...Est bien entendu un POISSON D'AVRIL!
(...Is of course an April Fools' Day prank!)
Poisson d'avril = April Fools
As it is in the UK too, in France, the 1st of April is dedicated to pranks, which we call "canulars", from someone pranking a friend or relative, all the way to the French Police and National News!
The only difference being that instead of saying "April Fools!" we say "Poisson d'avril!" which literally translates as "Fish of April!".