There's a brief article in the May-June 2007 issue of Audobon Magazine about the horrific practice of shark-finning. Nearly 73 million (yes, million) sharks are killed each year worldwide, many of them only for their fins. After being caught, fisherman slice the dorsal fins off of live sharks, then toss the sharks back in the ocean to die a slow and miserable death. Keeping only the fins leaves the fisherman more room on their boats.
There is a high demand for shark-fin soup in Asia and a growing appetite for it in the United States, as well. Many shark species are now considered "critically endangered," "endangered," "vulnerable," according to the International Union for Conservation of Nature and National Resources (IUCN). See their "Red List of Threatened Species." You can read the Audubon article (NOTE: there's an unpleasant photo there) and can do your part by avoiding shark-fin soup in restaurants and by expressing your concerns to any restaurants who offer it on their menus.
