What was the last message of the Dolphins in "The Hitchhiker's Guide to the Galaxy"?

The story behind the answer

In Douglas Adams’s The Hitchhiker’s Guide to the Galaxy, the dolphins—portrayed as more intelligent than humans—escape Earth just before it’s demolished to make way for a hyperspace bypass. Their parting note to humanity is the playful line, “So long, and thanks for all the fish,” acknowledging the meals we provided. This is explicitly described as their last message and later becomes the title of the fourth book in the series.

To remember it, link dolphins to fish and farewells: dolphins → fish; goodbye → “so long.” Don’t confuse it with “42,” which is the ultimate answer to life