You have a recipe that asks for Sardines or Anchovies to be mixed in. Only problem is that you don’t have any.
You can use fish sauce (found in the oriental section of most markets) It comes in a bottle and looks like brown ketchup.
For your recipe, start with about 1/4 teaspoon of fish sauce for each anchovy/sardine requested in recipe (about a teaspoon for 1/4 cup) You can adjust to your taste as needed.
If you want to make your dish vegetarian, you can use capers, tepenade, or even a bit of miso paste. Once again, start small and adjust to taste.