The official website for The Settlers of Catan

Question about Barbarians & Traders Treasure Fleet - Can I conquer my game partner’s Treasure Fleet by means of a Pirate Fleet?

Answer

No. Your Pirate Fleet always sends a Fleet of your game partner to the discard pile – not to you.

Note: The Pirate Fleets belong to the theme sets "Knights & Merchant" and "Science & Progress", which are part of the Catan Card Game – Expansion Set.