States valued a large merchant navy, as it took revenue from foreigners for shipping services and encouraged domestic exports on the cheap, at least in theory. Moreover, as the chief difference between a merchant vessel and a warship was merely the number of guns that it carried, a merchant fleet could be converted into a navy if war broke out.
The mental milieu of European policy makers in the 17th–18th centuries was that trade was a form of undeclared warfare: a zero-sum game. Underlying this was the silly assumption that there was a fixed volume of international trade; so, the fight there was over market share.
Britain and other countries sought to maximize their advantage while minimizing that of rival nations. The primary weapon in this war was a tariff on imported goods. Exports were lauded as money spinners, whereas imports were disdained as a financial drain. Most nations had “navigation laws” which aimed to restrict seaborne carriage to domestic ships.
Governments encouraged fisheries, both as a means for greater self-sufficiency in food supply, and for furnishing an export commodity. It also helped in training seamen and stimulated shipbuilding.