Onilahy River

Onilahy is a river in Atsimo-Andrefana and Anosy (Toliara Province), southern Madagascar. It flows down from the hills near Betroka to the Mozambique Channel. It empties at St. Augustin (23°34′00″S 43°45′00″E / 23.5666667°S 43.75°E / -23.5666667; 43.75Coordinates: 23°34′00″S 43°45′00″E / 23.5666667°S 43.75°E / -23.5666667; 43.75), and into the Bay of Saint-Augustin.

Onilahy River
JNB-MRU 22.08.2009 13-16-31.jpg
Onilahy River estuary
Madagascar rivers.svg
Map of Malagasy rivers (Onilahy flows from the south-central part to the southwestern coast).
regionAtsimo-Andrefana, Anosy
CitiesMandrofify, Ambolofoty
Physical characteristics
 • locationnear Betroka
MouthBay of Saint-Augustin
 • location
Saint Augustin, Mozambique Channel
Length525 km (326 mi)
Basin features
Protection status
Official nameZones humides de l'Onilahy
Designated22 May 2017
Reference no.2304[1]

Two species of cichlids are endemic to the river basin, but Ptychochromis onilahy is probably already extinct[2] and the remaining range of Ptychochromoides betsileanus covers less than 10 square kilometres (3.9 sq mi).[3][4]

Onilahy Bassin


Sources of the Onilahy river are near Beadabo. It flows through Ankilimary, to Benenitra, Ehara, Bezaha and Antanimena. It is crossed by the RN 10 near Tameantsoa. The mouth of the Onilahy river is situated in the Indian Ocean at Saint Augustin, Madagascar, 35 km south of Toliara (Tuléar).

Its main affluentes from its south are Sakamena river, Evasy, Ianapera river, Isoanala river and Ihazofotsy. From the north these are Sakondry, Taheza, Sakamare and the Imatoto river.


  1. ^ "Zones humides de l'Onilahy". Ramsar Sites Information Service. Retrieved 25 April 2018.
  2. ^ Stiassny, M., and Sparks, J. S. (2006). Phylogeny and Taxonomic Revision of the Endemic Malagasy Genus Ptychochromis (Teleostei: Cichlidae), with the Description of Five New Species and a Diagnosis for Katria, New Genus. American Museum Novitates 3535.
  3. ^ Ravelomanana, T.; Sparks, J.S. (2020). "Ptychochromoides betsileanus". IUCN Red List of Threatened Species. 2020: e.T18832A177066640. doi:10.2305/IUCN.UK.2020-3.RLTS.T18832A177066640.en. Retrieved 12 November 2021.
  4. ^ Walaszczyk, I.; Marcinowski, R.; Praszkier, T.; Dembicz, K.; Bieńkowska, M. (August 2004). "Biogeographical and stratigraphical significance of the latest Turonian and Early Coniacian inoceramid/ammonite succession of the Manasoa section on the Onilahy River, south-west Madagascar". Cretaceous Research. 25 (4): 543–576. doi:10.1016/j.cretres.2004.05.001. ISSN 0195-6671.