Ilish, or Hilsa as it is known in English, is a Bengali delicasy.
The spawning habits of this fish reminds me of the salmon runs in Alaska. In the city where I live in Michigan, USA, Bangladeshi & South Indian grocery stores stock flash frozen Hilsa. In Bengal, India, this fish is always eaten fresh, never frozen.
Hilsa is notorious for its bones, so it definitely is an acquired taste. My Father would soak this fish overnight, bones and all, in vinegar and salt and roast it the next day in the oven. The resulting aroma and melt-in-your-mouth deliciousness is something I will never forget. The vinegar works overnight to melt the bones, so every part of the fish is edible.
This preparation with a mustard/poppy seed sauce is the typical way this fish is cooked in the Bengali kitchen.