Carp naturally require minerals found within Rock Salt and they find it very attractive, almost irresistible at some times of the year.
It may be that the carp are able to mitigate osmotic loses via seeking out Rock Salt. It seems particularly effective in spring time when the fish may use it for physiological support and to help rid themselves of ecto parasites, such as leeches and fish lice, that have built up over the colder months.
One thing is clear carp find Rock Salt very attractive.
If using Rock Salt in your fishery for management or angling
please use it sensibly. Too much salt can be detrimental to the aquatic ecosystem.