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

Brian Wade Jamandre , Nico Jose Leander, Faith Santiago-Tadeo, Andie John Tadeo, Marvin Aquino, Rodolph Charles Rosel, Christian Molina, Shara Mae Puh-hay, Bryan Baldemor, Marycris Fiatacag

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.