Spatially explicit conservation planning for reef fish: mapping ontogenetic habitat hotspots to guide reserve design in the southern Penghu Islands

Context Marine reserve design in the Pacific region must address ecological requirements of key fishery species and balance conservation with local resource use. In the southern Penghu Islands, Taiwan, effective protected area planning is challenged by limited information on fish habitat preferences and spatial distribution across life stages. Aims We aimed to identify abundance hotspots and ontogenetic shifts in habitat use for economically important reef fish, to inform targeted conservation strategies and adaptive management. Methods Underwater video surveys were conducted across four islands, focusing on three families targeted by local fisheries – groupers (Epinephelidae), parrotfish (Labridae: Scarinae), and snappers (Lutjanidae). Bayesian models quantified habitat selection patterns for juveniles and adults. To overcome local constraint, these preferences were integrated with satellite-derived benthic maps to spatially project a Habitat Suitability Index (HSI) for critical nursery and adult habitats. Key results Mean fish density declined by 50% from 2013 to 2015, highlighting an urgent need for management interventions. Fish species exhibited pronounced ontogenetic shifts, with juveniles primarily occupying algae-dominated habitats. Habitat-constrained HSI mapping revealed significant overlap between key nursery grounds and areas currently outside no-take zones. Conclusions The spatial segregation of nursery and adult habitats, combined with observed population declines, highlights the need for spatially explicit reserve boundaries and stage-structured management in island reef systems. Implications Expanding no-take areas to protect identified abundance hotspots and establishing dedicated nursery habitat protection are recommended to enhance ecological outcomes and support long-term fishery sustainability.