Many recipes for seafood chowders, soups and sauces call for clam juice, sometimes called clam nectar. If you cannot find clam juice in your grocery store, of if you are allergic to clams, there are several substitutions for clam juice that will not negatively impact your recipe.
You can easily substitute an equal amount of fish stock for clam juice in a recipe. Some quality fish stocks are commercially available. You can also make your own fish stock from fish scraps, fish bones and heads, and even shrimp shells. Simply simmer fish trimmings with an onion, carrot, stalk of celery, some salt and pepper and a dash of lemon juice in water for 20 to 30 minutes. Fish stock freezes well for future use in all your fish and seafood recipes.
If you can't find fish stock in your grocery store and don't want to make your own, chicken broth can also be substituted for clam juice. Use equal parts chicken broth and water for the clam juice. For example, if your recipe called for 1 cup of clam juice, use 1/2 cup water and 1/2 cup chicken stock. This lends the saltiness that you would get from clam juice without adding an overpowering chicken flavor to your final dish.
You may also use vegetable stock -- technically vegetable broth -- in place of clam juice. The substitution here is somewhat less exact than for chicken or fish stock. Most cooks recommend using half veggie broth and half water, and then adding small amounts of tomato paste to taste. This substitution will be most effective with recipes that include tomato, such as tomato-based clam chowders and seafood stews.
Other substitutions for clam juice include a stock made of chicken broth and water with shrimp shells simmered in it; the water -- not oil -- from a can of tuna; white wine -- about half the amount of clam juice called for; and commercial fish or seafood bouillon cubes.