Although the fishes that live in the deep oceans are frequently described as deep-sea fishes it has become customary to refer to those that are exploited in the fisheries as deep-water fishes. This avoids confusion with the term deep-sea fishery meaning a fishery that takes place in distant waters. It is becoming accepted that a deep-water fish is one that lives, at least for most of its life cycle, at depths greater than 400 metres. The exploited fishes of the continental shelf are generally divided into two categories, pelagic and demersal. In the deep sea the pelagic extends from the surface to abyssal depths and it is usual to divide it into three zones. The epipelagic zone includes all those fish living in the upper photic layer of the ocean, such as the tuna fishes. The mesopelagic zone spans the depth range from below the photic zone down to about 1000 metres and supports an abundant and diverse fish fauna.