Rinse and dry the fish. Set the fillet in a deep 9-by-13-inch baking dish. In a large bowl, whisk the water with the salt until the salt is dissolved, then add the dill seeds, anise seeds, coriander seeds, herb sprigs, garlic and shallot. Pour the brine over the fish and evenly distribute the aromatics. Top the fillet with a sheet of paper towel, pressing gently to moisten the sheet. Wrap the entire dish in plastic wrap. Refrigerate for 3 days, turning the fish in its brine, topping it with a fresh paper towel and rewrapping in plastic wrap after 36 hours.