Icelandic subsistence farming from the Middle Ages well into the 20th century was restricted by the short production period (summer) compared to the long cold period. Apart from occasional game, the food produced in the three months of summer (including preserving meats and cheeses) had to suffice for nine months of winter. Researchers have estimated that, based on these methods of subsistence, Iceland could support a population of around 60,000. For centuries, farming methods changed very little, and fishing was done by men using hooks and lines from rowboats constructed from driftwood. Farmers also owned the boats, so fishing was limited to periods when the farmhands were not needed for farm work. Fish was not just a food, but a trade good, and it was exchanged for products brought by foreign merchant ships. The people were dependent on trade for cereals, such as rye and oats, transported to Iceland by Danish merchants. Until the 19th century, the vast majority of Icelandic farmers were tenant farmers on land owned by the Icelandic landowner elite, the Catholic church, or (especially after the confiscation of church lands during the Reformation) the king of Denmark. Tenant farmers used surplus fish, tallow, and butter to pay the landowner his dues.