Hilsa (ilish) any of the members of the genus Tenualosa of the family Clupeidae, order Clupeiformes. Locally known as Ilish, the fish has been designated as the national fish of Bangladesh. The body is strongly compressed and moderately deep with dorsal and ventral profile equally convex. The upper jaw has a distinct median notch. Regularly arranged medium-sized scales cover the metallic silver-coloured body. Body length may reach up to 60 cm, but commonly found specimens measure 35 to 40 cm. A large-sized hilsa weighs about 2.5 kg. Females grow faster, and are usually larger than males. The hilsa is known to be a fast swimmer, and attains maturity in one to two years.
 

Hilsa has a wide range of distribution and occurs in marine, estuarine and riverine environments. The fish is found in the Persian Gulf, Red Sea, Arabian Sea, Bay of Bengal, Vietnam Sea and China Sea. The riverine habitat covers the Satil Arab, and the Tigris and Euphrates of Iran and Iraq, the Indus of Pakistan, the rivers of Eastern and Western India, the Irrawaddy of Myanmar, and the Padma, Jamuna, Meghna, Karnafully and other coastal rivers of Bangladesh.

The fish is anadromous, with a life cycle that follows the general pattern of breeding upstream in fresh water and the larvae hatching from the free-floating eggs. The immature young stages grow in river channels and then descend to the sea for a period of feeding and growth before returning to the rivers as mature breeding adults to complete the cycle. The hilsa is a highly fecund fish. A large-sized female may produce up to 2 million eggs. Although hilsas spawn more or less throughout the year, they have a minor spawning season during February-March and a major season in September-October. Immature hilsa fish (6-10 cm), known as jatka, are extensively caught during their seaward migration in some of the major rivers of the country.
Hilsa is primarily a plankton feeder and its food includes blue-green algae, diatoms, desmids, copepods, cladocera, rotifers, etc. The feeding habit may vary according to the season and age of the fish.

Nearly 16.4% of the country's total fish production is contributed by this fishery. In terms of production and quantity exported, hilsa has played a significant role in the economy of Bangladesh in recent years. It is estimated that about 2 million fishermen and traders are engaged in hilsa fishing in the country.