Usually either stock made from shrimp or crabs; I've included recipes for both here:

Shrimp Stock

Ingredients for Shrimp Stock

  • About 4 quarts of shrimp shells (about what you'd get from shelling 2 pounds of shrimp)
  • 8 quarts cold water
  • 2 tablespoons oil
  • White Mirepoix:
    • 4 ounces onions, diced
    • 4 ounces leeks, white portion only, washed well, trimmed and chopped
    • 4 ounces celery, diced
    • 4 ounces parsnips, chopped
  • 2 lemons, halved
  • 1 cup parsley, coarsely chopped, stems and all
  • Sachet d'epices:
    • 8 bay leaves
    • 1/2 teaspoon dried basil leaves
    • 1/2 teaspoon dried oregano leaves
    • 1/2 teaspoon dried tarragon leaves
    • 1/2 teaspoon dried thyme leaves
    • 1/2 teaspoon cracked black peppercorns
    • The above ingredients are placed into a 4" square of cheesecloth and tied into a sack.

Instructions for Shrimp Stock

Rinse the shells briefly under cold water and drain well. sweat (cook over low heat in a small amount of oil. Cover directly with a piece of foil or parchment paper, then tightly cover the pot)the shells briefly in the oil, then add the mirepoix and sweat for 2-3 minutes. Add to cold water, add the parsley and sachet and bring to a boil. Reduce heat and simmer for 40 minutes. Strain the stock thoroughly in a china cap or strainer layered with cheesecloth. Cool the stock completely in an ice-water bath and use, refrigerate or freeze immediately.

Ingredients for Crab stock

  • 3 pounds crabs, broken in half (Blue crabs preferred but any hard shell crab will do
  • 8 quarts cold water
  • White Mirepoix
  • 1 whole head garlic, cut in half horizontally
  • 4 bay leaves
  • Sachet d'epices

Instructions for Crab Stock

Combine all the ingredients in a large stock pot and bring to a boil over high heat. Reduce heat and simmer for 1 hour, skimming if necessary. Strain through a china cap layered with cheesecloth, and discard the remainder. Cool the stock in an ice-water bath, transfer to a storage container and refrigerate overnight. The next day, skim off any fat that has risen to the top. Freeze any leftovers.