The giant fish Arapaima gigas is one of the most emblematic of these. Better known by the name of paiche in Peru or pirarucu in Brazil, it is one of the word’s largest freshwater fish. Certain specimens can reach 4 metres in length for a weight of 200 kg. However, this graceful predator is a victim of overfishing, and is now on the IUCN Red List of Threatened Species1.
Catches in the wild have been declining substantially and fish farming is gradually being developed in the Brazilian and Colombian Amazon to alleviate this. Captive reproduction is one of the main problems hampering aquaculture of Arapaima notably because it is almost impossible to distinguish the sex of the reproductive adults.
A new, practically non-invasive, method based on testing of a straightforward blood sample, enabled an IRD team and its Peruvian partner2 to determine accurately the sex of 100% of nearly 30 specimens. The technique should help improve breeding programmes of this species and at the same time facilitate study and monitoring of wild populations to improve conservation and management strategies.
Arapaima gigas is a giant air-breathing fish, emblematic in the rivers of South America. Known to Brazilians as pirarucu, it inhabits natural lagoons and low-current reaches of Amazonian rivers and is the largest freshwater fish of the South American continent. At adult stage, some specimens can reach several hundred kilograms for a length that sometimes exceeds four metres. This fish with tasty bone-free meat has been the target of intensive fishing since the early XVIIIth Century. One century ago, over 1 200 tonnes of Arapaima was landed each year in the Brazilian port of Belém alone. In 2006, declared catches for the whole of the Amazon Basin reached only 380 tonnes. Today, it is estimated that the natural population of Arapaima gigas ranges between 50 000 and 100 000 individuals. The species has been put on the IUCN Red List of Endangered Species, and fishing is now strictly regulated. Yet commercial demand is constantly on the increase and, in order to meet this, aquaculture stations for the species are starting up in the Peruvian and the Brazilian Amazon.
However, in terms of yield, rearing arapaima remains an uncertain operation owing to their specific behavioural and physiological characteristics. The species’ territorial behaviour, geared to building a nest which is guarded tenaciously to keep congeners at a far distance, necessitates the construction of quite large, spacious ponds. Another problem is that determination of the sex of individuals is almost impossible outside the reproductive period.
Experiments conducted by an IRD team have now shown sexing of Arapaima to be possible by means of a blood sample. The technique has the advantage of being practically non-invasive: it does not require anaesthesia, which often endangers fish survival. The blood sample was used to determine plasma levels of the hormones 17-ß oestradiol, whose concentration is higher in the female, and 11-ketotestosterone which is more abundant in the male. Also tested was the efficacy of another sexing method, based on detection and measurement of concentrations of the protein vitellogenin. Females synthesize this in the liver when they reach sexual maturity. It circulates in the blood stream as far as the ovary where it accumulates in oocytes to form the vitellus or yolk material. Determination of the hormones gave a 95% success rate for accurate sexing of 29 adult fish. The rate even reached 100% for the 10 immature specimens studied. With the vitellogenin test, applicable only to adults, the researchers accurately sexed 100% of the cohort. These results asserted the efficacy of the two methods for sexing Arapaima gigas. Fish farmers who embarked on rearing of this species have hitherto had no means of distinguishing the sex of each specimen. Their strategy consisted of introducing an abundant stock into a vast pond. This empirical method resulted in the first male-female pairs chasing the other fish off to the periphery of their territory and the fish farmer rarely obtained more than one reproduction batch per hectare of pond.
Previous studies, however, had shown that in captivity, a surface area of 500 m2 was enough for a Arapaima pair to establish its territory. With prior knowledge of the sex of the fish, determined by the technique developed by the IRD researchers, it is now possible to introduce equal numbers of male and female individuals into smaller ponds in order to attain much greater reproduction rates. Management and control of rearing of the Amazon’s giant fish improved in this way could find worthwhile commercial application for countries such as Brazil, Colombia or Peru. One breeding pair can produce, on average, 4000 fry per reproduction. With weight development of juveniles reaching 10 kg from the first year, it is therefore possible to produce 40 tonnes of fish starting from just one pair, making the aquaculture operation of this freshwater fish one of the most viable in the world. As well as commercial openings, the sexing methods tested by the IRD team hold potential as tools for conservation of wild populations of Arapaima. In the Peruvian Amazon, South of the city of Pucallpa, a restocking programme using captive-bred individuals is about to be launched in Lake Imiría which now probably harbours no more than a few tens of specimens in an area of over 5 000 ha. Prior sexing of wild individuals could make reintroduction using reared stock possible, with account taken of the male/female balance, thus optimising the reproduction capacity of the newly constituted population.
Grégory Fléchet – DIC
Grégory Fléchet | alfa
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