Place the shrimp shells and gumbo crabs on a baking sheet. Melt ¼ cup of margarine and pour it over the shrimp shells and crabs. Mix together with your hands to distribute the margarine.
Place the baking sheet in a 400-degree oven for 20 minutes.
Slice the yellow onion, bell pepper and garlic head in half. Chop the celery into approximately 4-inch pieces. Tie the parsley and herbs with butcher twine to make a bouquet garni. Place the peppercorns and broken bay leaves into a spice bag.
In a large stockpot, heat the vegetable oil over high heat.
Place the yellow onions, bell pepper and garlic cut side down in the stockpot. Add the celery and sear for about 5 minutes.
Add the green onions, parsley, herbs and spice bag and stir well.
Add the roasted shrimp shells and Gumbo crabs plus all the juice that accumulated while roasting. Add the oysters with their juice.
Pour 16 cups (4 quarts) of cold water into the stockpot and stir everything together well.
Bring the stockpot to a high boil for 5 to 10 minutes, uncovered.
Reduce the heat to a slow simmer and cover, stirring occasionally.
After 2 hours, remove the lid and continue at a low simmer for another hour.
If it looks like the liquid has reduced by about 50%, remove the stockpot from the heat.
After it cools for about 30 minutes, remove the solids with a slotted spoon into a strainer over a bowl large enough to hold 8 cups. Let the solids drain for about 15 minutes.
Discard the solids, then pour the remaining liquids through the strainer (optionally use some cheesecloth or a filter in the strainer).
This should yield about 8 cups.
If you are not going to use the stock right away, place it in freezer containers with a little headroom for expansion and refrigerate.
If you are not going to use the stock within 2 days, place the containers in the freezer. Make sure to label them with the name and date.