Declining fish sizes across the Lower Mekong Basin highlights urgent conservation needs

Fish body size is a key indicator to understand the status of fish stocks and thus necessary for fisheries management and conservation planning. However, changes in fish body size remains poorly understudied particularly in the Mekong Basin. Here, we examine the disparity between observed maximum total length ( TL max ) obtaining from the 7-year daily fish catch monitoring and literature-based TL max in FishBase for 257 fishes of the Lower Mekong Basin. We found a significant decrease in the observed TL max compared to literature-based TL max in FishBase, and this decrease was significantly more pronounced in large-bodied fishes and was substantially higher for endangered and critically endangered species. Our result also revealed a decrease in the fish body size belonging to the IUNC ‘Not Evaluated’ category. The difference between observed and literature-based TL max was also significantly associated with higher trophic position fishes. Moreover, large-bodied and threatened fishes observed in the observed fish catches were much smaller than their literature-based TL max , whereas small-bodied and least-concerned species showed relatively stable sizes compared to FishBase. The decrease in the TL max of large-bodied and threatened fishes is primarily attributed to indiscriminate fishing, and further exacerbated by environmental changes such as hydropower dam development, land use change and climate change. This evidence highlights the urgent need for conservation actions to protect the Mekong's large-bodied and threatened fishes. Further, species belonging to the IUCN ‘Not Evaluated’ category deserve more attention regarding their threat and conservation statuses so that appropriate actions are taken to sustainably manage their populations. • Fish body size is a key indicator for the health of fisheries and fish populations. • There is a decrease in observed lengths vs literature lengths of Mekong fishes. • Size decrease is pronounced in large-bodied and threatened fish species. • Lengths of higher trophic position fishes are shorter than literature lengths. • Fish sizes of ‘Not Evaluated’ in IUCN are decreasing, requiring research attention.