Population Size and Habitat Use by Tachypleus tridentatus in the Taiwan Straits

The adult population of Tachypleus tridentatus on the main island of Taiwan has long been considered extinct. However, it remains unclear whether this apparent absence reflects true extirpation or simply a lack of sustained large-scale monitoring. To address this knowledge gap, a multiyear field investigation was conducted from 2018 to 2024 across three key subregions of the Taiwan Strait, namely, Kinmen, Penghu, and mainland Taiwan. Mark-recapture methods were used to estimate the adult population size using two statistical approaches. Population size estimated from the Schnabel method presented 73,629 (95% CI: 59,555-96,415) in Kinmen, 1551 (968-3899) in Penghu, and 1099 (623-4627) in Taiwan main island. Multinomial N-mixture models showed a population size of 65,428 (63,704-67,156) in Kinmen, 3193 (2818-3588) in Penghu, and 1198 (387-2269) in Taiwan main island. Species distribution modeling (MaxEnt) and connectivity network analyses showed the major habitats in the Taiwan Bank and Changyun Ridge. Recapture records from five tagged individuals supported the predicted dispersal pathways, with the longest recorded straight-line movement exceeding 110 km, showing ecological connectivity within the Taiwan Strait. Based on these findings, three key conservation strategies are proposed, namely, implementation of standardized long-term monitoring, habitat restoration and stock enhancement efforts, and preparation for potentially assisted migration under future climate scenarios. The findings have provided a detailed assessment of adult T. tridentatus populations in the Taiwan Strait and offer actionable guidance for regional and international conservation planning in the context of environmental changes.