Tuna, especially bluefin tuna, are in high demand. From high priced sashimi grade (raw) fish to cooked canned tuna, we eat a lot of tuna and wild stocks are being depleted. Bluefin are the highest priced tuna and the most depleted (estimated at ~4% of historic population levels). These muscular, fast fish are difficult to reproduce in a lab so farmers are catching young fish in the wild and raising them in pens off shore until they are large enough to harvest. This process is called "tuna wrangling" and requires a lot of fish to feed the young tuna. Bluefin are carnivorous and so require fish to grow and produce quality meat. Over the years the efficiency of feeds has improved but they still require to eat more fish (usually sardines or anchovies) than are produced.
Bivalves like the oysters pictured here are some of the most environmentally sustainable forms of aquaculture. Bivalves are filter-feeders that feed by filtering algae and other organic matter from the water and do not require active feeding. Cultivation can actually improve the quality of coastal waters by helping to reduce high nutrient levels caused by agricultural run-off, for example.
(include oysters, clams, mussels, scallops, etc.)
Carp (Cyprinidae family)
Carp are a popular fresh water fish that have been framed in China for centuries. Carp as a group fill various trophic roles (carnivores, herbivores, detritvores) so combining multiple species in one pond can make and added feeds more efficient. Similar to tilapia, carp are grown on land in fresh water ponds which do require significant fresh water inputs and land space. While these are some of the more sustainably farmed finfish, intensification of efforts can result in unsustainable practices.
Shrimp (or prawns)
Shrimp are a popular species to cook but have historically been very unsustainable. Harvesting from the wild destroys benthic habitats through trawling and picks up more bycatch that target shrimp species. Early farming also destroyed habitats when large areas of mangroves were removed to make shrimp farms, recently shrimp farms have moved into land-based ponds which help preserve important coastal nursery habitats but also include environmental issues such as use of freshwater resources, pollution from waste water and transmission of disease and parasites. Research is still needed to make this a sustainable venture.
Salmon (Salmonidae family)
As carnivorous organisms, salmon require fish-based feed inputs similar to tuna and shrimp. These cold water species have historically been grown in hatcheries to supplement wild populations. Therefore, despite their complicated life histories, the transition to full salmon aquaculture was intuitive. Salmon are generally grown up in large marine pens where this exposure to the natural environment has cause much controversy. Risks to natural populations increase with high densities of genetically altered fish, transmission of diseases and parasites and mixing of modified genes threaten unstable wild populations.
Tilapia species include both fresh water and marine species and for this reason can be raised with less impact that marine types of aquaculture. Additionally, many tilapia species are herbivores so feeds are less expensive both financially and environmentally. Perhaps because of these benefits, tilapia farming has exploded and despite potentially sustainable techniques, can be grown at unsafe levels. Cheating on feeds can severely compromise the health benefits of tilapia to customers and over stocking ponds can be unsafe for the environment.
Algae are harvested for a variety of uses from direct human consumption to supplements, thickeners and cleaners. Algae is the most sustainable group to farm since these require no feed inputs and can be provide habitat for other organisms. Algal species are considered a stable crop and so can help to stabilize local economies as well.