user: Question: The ship SSBergensfjord built before 1973 has what tonnage?
Columns: ['Ship', 'Built', 'In service for NAL', 'Type', 'Tonnage']
Types: ['text', 'real', 'text', 'text', 'text']
Sample row: ['SSKristianiafjord', 1913.0, '1913–17', 'ocean liner', 'GRT']
SQL:

assistant: SELECT "Tonnage" FROM "Table" WHERE "Built" < 1973.0 AND "Ship" = 'SSBergensfjord';