What was the total water weight of the recipe? If it was much over 500g and still tasted too salty, then you're probably right that the carcasses brought a lot of salt with them.
Edited to add: salting to taste is usually the best idea with a broth, since other things you do to it will affect its apparent salinity. Including the acid level, and of course any reduction. If you're using it as stock it's best to leave it out entirely.