Mainland production numbers go to new islands (Settlers of Catan app)
The rules are given at Catan Maps (and I believe somewhere in app, but I don't have it installed at the moment):


  After setting up the board as shown, set aside the following four number tokens, shuffled in a face-down pile:  3, 5, 8, 9, 10
  
  [...]
  
  As soon as a player’s ship reaches the intersection of a terrain hex belonging to one of the small islands, a number token is assigned to this hex. The first 5 number tokens are taken randomly from the face down supply.
  
  Once the supply is used up, the player whose turn it is must remove a number token from the main island. At first, only number tokens are available that obey the following 3 rules:
  
  
  The number token must belong to a terrain hex adjacent to which the active player owns a settlement or city.
  Each settlement or city on the main island must keep at least one terrain hex with a number token.
  On a small island, the numbers 6 and 8 must not be next to each other.
  
  
  If it is not possible to comply with all three rules, you may break the rules in consecutive order from 1 to 3.


The page at Catan Maps also notes that this scenario was included in the first edition of Seafarers, but was removed because it requires so many different sets.