Fish populations of the Mediterranean are less abundant than those of the oceans. Gradients of temperature and salinity resulting from the depth and the closure of the ecosystem, however, promote the life cycle of the several important species on the continental shelves. The migratory habits of many important species bring them into contact with many Mediterranean islands and coastlands.
Since the routes of the shoals are far from predictable, places where their movements are topographically constrained are of obvious importance. Numbers are very variable from year to year: gluts occur, but dearth is so frequent as to make it unwise to make fish protein more than supplement to a subsistence diet. The nutritional usefulness is greatly increased by processing to make the resource sustainable in time of general dearth, the movable inland or by sea: drying and salting are the principal techniques, and the evaporate salt of pans on the important symbiotic resource. The salt in salt fish (with the minerals in the fish) was probably of as much dietary importance as the protein.
Even in conditions of glut, and assuming very favorable conditions for fishing, total yields cannot have constituted an important aggregate contribution to the protein needs of even small ancient populations, compared with cereal or legume staples. They did, however, play a significant role in diversifying a diet based on those staple, which was important both nutritionally and culturally in the classic Mediterranean pairing of staple and relish. Salted or pickled fish was the opson par excellence, which was widely largely caught by way of fly fishing in the open sea.
Fishing in that time was chancy and hazardous, but essential for the most prized fish. Many local markets were supplied from the rocky shores. The fisheries of the formerly extensive wetland lagoons of the Mediterranean costs were the easiest to develop artificially, because they were sheltered, shallow, and had controllable inlets and agreement. Both archaeological and literary evidence shows the extent to which Roman pisciculture developed, and the elaboration of fishponds for both fresh and salt water fish.