Pacu (Portuguese pronunciation: [paˈku]) is a common name used to refer to several common species of omnivorous South American freshwater fish that are related to the piranha. Pacu and piranha don't have similar teeth, although the main difference is jaw alignment; piranha have pointed, razor-sharp teeth in a pronounced underbite, whereas pacu have squarer, straighter teeth like a human in a less severe underbite, or a slight overbite. Additionally, full-grown pacu are much larger than piranha, reaching up to 0.9 m (3 feet) and 25 kg (55 pounds) in weight, in the wild.
Pacu is a term of Brazilian Indian origin. When the large fishes of the Colossoma genus entered the aquarium trade in the U.S. and other countries, they were erroneously labeled pacu. In the Amazon, the term pacu is reserved to smaller and medium sized fishes in the Metynnis, Mylossoma and Myleus genera. The Colossoma macropomum fish are known as tambaqui, whereas Piaractus brachypomus is known as pirapitinga.
Pacu, along with their piranha cousins are a characin fish, meaning a kind of tetra, belonging to the Characiformes order. The ongoing classification of these fish is difficult and often contentious, with ichthyologists basing ranks according to characteristics that may overlap irregularly (see Cladistics). DNA research sometimes confounds rather than clarifies species ranking. Ultimately, classifications can be rather arbitrary.
Pacu, along with piranha, are currently further classified into the Serrasalminae subfamily. Serrasalminae means "serrated salmon family" and is a name which refers to the serrated keel running along the belly of these fish. However, dental characteristics and feeding habits further separate the two groups from each other.
Each of these groups contain one or more separate species. For example, the fish often found in pet stores known as the "Black Pacu" and the "Red-bellied Pacu" belong to the species Colossoma macropomum and Piaractus brachypomus, respectively. A species popular among aquaculturists is the Piaractus mesopotamicus, also known as "Paraná River Pacu".
Location and habitat
Pacus inhabit most rivers and streams in the Amazon and Orinoco river basins of lowland Amazonia, where they form part of the highly diverse Neotropical fish fauna. They have also recently been found in the riverine systems of Papua New Guinea, where it is believed the fish has been introduced to aid the local fishing industry.
Importance to humans
Pacu are commonly sold as "Vegetarian Piranhas" to home aquarium owners. With the proper equipment and commitment, pacu have been known to make responsive pets. One such example was Swish, a 75 cm (30-inch) pacu owned for over 20 years by a Chinese restaurant (Kau Kau) in the Chinatown district in Seattle, Washington; one aquarium technician said of Swish, "He'd rub his body on your arms, kind of like a dog."
However, there is some question of whether the fish are a good choice for the typical hobbyist. While they are not aggressive carnivores like the piranha, their crushing jaw system, used primarily for eating seeds and nuts, can be hazardous. One toddler needed surgery after a pacu (misreported as a piranha) bit her finger at Edinburgh Butterfly and Insect World in Scotland. Commenting on the incident, Deep Sea World zoological manager Matthew Kane warned, "Pacus will eat anything, even children's wiggling fingers."
If a large population of pacu enters an eco-system to which it isn't native, the fish can have a very adverse effect. Pet stores sell pacu as small as 5–8 cm (2-3 inches) long and neglect to warn customers that fish growth is not inhibited by tank size, contrary to popular fish lore. "Most UK dealers now refuse to stock this species due to the large size and expensive aquarium requirements it demands," according to Practical Fishkeeping magazine's Matt Clarke. Incapable of maintaining large aquaria, overwhelmed hobbyists are suspected of illegally releasing their pacu into wild waterways. As tropical fish, pacu will die in cold weather; as newcomers to an ecosystem, pacu may out-compete native species for available food, habitat, and other resources, or displace them by introducing exotic parasites or diseases. Most wildlife resource authorities prohibit releasing exotic fish, including pacu, into the wild. Officials of one Texas lake have put a $100 bounty on the pacu caught there.
As exotic species
In the United States
Discoveries in the United States have been reported in Alabama, Arizona, Arkansas, California, Colorado, Georgia, Idaho, Illinois, Indiana, Kentucky , Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Missouri, Nebraska, New Hampshire, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, South Carolina, Texas, Utah, Wisconsin, and Wyoming.  State wildlife authorities typically advise home aquarists who wish to get rid of overgrown pacu to cut the heads off the fish and dispose of them as garbage. However, Habitattitude, a U.S. national initiative led by the Aquatic Nuisance Species (ANS) Task Force, recommends humanely disposing the fish through a veterinarian or pet retailer, returning them to retailers, or donating them to a local aquarium society, school, or aquatic business. Additionally, aquarium-raised fish can be eaten (see note in Food fish for cautions and instructions).
In New Guinea
Pacu were introduced in 1994 to the Sepik River, and in 1997 to the Ramu river as a food source, due to overfishing of native species. Local people blame the fish for outcompeting native species, including juvenile crocodiles, as well as for several attacks on humans.  Two fishermen reportedly died from having their testicles ripped off by pacus.
In Appendix B of Through the Brazilian Wilderness (see also online version), Theodore Roosevelt advised, "For small fish like the pacu and piranha an ordinary bass hook will do." Concerning the pacu, he added:
"A light rod and reel would be a convenience in catching the pacu. We used to fish for the latter variety in the quiet pools while allowing the canoe to drift, and always saved some of the fish as bait for the big fellows. We fished for the pacu as the native does, kneading a ball of mandioc farina with water and placing it on the hook as bait. I should not be surprised, though, if it were possible, with carefully chosen flies, to catch some of the fish that every once in a while we saw rise to the surface and drag some luckless insect under."
More recently, South American rivers including the Amazon have become a popular destination for recreational anglers, who go to fly fish for pacu. The International Game Fishing Organization has sponsored fly-fishing courses for native Brazilian fishermen, typically accustomed to subsistence fishing, so they can work as guides to fishing tourists.
When bait-fishing in pacu-stocked ponds, anglers in Malaysia are advised to use circle hooks, size 2 or larger, and braided leather rather than nylon leaders which are easily broken by the pacu's teeth. Since pond pacu often nibble at the bait before taking it, anglers should let them swim away with the bait. If the angler simply allows the line to tighten, the circle hook will slide to the side of the fish's mouth and embed its point there.
Theodore Roosevelt wrote of catching and eating pacu in his book Through the Brazilian Wilderness. He described them as "good-sized, deep-bodied fish," and noted, "They were delicious eating."
Today, the Amazon river is experiencing a crisis of overfishing. Both subsistence fishers and their commercial rivals compete in netting large quantities of pacu, which bring good prices at markets in Brazil and abroad.
Aquaculture may relieve the overfishing crisis, as well as improve food security by boosting fish supplies. Various species of pacu are increasingly being used for warm-water farm fishing around the world. Pacu are considered ideal for their tolerance of the low-oxygen water in farm ponds. They also don't require a lot of expensive protein in their diet, and can be raised year-round in warm or temperature-controlled environments.
Research shows that the "flavor of [farmed] pacu is comparable to that of hybrid striped bass, tilapia, and rainbow trout, but superior to catfish." In South America, pacu are prized for their sweet, mild flavor.
Aquarium-raised pacu can be cooked and eaten, but care should be taken to ensure that no chemicals or medicines were used in the aquarium. Heather Candelaria provides a recipe and preparation instructions on the Greater Seattle Aquarium Society's Web site.
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