The Amazon and its tributaries are home to almost one tenth of
the world’s freshwater fish biodiversity, 2500 species being
recorded. The giant fish Arapaima gigas is one of the most
emblematic of these. However, this graceful predator is a victim
of overfishing, and is now on the IUCN Red List of Threatened
Species. 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 partner 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 ma