Medieval Europeans built advanced fish ponds to provide a meat alternative for religious fast days.

During the Middle Ages, the Catholic Church's liturgical calendar required fasting from 'red meat' for approximately 150 days a year. This included every Friday, the forty days of Lent, and various other holy days. Because fish was classified as 'cold-blooded,' it was exempt from these restrictions, leading to a massive surge in demand across Europe between the 11th and 14th centuries.To meet this demand, Cistercian and Benedictine monks became pioneers in hydraulic engineering and aquaculture. They constructed elaborate systems of interconnected ponds, known as 'stews,' which allowed for the controlled breeding of species like Common Carp and Northern Pike. The Abbey of Cluny in France was a leader in this field, managing vast networks of ponds that could support over 400 monks and thousands of pilgrims annually.Archaeological excavations at sites like Bordesley Abbey in England have revealed sophisticated sluice gates and timber-lined channels used to regulate water flow and oxygen levels. These systems were so effective that they transformed the European diet and economy. By the 13th century, freshwater fish had become a high-status luxury item, and the techniques developed by medieval monks laid the foundational principles for modern commercial aquaculture.