Carp are actually related to goldfish and can be caught right in New York City, like in Kissena Lake in Queens. I assume these were locally caught but could not get a straight answer from where exactly.
Why carp? According to the interweb: “In Poland, Christmas Eve is a day first of fasting, then of feasting. The Wigilia feast begins at the appearance of the first star. There is no red meat served but fish, usually carp … Carp provides a main component of the Christmas Eve meal across Poland; carp fillet, carp in aspic etc.”
Here is a recipe for carp aspic in case you are curious.