“It is said that fish is an excellent brain food. It is probably because they travel in schools” is a pun that was printed in the Philipsburg (MT) Mail on August 10, 1893. “Fish make excellent brain food, probably because they travel in schools” was printed in an 1894 newspaper.
 
A riddle form was cited in 1897, but became popular in the 1960s:
 
Q: Why are fish so smart?
A: Because they travel in schools.
   
   
Chronicling America
10 August 1893, Philipsburg (MT) Mail, pg. 2, col. 4:
It is said that fish is an excellent brain food. It is probably because they travel in schools.
 
Chronicling America
7 February 1894, Juniata Sentinal and Republican (Mifflintown, Juniata County, PA), pg. 4, col. 3:
Fish make excellent brain food, probably because they travel in schools.
 
14 March 1897, Los Angeles (CA) Herald, “Humor of the Hour,” pg. 24, col. 3:
Tommy—“Pop, why do fish make brains?”
Tommy’s Pop—“I dare say because they travel in schools. Now run along and play.”—Philadelphia Record.
 
13 March 1960, Boston (MA) Sunday Herald, sec. 1, pg. 6, col. 7:
Mary: Why are fish so smart?
John: I don’t know. Why?
Mary: Because they travel in schools.
From Felicia Colerte (11)
Holbrook
 
Google Books
June 1963, Boys’ Life, “Think and Grin,” pg. 70, col. 2:
Bill: Why are fish so smart?
Will: Because they travel in schools. — Keith Landers, Keene, N.H.
 
1 September 1963, Indianapolis (IN) Star, “Riddle Box,” sec. 8, pg. 6, col. 6:
Why are fish smart?
Because they travel in schools.
     
Google Books
Milton Berle’s Private Joke File:
Over 10,000 of His Best Gags, Anecdotes, and One-liners
By Milton Berle
Edited by Milt Rosen
New York, NY: Three Rivers Press
1989
Pg. 263:
Fish must be brain food, because they travel in schools!
       
Google Groups: alt.humor.puns
Puns of the Weak 01-24-03
Stan Kegel
1/24/03
(...)
Why are fish in the sea smarter than animals on land.
Because they travel in schools. (Jokes Galore)
 
Twitter
Peyman
@pparsi
why are fish in the sea smarter than animals on land? is it because they travel in schools?
1:07 AM - 26 Jul 2009
     
Google Books
A Tribute to Teachers: \
Wit and Wisdom, Information and Inspiration About Those Who Change Our Lives
By Richard Lederer
Portland, OR: Mario Street Press
2011
Pg. ?:
Why are fish considered to be brain food?
Because they travel in schools (and sometimes take debate).
 
Twitter
lorraine Jo King
@LahLahBean
#sillysaturday
Why are fish in the sea smarter than animals on land.
Because they travel in schools.
12:15 AM - 23 Jun 2018