“Why are fish easy to weigh?”/“They have their own scales!”

A fish riddle has been cited in print since at least 1983:
 
Q: Why is it easy to weigh a fish?
A: Because it has its own scales.
 
“How do you tune a fish?”/“With its scales” is a similar fish riddle.
 
         
11 February 1983, The News Beacon (Fair Lawn, NJ), “Children’s Comedy Corner,” g. 4, col. 2:
Why is it easy to weigh a fish?
Because it has its own scales.
       
20 July 1991, Vancouver (BC) Sun, “SunRays” by Marilyn Stusiak, pg. A13, col. 2:
Why is it easy to weigh a fish?
Because they have their own scales.
 
Google Books
The Penguin’s Put-Downs, Jokes and Riddles
By Scott Franklin and Batton Lash
New York, NY: T. Doherty
1992
Pg. 65:
Why does The Penguin find it so easy to weigh fish?
Because they have their own scales!
   
Google Books
Little Humor:
Over 100 Big Laughs from the Mouths of Babes
By Lorry Erickson
Reiman Publications, L.P.
2003
Pg. 88:
Why is it easy to weigh fish?
They all have their own scales.
   
Google Groups: alt.humor.puns
Puns of the Weak 09-05-03
Stan Kegel
9/5/03
(...)
Why are fish so easy to weigh?
They come with their own scales! (Erin, 10)
 
Daily Shaskar
Funny question and answer: We bet you can’t give such unique answers… Wanna try????
Dailybhaskar.com | Last Modified - Sep 10, 2014, 03:13 PM IST
(...)
Q. Why is it easy to weigh a fish?
A : Because it has its own scales.
 
Reddit—Jokes
Posted by u/mgjcsaldivar July 25, 2017
Why is a fish easy to weigh?
Because it has its own scales!
   
Reddit—Jokes
Posted by u/Slats7 September 18, 2017
Why are Fish easy to weigh?
Because they have their own scales.
 
Reddit—Dadjokes
Posted by u/ct2k7 December 5, 2019
Why are fish easy to weigh?
They have their own scales!