Sampling of the debris.
You can not make statistical studies on the detritus of the area without an accurate way: we have thus led the sieves made in school. We have the largest knitted or crocheted top and bottom gradually decreasing.
Then collect debris in a plastic jar the size of a liter, equivalent then to a cubic decimeter. The weigh and derive the specific weight of the debris picked up. Agitiamo sieves and then we weigh the individual separated debris. Eventually we extract all the shells to determine the weight or volume percentage of shells per cubic decimeter