Cages, pens, floating tanks, and raceways are all water-based aquaculture systems. Cages are totally enclosed on all, or all but top, sides by mesh or netting, whereas in pen culture the bottom of the enclosure is formed by the lake or sea bed. Floating, submersible and submerged cage designs are used in mariculture. All consist of a collar or frame which supports the net bag. Cages and pens probably first developed in Asia several hundred years ago, although it is only recently that they have become popular. Mariculture in cages began in Japan in the s but developed largely as a result of the salmon farming industry in northern Europe and North America during the past two decades.