The term “aquaculture” refers to a broader spectrum of practices than many realize. In freshwater and marine ecosystems, finfish and shellfish are raised in a variety of different man-made structures. Mesh nets, lines, cages, and rafts are used for shellfish aquaculture, which in New England produces mostly mussels, clams, and oysters. These systems are placed in estuaries and bays, which are natural habitats for shellfish. Seed (very young shellfish) are obtained from hatcheries or collected from wild populations and allowed to settle on the aquaculture structure being used, where they can filter food and nutrients from the surrounding marine environment and grow to a commercially harvestable size.
Saltwater finfish aquaculture, which in New England consists mostly of salmon farming in Maine, is carried out in large net-cages placed in coastal marine waters, which are stocked with fry from freshwater hatcheries on land. There have also been efforts to expand this technology offshore in the U.S. for raising or growing out other finfish species and some extensive operations based on offshore structures elsewhere in the world. Because the fish are not free to forage or hunt for themselves, finfish aquaculture requires feed from the fish farmer. On land, aquaculture is carried out in natural habitats such as ponds as well as artificial environments like raceways and tanks. Freshwater finfish aquaculture in New England consists mainly of trout and salmon hatcheries in Vermont, New Hampshire, and Massachusetts, where juvenile fish are raised to a certain size in tanks on land and then released into streams and ponds.
These different practices can be categorized by the level of human intervention involved in the animal’s growth and life history. Extensive aquaculture, which requires minimal intervention, usually involves introducing species from a hatchery or a wild stock into a new natural or slightly altered environment and then leaving them alone until they are ready to be harvested. An example of extensive methods can be found in ancient Chinese carp aquaculture, in which the carp ponds were stocked with fry from wild populations but left to feed on naturally occurring or introduced algae. Many shellfish farms – such as theNantucket Oyster Company in Massachusetts – also practice extensive methods, in which wild or hatchery seed is set on a man-made raft or line suspended in a natural marine environment. Because shellfish are filter feeders, they do not need any supplemental feed. Since rearing seafood in this manner requires modest energy inputs, extensive aquaculture is among the most efficient and sustainable food production processes in the world.
Intensive aquaculture is at the opposite end of the spectrum. Both feed and habitat are artificially supplied in intensive aquaculture systems, which are generally less sustainable because the higher level of human intervention requires higher energy, water, and land use. Within intensive systems, farming low trophic-level species (herbivorous and omnivorous fish such as tilapia and catfish) is much more sustainable than farming high trophic-level species (piscivorous fish like salmon) – a future blog post in this series will explore the issue of sustainability further. Cage aquaculture – such as that used by True North Salmon Company in Maine – is often considered intensive because even though fish may be raised in a natural habitat (rather than on land in a recirculating system or raceway) they are entirely dependent on humans for survival.
Semi-intensive aquaculture involves an intermediate level of human intervention, in which feed may be provided in a natural environment or an environment is provided but feed is not. In some cases, feed may be added to a natural environment to augment growth or nutrition of the cultured species, but at levels below those required to fully sustain the population. Some people consider the New England lobster industry to fall in this category as current populations are nurtured and grow as a result of millions of pounds of food put in the water in the form of baits in traps. Of course, if the lines between these three categories seem fuzzy, it’s because they are – every farm is different, and there are many ways to farm the same aquatic animal. Extensive, intensive, and semi-intensive are general terms that attempt to roughly describe the amount of energy needed to produce the final seafood product.
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