direction they tend to go when moving.  So, it's plain to see that a mermaid is not, in fact, half fish.  Mermaids are clearly half mammal and half mammal.  

Case closed.