 In all cases, the master and the chief engineer officer of the ship shall be Bulgarian nationals or nationals of a Member State of the European Union, or a contracting state to the Agreement on the European Economic Area, or of the Swiss Confederation. 
"Chapter five - MASTER AND SHIP CREW [...]
Formation of the Crew
Art. 88. (amend. - SG 41/01; amend. - SG 113/02; amend. - SG 85/10)
(1) The ship crew of merchant ships shall be formed by qualified seafarers. At least 25 percent of the management and operational level positions and at least 25 percent of the executive level positions shall be occupied by Bulgarian nationals. [...]
(4) In all cases, the master and the chief engineer officer of the ship shall be Bulgarian nationals or nationals of a Member State of the European Union, or a contracting state to the Agreement on the European Economic Area, or of the Swiss Confederation. [...]" (Merchant Shipping Code of Bulgaria, SG 113/02)
Merchant Shipping Code of Bulgaria, SG 113/02
[<a href="https://www.marad.bg/sites/default/files/upload/documents/2021-06/Merchant%20Shipping%20Code.pdf">Code</a>]