Hi Nigel, welcome to the forum. Indeed very old Dutch silver spoon with the town mark of Amsterdam.
I believe the fish facing to the right could be a flounder, the maker's mark of Jacobus Buth
, registered with the Amsterdam silver guild 1690-1749, known cast spoon maker.
Amsterdam year letter K for 1696, Silverware of 1st standard was stamped with the province Coat of Arms of Holland; the lion rampant facing left in a crowned shield,minimum of 934/1000 fineness.
An exact copy of your spoon by the same maker is in the collection of the Zilvercollectie in het Kasteel van Laarne. See image