Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish. Shellfish include various species of molluscs (e.g. bivalve molluscs such as clams, oysters, and mussels and cephalopods such as octopus and squid), crustaceans (e.g. shrimp, crabs, and lobster), and echinoderms (e.g. sea cucumbers and sea urchins). Historically, marine mammals such as cetaceans (whales and dolphins) as well as seals have been eaten as food, though that happens to a lesser extent in modern times. Edible sea plants such as some seaweeds and microalgae are widely eaten as sea vegetables around the world, especially in Asia. In the United States, although not generally in the United Kingdom, the term "seafood" is extended to fresh water organisms eaten by humans, so all edible aquatic life may be referred to as "seafood".
Fishing scenes are rarely represented in ancient Greek culture, a reflection of the low social status of fishing. However, Oppian of Corycus, a Greek author wrote a major treatise on sea fishing, the Halieulica or Halieutika, composed between 177 and 180. This is the earliest such work to have survived to the modern day. The consumption of fish varied in accordance with the wealth and location of the household. In the Greek islands and on the coast, fresh fish and seafood (squid, octopus, and shellfish) were common. They were eaten locally but more often transported inland. Sardines and anchovies were regular fare for the citizens of Athens. They were sometimes sold fresh, but more frequently salted. A stele of the late 3rd century BCE from the small Boeotian city of Akraiphia, on Lake Copais, provides us with a list of fish prices. The cheapest was skaren (probably parrotfish) whereas Atlantic bluefin tuna was three times as expensive. Common salt water fish were yellowfin tuna, red mullet, ray, swordfish or sturgeon, a delicacy which was eaten salted. Lake Copais itself was famous in all Greece for its eels, celebrated by the hero of The Acharnians. Other fresh water fish were pike-fish, carp and the less appreciated catfish.
Fresh fish is a highly perishable food product, so it must be eaten promptly or discarded; it can be kept for only a short time. In many countries, fresh fish are filleted and displayed for sale on a bed of crushed ice or refrigerated. Fresh fish is most commonly found near bodies of water, but the advent of refrigerated train and truck transportation has made fresh fish more widely available inland.
The UK Food Standards Agency recommends that at least two portions of seafood should be consumed each week, one of which should be oil-rich. There are over 100 different types of seafood available around the coast of the UK.
Over 33,000 species of fish and many more marine invertebrate species have been described. Bromophenols, which are produced by marine algae, gives marine animals an odor and taste that is absent from freshwater fish and invertebrates. Also, a chemical substance called dimethylsulfoniopropionate (DMSP) that is found in red and green algae is transferred to animals in the marine food chain. When broken down, dimethyl sulfide (DMS) is produced, and is often released during food preparation when fresh fish and shellfish are heated. In small quantities it creates a specific smell one associates with the ocean, but which in larger quantities gives the impression of rotten seaweed and old fish. Another molecule known as TMAO occurs in fishes and give them a distinct smell. It also exists in freshwater species, but becomes more numerous in the cells of an animal the deeper it lives, so that fish from the deeper parts of the ocean has a stronger taste than species who lives in shallow water. Eggs from seaweed contains sex pheromones called dictyopterenes, which are meant to attract the sperm. These pheromones are also found in edible seaweeds, which contributes to their aroma. However, only a small number of species are commonly eaten by humans.
There is broad scientific consensus that docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) found in seafood are beneficial to neurodevelopment and cognition, especially at young ages. The United Nations Food and Agriculture Organization has described fish as "nature's super food." Seafood consumption is associated with improved neurologic development during gestation and early childhood and more tenuously linked to reduced mortality from coronary heart disease.
There is some debate over the particular health benefits of fish, especially regarding the relationship between seafood consumption and cardiovascular health. However, government recommendations promoting limited seafood consumption are relatively unified. The US Food and Drug Administration recommends moderate (4 oz for children and 8 - 12 oz for adults, weekly) consumption of fish as part of a healthy and balanced diet. The UK National Health Service gives similar advice, recommending at least 2 portions (about 10 oz) of fish weekly. The Chinese National Health Commission recommends slightly more, advising 10 - 20 oz of fish weekly.
Ciguatera fish poisoning (CFP) is an illness resulting from consuming toxins produced by dinoflagellates which bioaccumulate in the liver, roe, head, and intestines of reef fish. It is the most common disease associated with seafood consumption and poses the greatest risk to consumers. The population of plankton which produces these toxins varies significantly over time and location, as seen in red tides. Evaluating the risk of ciguatera in any given fish requires specific knowledge of its origin and life history, information which is often inaccurate or unavailable. While ciguatera is relatively widespread compared to other seafood-related health hazards (up to 50,000 people suffer from ciguatera every year), mortality is very low.
Man-made disasters can cause localized hazards in seafood which may spread widely via piscine food chains. The first occurrence of widespread mercury poisoning in humans occurred this way in the 1950s in Minamata, Japan. Wastewater from a nearby chemical factory released methylmercury that accumulated in fish which were consumed by humans. Severe mercury poisoning is now known as Minamata disease. The 2011 Fukushima Daiichi Nuclear Power Plant disaster and 1947 - 1991 Marshall Islands nuclear bomb testing led to dangerous radionuclide contamination of local sea life which, in the latter case, remained as of 2008.
Byzantine supply chains, frequent bycatch, brand naming, species substitution, and inaccurate ecolabels all contribute to confusion for the consumer. A 2013 study by Oceana found that one third of seafood sampled from the United States was incorrectly labelled. Snapper and tuna were particularly susceptible to mislabelling, and seafood substitution was the most common type of fraud. Another type of mislabelling is short-weighting, where practices such as overglazing or soaking can misleadingly increase the apparent weight of the fish. For supermarket shoppers, many seafood products are unrecognizable fillets. Without sophisticated DNA testing, there is no foolproof method to identify a fish species without their head, skin, and fins. This creates easy opportunities to substitute cheap products for expensive ones, a form of economic fraud.
A major international scientific study released in November 2006 in the journal Science found that about one-third of all fishing stocks worldwide have collapsed (with a collapse being defined as a decline to less than 10% of their maximum observed abundance), and that if current trends continue all fish stocks worldwide will collapse within fifty years. In July 2009, Boris Worm of Dalhousie University, the author of the November 2006 study in Science, co-authored an update on the state of the world's fisheries with one of the original study's critics, Ray Hilborn of the University of Washington at Seattle. The new study found that through good fisheries management techniques even depleted fish stocks can be revived and made commercially viable again.