I have received a number of responces about Courland jews surnames.
One pointed to German roots of Courland jews (see also
http://bernshtam.name/bernshtam/Courland.htm ), and another
pointed to the Napoleonic decree (1808) abut jews surnames.
One more interesting point:
I have found only germans (except jews) with the surname Berenshtam/Bernshtam.
There are 2 famous families of russian germans.
Now I'm trying to find exact place of origin of these people, hoping
to find in this way origin area of my ancestors.
So I think that courland jews received surnames before they arrived to
Courland - i.e. 17th-18th centuries.