The relationship between biodiversity and ecosystem functioning (BEF) is a central topic in ecology. A better
understanding of its underlying mechanisms is helpful for biodiversity conservation. Using the trawl survey data of marine fish communities, we applied structural equation modelling to examine the existence of fish BEF relationships in Beibu Gulf. We found that ecosystem functioning, i.e., fish stock, was not related to fish richness, while community evenness was the significant factor. Specifically, high fish stock was associated with fish assemblages dominated by a few generalist fish species at high trophic levels, which can exploit both the benthic and pelagic energy source. Our study provides a deep understanding of the mechanisms determining marine ecosystem functioning and informs fishery management.