Agreed, the person who abandons can only as such abandon his interest, and if he is a possessor but not the owner he can only as such abandon his possessory title.

However it is commonplace that a master may sell goods on board under an agency of necessity (if the goods are deteriorating), so I think it a little odd that he cannot also abandon them under a necessity. If the shipowner also owned the goods, then the captain's abandonment must surely affect the owner (the master being the shipowner's agent in any event). I can envisage all sorts of anomalies arising...