Diets that incorporate seafood can be healthy for us and kind to the environment. However, all seafood is not comparable, so choice is important. Mollusks such as oysters, mussels, and scallops are good choices if they are grown suspended in water, because dredging damages seafloors. On fish farms, this involves growing organisms on nets, trays, or racks. Because mollusks are filter feeders, farmed mollusks can actually improve water quality. Although shrimp are also filter feeders, shrimp farms, especially in Southeast Asia, are often built in coastal areas where mangroves are destroyed to make room for farms. As long as excessive quantities of grain or wild fish are not used for feed, aquaculture can be a very good alternative to open ocean fishing because it can reduce by���catch, the pressure on wild stocks, and the fossil fuel use required at sea. Exceptions are farms that raise transgenic salmon, which often spread disease, or where the farmed fish become oversized and outcompete the native fish. These salmon consume massive quantities of fish feed, so better fish choices are farm���raised talapia, striped bass, and sturgeon. In the wild, Pacific halibut, salmon, sablefish, and sardines are good choices because they are fished selectively. Wild grouper, shark, swordfish, tuna, and orange roughy must be consumed cautiously because water they inhabit usually contains high levels of mercury, PCBs, dioxins, or pesticides, which can biomagnify in food webs and bioaccumulate in fish tissues. Wild Chilean sea bass, Atlantic cod, and Atlantic halibut should be avoided because their populations have been overfished. Orange roughy have been overfished, reproduce slowly, and are fished with bottom���trawlers that frequently damage bottom habitat. The main environmental problem caused by harvesting bottom-dwelling mollusks is ________.