The hilsa is a type of shad, related to fish like herrings and sardines. Shad like hilsa are anadromous—they live in the sea, but ascend rivers to breed. This may account for the unique taste, as may the nature of the water in the river where the fish breeds.Three favourites are the hilsa from the Padma in Bangladesh, the Bhagirathi Hooghly and the Rupnarayan river. The Padma hilsa is claimed to be the tastiest (and the boniest—the hilsa is not for those who dislike bony fish), but each kind has its fervent loyalists. Hopefully the fish will be around to sustain their counterclaims.