Rediscovery and Habitat Assessment of the Endemic Philippine Medaka (Oryzias luzonensis) in Northern Luzon

This is a Preprint and has not been peer reviewed. This is version 1 of this Preprint. You must log in to post a comment. There are no comments or no comments have been made public for this article. This is a Preprint and has not been peer reviewed. This is version 1 of this Preprint. Add a Comment You must log in to post a comment. Comments There are no comments or no comments have been made public for this article. The endemic Philippine medaka Oryzias luzonensis, confined to the Laoag (Padsan) watershed in Ilocos Norte, Northern Luzon, had not been recorded in the wild for over 40 years, prompting its classification as Endangered by the IUCN in 2021. Field surveys conducted from January to May 2024 across six sites using traditional push nets successfully rediscovered the species in three Solsona municipality sites, yielding 54 specimens (32 at Maananteng, 12 at Aguitap, 10 at San Juan; mean length 2.52 ± 0.21 cm). No individuals were found in three Dingras sites, which exhibited moderate to severe habitat degradation from agricultural intensification, sand and gravel mining, and channelization. Habitat survey showed significant environmental differences among sites for temperature and turbidity (F₅,₁₂ = 28.7, p < 0.001), with O. luzonensis presence associated with cooler, clearer waters. Only juvenile and subadult specimens were captured, suggesting potential recruitment limitations. This rediscovery confirms species persistence but highlights extreme vulnerability, necessitating immediate habitat protection and invasive species control to prevent extinction of the Philippines' sole medaka species. https://doi.org/10.32942/X2BH18 Biodiversity, Life Sciences Endemic fish, conservation, freshwater ecosystems, Philippines, habitat degradation, Oryzias Published: 2025-07-28 16:20 Last Updated: 2025-07-28 16:20 CC BY Attribution 4.0 International Language: English