Truncate and Indented

  • Good compromise between Lunate and Rounded
  • Less drag than Rounded
  • More low-speed thrust than Lunate
  • Often found on faster fish
Image Source: Lake Superior Streams
Rainbow Trout
This Rainbow trout has a slightly-indented truncate caudal fin.
Sources:
​(Page 168, last paragraph: J. Morrissey & J. Sumich: https://books.google.com/books?id=TW2SAkgJOdMC&pg=PA168&lpg=PA168&dq=truncate++caudal+fin&source=bl&ots=fc5ntwYEEo&sig=ACfU3U2fUa_xjurEXZUTn9WLlB3W3oR0gw&hl=en&sa=X&ved=2ahUKEwjfsO_oqt3hAhXbJTQIHYUlAN04ChDoATAHegQICRAB#v=onepage&q=truncate%20%20caudal%20fin&f=false)

https://manoa.hawaii.edu/exploringourfluidearth/biological/fish/structure-and-function-fish