Trawling the archives: Long-term trends in fish taxonomic and functional diversity in UK coastal community

Effective marine conservation and fisheries management require long-term monitoring. Traditional metrics, such as species richness, abundance and diversity indices, are typically used to describe changes in community structure; however, these do not account for species' functional roles and their contribution to ecosystem processes. In this study, we investigated changes in functional diversity (FD) of fish species in a UK temperate coastal ecosystem at both the community and species levels from 1988 to 2023, using scientific trawl surveys. Species richness and functional evenness were found to have significantly increased over the study period, suggesting diversification and a more uniform distribution of functional traits. However, a decrease in functional redundancy was found, indicating a reduction of ecological buffering capacity, making the ecosystem more vulnerable to species loss. While functional richness increased with species richness, the temporal trends for functional richness, divergence, and dispersion remained stable, suggesting that new species contributed novel functions but did not expand the overall range of ecological roles. However, species-level changes in species size and abundance indicated potential shifts in ecosystem functioning that were not captured by community-level taxonomic or FD indices alone. Notably, several important commercial fish species have decreased in size but increased in abundance, while the size and abundance of medium-sized predators have increased, potentially reflecting mesopredator release. These changes in size and abundance may be knock-on effects from a combination of non-sustainable fishing practices and a loss of essential fish habitat. By characterising the fish community in functional trait space, this study provides a record of historical biodiversity trends in a temperate coastal ecosystem to support future monitoring in this region and similar coastal systems. The findings of this study highlight the importance of integrating FD metrics with traditional methods like species richness and abundance to better assess long-term changes in fish communities and inform conservation strategies.