Persistent Decline of the Piscivorous Cyprinid, <i>Opsariichthys uncirostris</i> , in Lake Biwa Despite Recovery Trends Among Other Native Species

ABSTRACT Freshwater ecosystems are increasingly recognised as highly susceptible to anthropogenic disturbances. Because of the challenges associated with accurate population assessment, local fishery records are often utilised as proxies to infer fish population trends. However, for species with limited market size and lower commercial value, fishery records may not reliably reflect true population dynamics. This study aims to evaluate long‐term population trends of three‐lips ( Opsariichthys uncirostris ) using shore‐based lure angling surveys and to assess how recent climatic changes, particularly rising water temperatures and declining water levels, may be contributing to its continued decline along the western shore of Lake Biwa from 2011 to 2023. The results indicated a sustained decline in detection probability over the past 13 years, including in habitats with suitable substrate conditions, such as gravel or sand lake bottoms. This decline could be associated with higher water temperatures and lower water levels, especially in summer and the reproductive season. The use of shore‐based lure angling provided an alternative assessment approach independent of fishery catch data. This approach corroborated the interpretation that the long‐term reduction in fishery catches reflects a real population decrease. Although the mechanisms underlying the continued population decline remain unclear, the persistently low detection probabilities represent a critical conservation concern, particularly along the northwestern shore, where individuals intensively migrate into tributaries for reproduction.