If using fresh sardines, scrape off any scales with a blunt knife; cut off the fins. Cut off the head and tail of each fish and slit it open down the stomach. Pull out the backbone and the guts (a messy job but quite easy). Open out the fish, and cut the two fillets apart. Rinse the sardines well under cold water to remove any blood, and pat dry. Coarsely chop the sardines (fresh or canned).
Trim the fennel bulb, and reserve the fronds. Halve and core the fennel bulb and cut into ¼-inch dice. Chop enough of the reserved fronds to make ¼ cup, and reserve for garnish.
Bring 6 quarts of water to a boil in a large pot and add 3 tablespoons kosher salt.
Meanwhile, heat ¼ cup of the oil in another large pot over medium heat. Add the fennel seeds and cook, stirring, until fragrant and lightly toasted, about 1 minute. Add the diced fennel and cook, stirring occasionally, until softened, about 5 minutes. Add the sardines and cook, stirring occasionally, until just opaque throughout, 1 to 2 minutes. Remove from the heat.
Drop the pasta into the boiling water and cook until just al dente. Drain, reserving about ½ cup of the pasta water.
Fry bread crumbs until golden brown.
Add the pasta and ¼ cup of the reserved pasta water to the sardines and stir and toss over medium heat until the pasta is well coated (add a splash or two more of the reserved pasta water if necessary to loosen the sauce). Stir in the remaining 2 tablespoons olive oil, then stir in half of the chopped fennel fronds, half the fennel pollen, if using, half the orange zest, and half the bread crumbs.
Transfer the pasta to a serving bowl and scatter the remaining fennel fronds, pollen, zest, and bread crumbs over the top. Serve immediately.