Fearing shortage of culturally significant fish, medieval societies reacted in several ways. In a culture of markets, scarce supply motivates sellers to demand and/or buyers to offer more for the commodity: anecdotes from the twelfth century and serial prices from the thirteenth indicate fish prices rising even past 1350. Ownership of the productive resource itself could capture some of those sellers’ gains, not to mention the prestige and power medieval society associated with landownership: elite acquisition of fishing rights had begun with early creation of private lordships, but by and after the twelfth century it also promised income from direct exploitation or from leasing operations to artisans (depriving local subsistence fishers). In contrast, relict and then emergent claimants to public authority could gain by regulating resource exploitation ‘for the public good’. From the 1200s onwards kings, territorial princes, and self-governing communes asserted control over fishing rights and activities, first on acknowledged public waters (large rivers, coastal waters) and eventually over practices and uses of private natural waters. The chapter explores grounds for regulating fisheries as a ‘public’ resource to allocate their value, settle disputes, ensure consumer safety, and occasionally to encourage what might now be called ‘sustainable’ uses within recognizable limits. Like the artisanal fisheries toward which they were directed, these cultural measures retained close ties between local natural ecosystems and consumers of fish.

The high-medieval demographic and economic growth in which fishers and their customers shared had detectable environmental consequences. Prevailing agricultural practices plus increased human and other wastes damaged river systems and polluted both flowing and still waters. Contemporaries were aware of some such effects; others emerge only in modern scientific archaeology. Rulers and others blamed perceived declines in the quantity and quality of fish on overfishing. Present-day studies of long-running assemblages of fish remains detect local depletion of favoured varieties and shrinking average size of more common species. Some fishes (eel) and some fisheries (for herring) of previously limited importance increased their contribution to European diets. An exotic species, common carp, hitherto present in Europe only in the lower Danube, spread westwards into waters made warmer and siltier by human activities. In large thirteenth-century assemblages (but with regional variations), more accessible herring, eel, codfishes, and small cyprinids become dominant. Not all change had human origin; natural dynamics also played a role. High medieval centuries saw the crest, then decline, of climatic warming, with concomitant regional differences in precipitation, seasonality, riverine and estuarine hydrology, and even shifts in stratification and water chemistry of the Baltic. Changed habitats let heat-tolerant fishes spread west, while a herring-dominated regime in the Baltic peaked and slowly yielded to greater presence of cod. Knowingly or not, humans and animals had to adapt.