One of the largest frogs of the genus Rana, which can reach up to 20 cm in snout-to-vent length (SV-L), and up to 800 g in weight.
Body robust, with wide, flat head. Skin smooth and plain (no wrinkles, warts or spikes), dominantly pale green with brown spots on the dorsal and lateral parts of the body, including limbs and head. Abdominal side white. In adult males, upper abdomen temporarily turns yellowish as sexual maturity approaches. Anterior limbs short. Posterior legs robust and long (representing up to 50 percent of total body length, and up to 40 percent of total body weight). Both anterior and posterior limbs possess inter-digital swimming webs. Sexual dimorphism is present in this species, although it is only conspicuous when pre-adult stage is reached.
The American bullfrog is native to North America. Its natural geographical distribution extends from the south-eastern Canadian province of New Brunswick in the north, southwards through eastern United States to Florida, Alabama and the south-eastern part of Texas. It has been introduced to a number of other regions, including Central and South America, the Caribbean islands, and Northeast and South-eastern Asia.
The first documented attempts to cultivate the American bullfrog date back to the late XIX century in the United States. Primitive farms consisted of fenced earthen ponds, where tadpoles were stocked to feed on natural productivity. Young frogs would congregate along the edge of the pond to feed on insect larvae hatched on decomposing meat or other kitchen wastes deliberately placed as spawning substrates. Poor yields soon discouraged such pioneering attempts. By the late 1930s, bullfrogs had been introduced in Brazil, and the first frog farm outside the United States was established. Success was marginal due to the lack of sufficient food, cannibalism and disease problems. Bullfrog farming re-gained popularity in Brazil in the mid 1970s. Small ventures multiplied and academic groups got involved in research until late 1980s, developing a range of culture facility designs and culture techniques for bullfrogs, whose systems are described in the production section of this fact sheet.
Taiwan Province of China imported the first batch of bullfrogs as a potential alternative aquaculture species in the 1950s. A research programme aimed at the cultivation of Rana catesbeiana was then established.
The first attempts to cultivate this species in Mexico began in the early 1960s, when this species was introduced. The first farm was established with the objective of meeting the growing demand for live frogs in the United States. The farm had breeding, tadpole rearing and fattening facilities and eventually became an extensive tadpole-producing facility for stocking natural water bodies that later became frog fishing grounds.
Between 1968 and 1990, two research groups in United States, at the University of Michigan, and at Louisiana State University, carried out research on culture techniques for bullfrogs as laboratory animals, and on its reproductive physiology, pathology and nutrition.
In the early 1990s Thai farmers also began culturing frogs, and R. catesbeiana (along with Rana tigerina, a species native to Southeast Asia) became a popular culture species, technically supported by Chulalongkorn University.
Between 1991 and 2001, the Center for Research and Advanced Studies (Cinvestav) of Mexico, carried out a comprehensive applied research programme for its cultivation, resulting in further advances in reproductive physiology, nutrition, pathology and ecophysiology, which led to the development of an inundated-type culture system that is currently in commercial use in a number of regions of Mexico and Central America.
Bullfrogs readily adapt to and occur throughout a wide range of climatic conditions. They are found in temperate regions, where they hibernate to withstand below-freezing ambient temperatures, and also in tropical regions, where temperatures reach 40 °C during summer months.
The reproductive physiology of bullfrogs is highly influenced by temperature, photoperiod, ambient humidity, and barometric pressure. Consequently, the closer a population is to the Equator, the wider the length of its reproductive period. While year-round spawning is common in bullfrog stocks from Panama and Ecuador, wild stocks in Mississippi (United States of America) have a three-month reproductive season.
Metamorphosis of tadpoles is dependent upon two types of ecological factors:
In general, bullfrogs from the northern hemisphere (i.e. Asia, North and Central America) reproduce from March through August, while populations from countries south of the Equator (i.e. Brazil, Uruguay and Argentina) reproduce from September through February. The fecundity of bullfrogs is a function of age, rather than body mass. In farmed females, between 1 300 and 33 000 laid eggs have been recorded from first-spawning and 3-year females, respectively.
Embryonic development and timing to metamorphosis are highly temperature-dependent. In tropical regions (>26 °C) hatching occurs within 48 hours. The concentration of dissolved oxygen is of particular importance at this stage, therefore egg masses have to be kept near the surface. DO levels should be >3 mg/litre. Newly-hatched larvae are approximately 10 mm in length and attach to smooth surfaces by means of an oral sucker. They have a yolk sac that is consumed in approximately 72 hours at ≥26 °C. Immediately after the yolk sac is exhausted, larvae become free-swimming, filter feeding tadpoles.
Ontogenic transition of feeding habits include filter-feeding of phytoplankton, grazing on benthic algae, detritophagy and a gradual shift towards animal protein ingestion, before reaching the metamorphic climax when nourishment becomes briefly autogenic. After tail resorption, froglets become carnivores.
Farmed frog stocks can achieve a survivorship of more than 50 percent. Genetic variability expresses through a significant asynchrony in growth and timing to sexual maturity within every cohort, therefore, under tropical conditions, fitter individuals can reach market size (>180 g), in 3 months after metamorphosis, and sexual maturity in as early as 7 months after hatching, while weakest may take over one year.
Various strategies are employed by bullfrog farmers to obtain seed. These include collection of egg masses or tadpoles from the wild, as well as semi-natural breeding in captivity (semi-controlled reproduction), and more recently, environmental and hormonal manipulation for controlled reproduction.
Collection from the wild
Many small farmers of Latin America, in areas where wild bullfrog populations occur, collect tadpoles during the breeding season. Smaller tadpoles (<50 mm) are transported in oxygen-saturated water in plastic bags at 1 500-2 000/litre. Metamorphosing tadpoles are also collected. These are placed on soaked cotton sheets, and transported in perforated cardboard boxes at 250/m2 of box bottom area.
This consists of stocking adult males and females in breeding pens where practically no management other than feeding is practiced. Breeding pens are of two types. Asian farmers employ 10-20 m2 square concrete tanks, with 1 m walls and an 8-12 cm permanent water layer. Water flows through the tank continuously. Breeders are stocked at 1-5/m2 , and a sex ratio of 1 male: 1-3 females. Feeding consists of 40 percent protein floating pellets, provided once a day at 2.5-3 percent bodyweight/day.
Latin American bullfrog farms generally employ a semi-dry type of breeding pen. This consists of an area of variable dimensions (they range from 10 to 1 600 m2 ), constructed of cinder block or agricultural shade cloth. Some pens also have a top mesh to prevent avian predation. Between 75 and 90 percent of the total floor surface area of the pen is covered by grass, while the rest has shallow (<10 cm) concrete-lined puddles. These latter are used as spawning sites and can be small (1 m2 ), for one breeding couple at a time, or large enough for collective breeding (20-60 m2 ). Feeding points and shelters are also present. Feeding consists of a 40 percent protein pellet, fed once daily at 2.5-3 percent bw/day, together with 2-3 percent bw/day fly (Musca domestica) larvae.
Controlled reproduction involves the manipulation of ambient and water temperature, ambient humidity and photoperiod in indoor breeding facilities. Such strategies are usually sufficient to stimulate reproduction throughout the year, especially in thermally stable latitudes. However, hormonal stimulation is occasionally employed. Intensive bullfrog farms in Mexico and Guatemala employ indoor breeding facilities, which retain heat through a greenhouse effect. Buildings are divided into three sections:
Most farms use their tadpole rearing tanks to incubate fertilized spawns. However, some others have specific incubation facilities. These involve concrete buildings where eggs are placed in small (1 m2) plastic or fibreglass containers. Fertilized eggs are kept in a shallow (5-8 cm), static, chlorine-free, clean water layer in a shaded, ventilated (25-26 °C) site. No aeration is employed. Hatching takes place within 48 hours at 26 °C.
Once tadpoles reach the first-feeding stage, they are transferred to the tadpole rearing tanks. Phytoplankton inoculation is carried out for the first two weeks, and then supplementary feeding is provided 2-4 times a day. The recommended stocking density is 50/m2.
Water quality is maintained by running freshwater continuously through the tank (1-1.5 times/day), or through aeration. Culling is a routine procedure to lower intra-specific competition. Timing to metamorphosis varies greatly among individuals. Precocious tadpoles become froglets in 45 days, while most of the population achieves this in 90-120 days, and a small portion of the cohort (7-12 percent) will metamorphose after one year or more, even in tropical conditions.
Nursery tanks are only used in the wet, Mexican system. These are 5.0 x 2.5 x 0.40 m and attached to the tadpole rearing tanks (see photograph in the images gallery). Here froglets are trained to feed on inert, floating pellets. Through routine culling, weaker, slow-growing individuals are removed. Froglets are stocked at between 50 and 75/m2. A 2 cm water layer is maintained permanently. Young frogs are fed ad libitum at least 6 times a day with 1/16', 40 percent crude protein, floating crumbles, until they reach 30-45 g, which normally takes between 30 and 45 days at 26 °C.
Ongrowing systems can be classified into two major types: dry, and semi-dry and wet (inundated) systems.
These include the Brazilian designs mentioned in the following table.
Semi-dry and wet systems
This category comprises a number of systems employed in Southeast Asia, as well as some Latin American versions employed in Mexico, Guatemala, Argentina and Uruguay. The following table summarizes the main operational and productive characteristics.
Specific frog feed is used in some Asian countries, while Latin American farmers employ chiefly tilapia or trout feeds.
Once frogs reach market size, they are prepared for harvest. Frogs are starved for 24 hrs before harvest. In semi-dry systems, ongrowing rooms are drained and frogs are collected manually. Harvesting is carried out in much the same way in wet systems although, in some farms, iced water is pumped into the tanks to hypothermally anaesthetize the frogs to allow for better handling.
Processing of frogs is carried out in certified processing facilities. Frogs are hypothermally anaesthetized (12´at 4 °C) and then slaughtered. Processing includes bleeding, skinning and cutting off the lower limbs, which are then thoroughly cleansed using chlorinated water. Then the legs are individually bagged, frozen and packed by size in master boxes for transport. Frog legs are kept frozen at -15±2 °C which gives them a shelf life of up to 6 months.
Frogs for the live or scientific markets are manually rinsed and packed in 25-lb waxed perforated cardboard boxes. Live frogs are transported under cool and dry conditions by plane. Survival is usually up to 99 percent when the journey does not exceed 72 hrs.
The annual operating costs for a 36 tonnes/yr bullfrog farm (in Mexico) have been reported to total USD 67 560 (41 percent feed; 31 percent labour). Fixed investment costs were USD 130 000 (land USD 20 000; infrastructure USD 90 000; equipment USD 16 000; and initial broodstock USD 4 000).
The major disease problems affecting American bullfrogs are included in the table below.
In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.
Suppliers of pathology expertise
Assistance can be obtained from the following sources:
In 1980, it was estimated that 3 percent of the global market of frogs (all species) was supplied by aquaculture, while the current (2002) estimated contribution is 15 percent, taking into account the calculated growth rate of the industry.
Market statistics for frogs are scarce and unreliable. Some documented demand statistics place the United States of America as the highest consumer of frogs (all species), followed by France, Canada, Belgium, Italy and Spain.
There are three major market niches for bullfrogs, namely:
The demand for live frogs for food has increased especially amongst Asian groups living in Canada and the United States of America. Wholesaler prices range between USD 2.25 and USD 3.75/kg of whole frog, depending on weight. The main producers (capture and aquaculture) are Taiwan Province of China, Brazil and Mexico.
Live bullfrogs for educational or scientific purposes are sold in the United States of America to processing companies. Three size-categories are marketed: 4-5', 5-6', and 6-7' (S-VL); prices range from USD 1.30-3.25 each.
Continued research efforts focusing on nutrition, pathology and reproduction are expected to result in important improvements that will boost production in the future. However, there is still much to be done on genetics. Marketwise, prices will tend to gradually increase, as trading of wild-caught frogs becomes restricted, thus being replaced by farmed animals; nonetheless, important efforts should focus on marketing and advertising, since frog meat and its attributes are far from being widely known.
As in any other type of aquaculture, bullfrog farming can cause problems if not managed properly. The following table summarizes a series of externalities likely to be generated by frog farming, as well as the risks involved and some suggestions for their prevention or mitigation.
All countries request sanitary certification when buying frog legs but no CITES paperwork is required.
It is important to develop specific regulatory frameworks for bullfrog farming, as it is becoming an important aquaculture activity in many regions. It should also follow the general principles adopted by the Code of Conduct for Responsible Fisheries, Article 9 (see http://www.fao.org).
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