It is the earliest such work to have survived to the modern day. "Garum", also known as "liquamen", was the universal sauce added to everything. It was prepared by subjecting salted fish, in particular mackerel intestines, to a very slow thermal process. Over the course of two to three months, in an enzymatic process stimulated by heating, usually by exposure to the sun, the protein-laden fish parts decomposed almost entirely. The resulting mass was then filtered and the liquid traded as "garum", the remaining solids as "alec" - a kind of savoury spread. Because of the smell it produced, the production of "garum" within the city was banned. "Garum", supplied in small sealed amphorae, was used throughout the Empire and totally replaced salt as a condiment. Today similar sauces are produced in Southeast Asia, usually sold abroad under the description "fish sauce", or "nam pla". Aquaculture in China began before the 1st millennium BC with the farming of the common carp. These carp were grown in ponds on silk farms, and were fed silkworm nymphs and faeces. Carp are native to China. They are good to eat, and they are easy to farm since they are prolific breeders, do not eat their young, and grow fast. The original idea that carp could be cultured most likely arose when they were washed into ponds and paddy fields during monsoons. This would lead naturally to the idea of stocking ponds. The Chinese politician Fan Li was credited with authorship of "The Fish-Breeding Classic", the earliest-known treatise on fish farming. During the 7th- to 10th-century Tang dynasty, the farming of common carp was banned because the Chinese word for common carp () sounded like the emperors' family name, Li (). Anything that sounded like the emperor's name could not be kept or killed. The ban had a productive outcome, because it resulted in the development of polyculture, growing multiple species in the same ponds. Different species feed on different foods and occupy different niches in the ponds. In this way, the Chinese were able to simultaneously breed four different species of carp, the mud carp, which are bottom feeders, silver carp and bighead carp, which are midwater feeders, and grass carp which are top feeders. Another development during the Tang dynasty was a mutation of the domesticated carp, which led to the development of goldfish.