The ancient Romans might have traded live fish across the Mediterranean Sea by endowing their ships with an ingenious hydraulic system, a new investigation into a second century A.D. wreck suggests.
Consisting of a pumping system designed to suck the sea water into a fish tank, the apparatus has been reconstructed by a team of Italian researchers who analyzed a unique feature of the wreck: a lead pipe inserted in the hull near the keel.
Recovered in pieces from the Adriatic sea in 1999, the ship was carrying a cargo of processed fish when it sank six miles off the coast of Grado in northeastern Italy.
The small trade vessel, which was 55 feet long and 19 feet wide, was packed with some 600 vases called amphoras.They were filled with sardines, salted mackerel, and garum, a fish sauce much loved by the Romans.
Now the archaeologists suspect that some 200 kilograms (440 pounds) of live fish, placed in a tank on the deck in the aft area, might have also been carried by the ship during its sailing life.
"The apparatus shows how a simple small cargo vessel could have been turned into one able to carry live fish. This potentiality, if confirmed by future studies, shows that trading live fish was actually possible in the Roman world," Carlo Beltrame, a marine archaeologist at the Ca' Foscari University of Venice in Italy, told Discovery News.
Indeed, a number of historical accounts have suggested that the Romans might have transported live fish by sea. For example, the scientist and historian Pliny the Elder (23 – 79 A.D.), wrote that live parrotfish were shipped from the Black Sea to the Neapolitan coast in order to introduce the species into the Tyrrhenian Sea.
Measuring 51 inches in length and featuring a diameter of at least 2.7 inches, the unique lead pipe was located in a sort of "small bilge-well" and would have been connected to a hand operated piston pump (which had not been found within the wreck).
Ending with a hole right in the hull, the pipe intrigued the researchers.
"No seaman would have drilled a hole in the keel, creating a potential way for water to enter the hull, unless there was a very powerful reason to do so," Beltrame and colleagues reported in the International Journal of Nautical Archaeology.
According to the researchers, the reason wasn't the need for removing bilge water from the bottom of the boat through the pipe.
Indeed, bucket chain pumps were able to discharge bilge water from the side in a much safer way, possibly recovering between 110 and 225 liters (30 to 60 gallons) of water per minute.
"It seems unlikely that sailors aboard the small Grado ship abandoned the usual chain-pumping apparatus in favor of the more complex bilge pump," Beltrame said.
Rather than serving a bilge pump to send water out of the ship, the pipe could have supported a sucking pump to bring water onto the vessel, the researchers argued.
But what could have been the purpose of such an unusual hydraulic system?
According to Beltrame and colleagues, the ship was too small to justify the presence of the pump to wash the decks or extinguish fires (similar piston-driven suction systems were employed on warships such as Horatio Nelson's HMS Victory).
"Given the ship's involvement in the fish trade, the most logical hypothesis is that the piston pump worked to supply a fish tank with oxygenated water," said Beltrame.
The researchers calculated that the small trade vessel could have carried a tank containing around 4 cubic meters (141 cubic feet) of water.
This water mass would have created no problems for stability while housing some 200 kilograms (440 pounds) of live fish, such as sea bass or sea bream.
Connected to the lead pipe, the hand operated piston pump would have easily allowed the necessary exchange of the water mass.
According to the researchers, the water would have needed to be replaced once every half an hour in order to provide a constant oxygen supply.
"With a flow of 252 liters (66 gallons) per minute, the piston pump would have filled the tank in 16 minutes," Beltrame said.
According to Rita Auriemma, a marine archaeologist at the University of Salento, it is plausible that the hydraulic system in Grado ship served for live fish trade.
"The context in which the ship operated makes this the most logical explanation," Auriemma told Discovery News.
"The near Istria coast was known for numerous vivaria, large enclosures to breed fish. It is possible that the Grado ship transported live fish from these vivaria to large markets in the high Adriatic," Auriemma said.
Indeed, it would have taken about 10 hours to cross the nearly 30 miles of sea that divided the Istria vivaria to the river port of Aquileia, one of the richest Roman towns during the imperial period.
"Such a trip could have been sustained by the live fish only through an apparatus of continuous water exchange similar to that of the Grado ship," Beltrame said.