Arapaima gigas, the largest freshwater fish in the world, has fascinated scientists and locals alike for centuries. Also known as Pirarucu in Brazilian Portuguese, which translates to "Red Fish" due to its red caudal fin, this species is a vital part of the Amazonian ecosystem and an increasingly important species for aquaculture. This article provides a comprehensive overview of the Arapaima gigas, focusing on its diet, feeding habits, and the potential for sustainable aquaculture.
Introduction: A Giant of the Amazon
Arapaima gigas was first officially described in 1822 by Schinz. The genus name "Arapaima" comes from the indigenous Tupi language, where "arapa" means "fish" and "paima" means "leaf," referring to the fish's large, scaly scales. The species name "gigas" is derived from Latin, meaning "huge" or "gigantic," reflecting the enormous size this fish can attain.
This impressive fish species has a streamlined body, large fins, and distinctive coloration. They can reach a length of up to 450 centimeters and a weight of 200 kg. The body is primarily gray to gray-green, with red spots on the scales towards the tail or a reddish-orange color of the flesh. During the breeding period, male Arapaima gigas exhibit a striking color change, acquiring a sharp dark coloration on the top of their heads, extending to the dorsal region, almost to the attachment of the dorsal fin. The underside of the head gets yellow spots, while the flanks, belly, and tail get a red color.
Natural Habitat and Distribution
The Arapaima gigas inhabits several types of habitats within the Amazon Basin, such as floodplains, lakes, and major tributaries of the Amazon, including the Rio Madeira and the Rio Machado. They thrive in both white water and clear water, often in oxygen-poor areas such as swampy parts of the rainforest. Their habitat often contains an abundance of plants, especially in the flood plains where seasonal flooding occurs. The bottom of their habitat is usually sand and mud, especially in the lakes and river channels where they build nests and breed.
Naturally distributed in the sub-basins of the Amazon, Tocantins-Araguaia, and Essequibo rivers, spanning Brazil, Ecuador, Colombia, Peru, and the rivers of Guyana. Over time, it has been introduced to the Bolivian Amazon. At present, this species can also be found in Central America, North America, and even Asia, including China, Indonesia, the Philippines, Malaysia, Singapore, and Thailand. The preferred habitats of A. gigas are the low-gradient aquatic environments of the Amazon River and its tributaries, mostly lakes, in addition to connecting channels during seasons of low water levels. These environments are characterized by low water flow, high depth, turbidity, abundant floating and emerging macrophytic vegetation, which in some cases, can cover the entire body of water, and frequent hypoxic conditions.
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Diet and Feeding Habits in the Wild
Arapaima gigas are primarily piscivorous, meaning their diet consists mainly of fish. However, they also consume other prey such as birds, reptiles (e.g., small caimans and turtles), and even small mammals. In the wild, they can consume more than 8% of their body weight per day. For a 90 kg fish, this means about 7 kg of food per day.
As a carnivorous species, A. gigas has a natural diet that primarily consists of small fish, crustaceans, mollusks, and insects. During its early stages, particularly as fry, A. gigas primarily consumes plankton, later transitioning to insects. In the juvenile stage, the diet mainly consists of small fish and micro crustaceans, and when they reach the adult stage, they feed exclusively on fish, crabs, and prawns. Studies suggest that there is no a particular “prey target - size”. A. gigas captures its prey through a rapid movement of the head, often accompanied by a tail whip, producing a distinct high-pitched noise. This movement, involving the opercula lids, expels water taken in during the strike. Fish prefer to feed during dawn or dusk, although they may also feed during the day. When temperatures rise, they seek refuge among aquatic vegetation to escape intense sunlight, often remaining stationary at the water's bottom but periodically surfacing to breathe atmospheric oxygen.
Feeding in Captivity
In an aquarium environment, the diet of the Arapaima gigas should mimic their natural diet as much as possible. This can include fish, the most important part of their diet, but also shrimps and crustaceans, pieces of chicken or beef, and large insects. Fish should remain the main part of their diet. In the beginning, whenever you get a new predatory fish in the juvenile stages be very careful though to make sure it's getting enough to eat but Immediately begin to ween from live… It needs to eat a lot during these stages so sometimes it's tricky but you want to assure what it's getting is the right foods and feeders alone will not cut it.
Many predator fish can be pellet trained successfully, however, one can still feed much more than just pellets. A mixture of Hikari Jumbo CarniSticks, Hikari Massivore Delite, Azoo 9 in 1 Arowana Sticks, TetraCichlid Jumbo Sticks, large freeze-dried krill & a turtle mix with dried grass hoppers, mill worms etc. can be used. Raw shrimp, silver sides, shiners, fat heads, blue gill, cry fish, squid, snails, frogs, trout, salmon, herring, smelt, cichlids, krill, and sometimes dried catfood can be added three to four times a week on top of their daily pellet mix.
Breeding and Reproduction
They usually reproduce during the rainy season. The Arapaima gigas makes nests on hard ground without vegetation and organic matter. The nests have a diameter of approximately 47 cm and are 15 to 20 cm deep. This usually occurs in shallow waters on the shores of lakes, canals, and lagoons. The males guard the eggs and young for about a month. The woman protects the fry and male by patroling around them. A female Arapaima gigas can lay between 10,000 and 20,000 eggs per clutch. However, not all eggs are fertilized. The eggs are about 2.5 to 3 millimeters in size.
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During the reproductive period, male Arapaima gigas exhibit a striking color change. The males acquire a sharp dark coloration on the top of their heads, extending to the dorsal region, almost to the attachment of the dorsal fin. The underside of the head gets yellow spots, while the flanks, belly, and tail get a red color.
Conservation and Threats
The main natural enemies of the Arapaima gigas are humans, who hunt them for their meat, and caimans such as the spectacled caiman (Caiman crocodilus). In the 1970s, the arapaima population began to decline due to overfishing until it went commercially extinct near major Amazonian cities. A favorite food amongst the Amazonian natives, the arapaima is harpooned not only to be eaten but also for its scales, which can reach 4 inches (10 centimeters) in length and are used for jewelry. Their bony tongues can also be used as a scraper.
Due to overfishing and habitat degradation, Arapaima populations have declined significantly over the last century. In Brazil, commercial fishing of wild arapaima is banned, but they are being bred with some success in indoor ponds in Germany and artificial lakes in Peru. Fishing restrictions have been implemented in Brazil and Peru. Aquaculture programs now raise Arapaimas sustainably for meat. Efforts are ongoing to monitor wild populations, enforce catch limits, and educate locals about sustainable practices.
Aquaculture of Arapaima Gigas
Arapaima gigas, a native species of the Amazon River basin, holds the distinction of being the world's largest freshwater fish. Its suitability for aquaculture is underscored by a range of favorable traits, including rapid growth, ease of adaptation to commercial aquafeeds, good meat quality, high fillet yield without intramuscular bones, strong market demand, and widespread consumer acceptance. Aquaculture offers a sustainable source of fish while leveraging the Amazonian region's favorable conditions, such as climate, soil quality, and access to agricultural inputs for feed production. More specifically, the cultivation of A. gigas plays a vital role in mitigating various negative impacts. These include the reduction of wild fish populations, primarily due to overfishing in rivers, the expansion of agricultural land, and other human activities.
A. gigas is typically farmed in earthen ponds, although floating cage systems are also utilized. Land-based pond culture involves one to three well-defined production phases: pre-growth, growth, and final fattening. However, the specific phases may vary based on the size of the fingerlings at the start of fry rearing and pre-growth processes. Following the description made by Chu-Koo et al., the fry rearing stage begins with the capture of individuals from breeding ponds when they reach a minimum size of 2 cm. During this phase, fry initially consume zooplankton, followed by Artemia nauplii, and gradually transition to a balanced diet as part of a feeding training protocol. This phase typically spans between 17 to 32 days, depending on the specific protocol used, and concludes when the fish reach sizes ranging from 7 to 8 cm. Moving on to the pre-growth stage, fingerlings, with sizes between 8 to 15 cm (having completed the rearing phase), continue their adaptation to balanced aquafeed and initiate their development toward the fattening stage. In the initial fattening phase, juveniles from the pre-growth stage (typically weighing 150 to 200 g) are stocked at a density of 1 to 1.5 fish per square meter. This phase occurs in ponds measuring 500 m2, and the fish remain in this stage for approximately 3 months or until they reach 2 kg in weight, at which point they are selected and transferred to the second phase. In the final phase, fish are stocked at a density of 0.25 fish per square meter, typically in ponds ranging from 1,000 to 2,000 m2 in size. During this stage, A. gigas can reach a weight of 10 to 12 kg after approximately 8 months, or other weights dictated by market demand.
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Nutritional Requirements for Aquaculture
One main constraint pertains to the limited knowledge concerning fundamental and applied aspects of the digestive physiology of the species, as well as its nutritional requirements in captivity. It must be pointed out that nutritional knowledge should be tailored to the specific species, taking into account its unique physiological and behavioral characteristics, with minimal room for generalizations. In this regard, in recent years, several studies have contributed valuable insights, including the characterization of the gastrointestinal tract, determination of optimal protein and energy levels, formulation of feed rations, and assessment of ingredient digestibility in A. gigas. However, there remains substantial gaps in knowledge regarding other aspects of digestive physiology relevant to nutrition. These aspects include some addressing fundamental research (characterization of gastrointestinal tract), applied research (determination of optimal protein and energy levels, assessment of ingredient digestibility, nutritional requirements for lipids, carbohydrates, vitamins, and minerals,) as well as practical applications of such knowledge to the formulation of feed rations, incorporation of functional additives and the use of non-traditional ingredients, since a comprehensive understanding of digestive mechanisms and effective nutritional management is essential to formulate efficient aquafeeds capable of meeting the species' diverse nutritional needs throughout its life stages.
Conducting experimental work with this species is inherently challenging for several reasons: the large size of the specimens, the considerable cost associated with using A. gigas as biological material and the limited availability of laboratories equipped with suitable facilities and experimental systems for the study of large fish. Considering that nutritional requirements can vary depending on factors such as the size or stage of the species, protein source in the diet, production system, experimental conditions and environmental factors, there is a pressing need to complement existing research with information on nutrition at different stages of development.
Different studies have assessed that requirements of crude protein (CP) in A. gigas, that can vary between 56% and 30%, depending on the age/size of the individuals, although several other parameters may have influenced the results obtained, like the type of rearing facility (floating cage, earthen pond), the duration of the experiment (from 4 weeks to 12 months) or feeding system. The levels of CP can be reduced as fish grow up; in juveniles of 120 g Ituassú et al. determined an optimal CP level of 48.6%, although they found that feed conversion and protein efficiency indices were not affected by reducing protein levels up to 33%. Juveniles of around 500 - 650 g present a requirement of around 40 - 45% CP.
Regarding the essential amino acid requirements for A. gigas, two studies have been conducted based on the essential amino acid composition found in muscle tissue. These suggest the percentage of amino acids relative to dietary protein for specimens at various developmental stages and from both natural and aquaculture settings. Rodrigues et al. estimated amino acid requirements by the analysis of muscle composition in specimens sampled in the wild and in farms considering two size classes (1.66 ± 0.22 kg and 10.49 ± 1.07 kg, respectively). The estimated essential amino acid requirements were very similar between both groups, despite the differences in the muscle amino acid profile. Their results suggest that the highest estimated requirements for A. gigas should be for leucine, phenylalanine + tyrosine, arginine, and valine, mainly. In a different study, developed using juvenile specimens with an average weight of nearly 1 kg, the amino acids with the highest estimated requirements were arginine, phenylalanine + tyrosine, leucine, and isoleucine. On the other hand, in the few studies existing about requirements of functional amino acids, Ramos et al. determined that inclusion of 1.02% of glutamine in the diets of juvenile pirarucu (82.12 g) improved growth performance and influenced intestinal villi height and activity of important digestive enzymes, favoring nutrient digestion and absorption. Glutamine plays a role as a regulator of essential metabolic pathways and has potential to enhance the nutrition of neotropical carnivorous fish.
Rojas et al. suggested that the recommended level of total lipids in diets should be around 20% inclusion in diets with 4,000 kcal DE for the postlarvae and fry stages. This inclusion ensures an adequate energy/protein ratio (an average of 10 to 11 kcal DE/g CP) and optimal utilization of dietary protein.
Rearing Conditions for Aquaculture
Optimal water quality conditions are crucial to ensure the optimal growth of A. gigas and hence, is essential to monitor parameters such as temperature and pH in the culture units. It is worth noting that A. gigas relies on obligatory aerial respiration, obtaining the majority of its oxygen through its swim bladder. This adaptation allows the species to tolerate waters with low levels of dissolved oxygen, providing an advantage over species that primarily respire through gills. A. gigas can thrive even in environments with dissolved oxygen levels below 2 mg/L. Despite its reliance on pulmonary respiration, this species still excretes CO2 through its gills, necessitating low levels of this gas in the water for effective gas exchange. Elevated CO2 levels above 20 mg/L can adversely affect animal health and increase stress, particularly in juveniles. A. gigas also exhibits notable tolerance to high concentrations of total ammonium. Research has shown that levels ranging from 0.8 to 2.4 mg/L do not significantly impede the development of the species in culture units.
Regarding the water requirement for the culture of A. gigas, there are no studies on the subject. It is known that the evolutionary aspect of the respiration of A. gigas, gill respiration and accessory respiration through the vascularized swim bladder, allows it to develop in environments with low levels of dissolved oxygen, characteristic of Amazonian waters. This presumes a low water requirement for the culture of this species in conventional systems.
Considering an earthen pond of 10,000 m2 and an average depth of 1.5 m, the volume of water in the culture enclosure is 15,000 m3, which is necessary to obtain a biomass of 10,000 kg of A. gigas of 10 kg average market weight in 16 months of culture. To ensure adequate fish development, a continuous daily replacement of 5-10% of volume of water is recommended, estimating a water requirement of 37.5-73.5 m3/tons (1.30-2.55 L/min/tons). Furthermore, de Sousa et al. determined the greywater footprint value to produce A. gigas at 64.51 m3/tons, indicating the sustainability of the cultivation of this species from an environmental point of view.