its actually signifies the age of pisces and was adopted by christianity
As well as the obvious Jesus - fisher connection,
Jesus was (somewhere in the Bible) the alpha and the omega.
The fishy symbol is the alpha. I don't know about the omega. (But that doesn't look like a fish.)
Meat could not be eaten on many holy days, but fish was somehow exempt.
In mediaeval Europe, some people, especially the Basques did a rather good trade in salted Cod from the other side of the Atlantic, which they sold across Europe, but maybe Poland preferred to stick more to the freshwater stuff.