In his book De Re Rustica, Marcus Terentius Varro (116 - 27 BCE) wrote that "There are two kinds of fish-ponds, the fresh and the salt. The one is open to common folk, and not unprofitable, where the Nymphs furnish the water for our domestic fish; the ponds of the nobility, however, filled with sea-water, for which only Neptune can furnish the fish as well as the water, appeal more to the eye than to the purse, and exhaust the pouch of the owner rather than fill it".
The century before the birth of Christ was the great age of saltwater fishponds. The Late Republic was a period during which Rome's aristocracy competed with each other in their displays of conspicuous consumption. Fresh water fishponds were much more profitable than saltwater ponds, as well as being much less expensive to construct and maintain, but Rome's wealthy elite looked down upon them.
Part of the appeal of saltwater ponds was that Roman gourmets valued saltwater fish more highly than they did freshwater fish, but saltwater fishponds don't seem to have been very practical fish farms. Varro says that although "Quintus Hortensius had ponds built at great expense near Bauli, I was at his villa often enough to know that it was his custom always to send to Puteoli to buy fish for his dinner."
In his book Pisicinae, James Higgenbotham explains that the concentration of power in the hands of the Emperor Augustus and his successors "began a process through which the active competition among the ruling classes was reduced and power as well as property was transferred in greater measure to the imperial household." Many of the great seaside villas became incorporated into even larger imperial estates and the few new saltwater fishponds were mainly created for the emperors and their families.
The photo at the top of this article shows part of the saltwater fishpond in front of a grotto which belonged to the Emperor Tiberius. This fishpond is in many ways a typical saltwater fishpond.
One characteristic which most Roman saltwater ponds shared was that they were never really intended to be salt water ponds. Most of these ponds combined fresh water with saltwater to create brackish conditions. The grotto of Tiberius contains several freshwater springs which still flow. If the site didn't have any natural springs, fresh water was supplied by cisterns or aqueducts.
All Roman saltwater ponds relied on tidal flows to introduce seawater into the ponds. The rectangular pond in front of the grotto of Tiberius is surrounded on three sides by a low marshy area that was flooded during high tides. This pond is surrounded by a low wall which separated the pond from the marsy area during high tides. There was a single channel which connected the pond with the sea. Fish that prefer brackish water probably swam into the pond through this channel during high tides. The channel is represented by a straight blue line on the map below.
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