Crawfish are sweet, freshwater crustaceans that closely resemble lobsters. Like lobsters, they are highly valued for their sweet flesh, and they are harvested all over the world. In America, they are most closely associated with the food and culture of Louisiana, where they have been named the state's official crustacean. They are best when harvested or purchased on the day they are to be cooked, but can be kept fresh and healthy for two days with minimal effort.
Soak your sack of crawfish with fresh, cold water as soon as you get home. Always keep the crawfish moist, otherwise they will begin to die.
Prepare a picnic cooler, or a large plastic tub, with ice or a layer of gel packs at the bottom. Lay the sack of crawfish in the container on its side.
Cover the crawfish with ice or additional gel packs. If you use loose, unpackaged ice for at least some of your cooling, the steady trickle of water will help keep the crawfish moist.
Open the drain on your picnic cooler to ensure that the crawfish will not be immersed in standing water, which will kill them. If you are using a plastic tub, put a wire rack or other platform over the gel packs.
Drape the top of the bag with well-soaked towels or burlap sacks, to keep the air in the cooler moist and provide some protection from wind and light.
Store the cooler in a cool, dark place, such as a garage or basement, until you are ready to cook the crawfish. Change the ice or gel packs regularly to keep them cold, and mist or sprinkle the crawfish every few hours with water.
Crawfish can be held this way for two days with relatively few of them dying. If necessary, they can be held for up to four days, but by the fourth day the number of dead crawfish will grow significant.
Some instructions call for you to "purge" the crawfish by leaving them in water for a period of time. This is not generally necessary with crawfish that have been purchased from a fishmonger, who will have already done this.
Discard any dead crawfish, and do not cook or eat them. They deteriorate rapidly once dead, and may cause illness.