They will most likely serve sushi at the lido buffet. I cannot say when they will serve it. It's been five years since i've sailed on the Triumph and four years since i've sailed on the sister ship Victory.
Norwegian Pearl 12/19/09
Carnival Miracle 12/23/08
Carnival Legend 4/1/07
Carnival Liberty 1/1/06,11/21/06
Caribbean Princess 5/28/05
Norwegian Wind 3/20/04
Carnival Legend 12/20/02, 3/18/03
Carnival Fascination 11/1/02
Carnival Victory 3/17/02
Carnival Triumph 3/17/01
RCI Majesty Of The Seas 1/5/01
Carnival Ecstasy 1/15/99, 2/04/00
I too have never sailed the "Triumph", however the last 3 Carnival Cruises I been on all served sushi at one time or the other. In fact, the "Glory", and the "Valor" both had sushi bars as I recall. I think it's a safe bet that you'll get sushi aboard "Triumph".
Have a great cruise.