Many oysters were grown and eaten in the New York City area in the 19th century. By the 20th century, through pollution and other causes, the oyster catching industry began to die out. New York City was not called the “Big Oyster” in the 1800s or early 1900s, but the nickname became “retroactively” popular in the 2000s.
 
“BEFORE it was the Big Apple, New York was the big oyster, at a time American oysters were known for their size,” Mark Kurlansky wrote in the New York (NY) Times in June 24, 2001. Kurlansky later wrote a book, The Big Oyster: New York on the Half Shell (2006). “Before nyc was ‘the big Apple’ it was known as the ‘big oyster’ #true” was posted on Twitter by jamesalexander on September 20, 2013.
 
“It’s the Biggest Oyster Found in New York in 100 Years. And It Has Stories to Tell” by James Barron was published in the New York (NY) Times on September 2, 2018:
 
“So Big is a metaphor for New York City itself — which, as the author Mark Kurlansky explained in his 2006 book, “The Big Oyster,” had another nickname before it was the Big Apple.”
 
However, there is no evidence that New York City was nicknamed the “Big Oyster” before Mark Kurlansky’s 2001 article and 2006 book.
 
   
24 June 2001, New York (NY) Times, “City Lore: When the oyster Was Their World Today They’re Pricey, but for 19th-Century New Yorkers, They Were Cheap and Seemingly Inexhaustible” by Mark Kurlansky, pg CY3:
BEFORE it was the Big Apple, New York was the big oyster, at a time American oysters were known for their size. A dubious endorsement came from the English novelist William Makepeace Thackeray, who wrote that eating an American oyster was like eating a baby.
 
New York was for several centuries one of the world’s oyster centers. New York oysters were said to be less perishable, oysters that knew how to take it. In the 19th century, when Europe and America were in the throes of an oyster craze, more oysters were shipped abroad from New York than from any other American port.
 
OCLC WorldCat record
The Big Oyster : New York on the half shell
Author: Mark Kurlansky
Publisher: New York : Ballantine Books, ©2006.
Edition/Format:   Audiobook : English : 1st ed
Summary:
Before New York City was the Big Apple, it could have been called the Big Oyster. Author Kurlansky tells the remarkable story of the oyster, whose influence on the great metropolis remains unparalleled. For centuries New York was famous for its oysters, Gotham’s most celebrated export, a staple food for the wealthy, the poor, and tourists alike, and the primary natural defense against pollution for the city’s congested waterways. Filled with cultural, historical, and culinary insight, from the island hunting ground of the Lenape Indians to the death of the oyster beds and the rise of America’s environmentalist movement, here are the stories behind Peter Stuyvesant’s peg leg and Robert Fulton’s “Folly”; the oyster merchant and pioneering African American leader Thomas Downing; the birth of the business lunch at Delmonico’s; early feminist Fanny Fern, one of the highest-paid newspaper writers in the city; and even “Diamond” Jim Brady.—From publisher description.
     
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jamesalexander
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Before nyc was “the big Apple” it was known as the “big oyster” #true
10:08 AM - 20 Sep 2013 from New York, NY
 
10 August 2015, Wall Street Journal (New York, NY), “Mollusks and Me: Learning the Art of Eating Oysters; Ralph Gardner Jr. samples seven kinds of bivalves with Danny McDonald of Pier A Harbor House” by Ralph Gardner, pg. A16:
“In 1872, there was more money spent on the consumption of oysters in New York City than all other food products combined,” Mr. McDonald reported. “New York, before it became the ‘Big Apple’ was the ‘Big Oyster.’” 
 
10 May 2016, New York (NY) Times (online), “New York Today: The Big Oyster” by Alexandra S. Levine:
The Big Apple was once the Big Oyster.
 
New York Harbor was rich with oysters as far back as when Lenape tribes lived here, and the Dutch once called Ellis Island the Little Oyster Island and Liberty Island the Great Oyster Island.
   
2 September 2018, New York (NY) Times, “It’s the Biggest Oyster Found in New York in 100 Years. And It Has Stories to Tell” by James Barron, pg. A17:
So Big is a metaphor for New York City itself — which, as the author Mark Kurlansky explained in his 2006 book, “The Big Oyster,” had another nickname before it was the Big Apple. By some accounts, in the 17th century, New York Harbor held half of the world’s oysters. The city was filled with oyster stands before there were hot-dog stands or food trucks. A 12-course dinner in 1842 for Charles Dickens — who characterized old Ebenezer Scrooge as being as “secret, and self-contained, and solitary as an oyster” — began with oysters, glorious oysters.