Sampling intensity influences the estimation of functional diversity indices of fish communities

•Sampling intensity could influence the estimation of FD (functional diversity) indices.•The accuracy and precision of FD indices estimates increased with sampling intensity.•Functional richness and divergence indices were less sensitive than evenness indices.•The effects showed an obvious seasonal variability due to fish migration. Functional diversity has emerged as a key component of biodiversity in recent two decades, which has been extensively used to assess the health of aquatic ecosystems and predict the responses of fish communities to disturbances. However, sampling intensity may affect the estimation of functional diversity indices and thus lower the validity of functional diversity indices in monitoring long-term changes in ecosystem status. In this study, we used a simulation study to investigate the effect of sampling intensity on the estimation of functional diversity indices of fish communities in the Haizhou Bay, China, based on thirteen fish functional traits that reflect the characteristics of feeding, habitat use, swimming behavior, ecological adaptation, reproduction and life history. In general, sampling intensity influenced the estimation of functional diversity indices, and the accuracy and precision of the estimation of functional diversity indices increased with the sampling intensity. Functional richness and functional divergence indices were less sensitive to sampling intensity than functional evenness indices. The effects of sampling intensity on the estimation of functional diversity indices also showed an obvious seasonal variability, reflecting seasonal differences in trait distribution of composite fish species. This study indicates that it is essential to evaluate and identify the seasonal sampling intensity required in monitoring programs, especially for long-term monitoring program of fisheries ecosystems.