Identifying functional homogeneity in a dynamic environment: Application to soft-substrate fish assemblages off the Portuguese coast

In fish community assessment, measures of functional integrity have shown to be the most suited to detect anthropogenic impacts in a background of natural variation. However, minimising background noise involves the establishment of homogeneous areas, which is still mostly done based on taxonomic approaches or by relying on abiotic factors. The present study proposes a framework to identify functional homogeneity in demersal soft-substrate fish assemblages along the Portuguese coast, by relying on functional metrics such as trophic guild composition. Using metric pairwise constrained k-means (MPCK-means), sampling stations were clustered based on individual categories of functional metrics, while taking into account the minimum achievable homogeneity contained in 5 years of bottom trawl surveys. All individual clustering solutions resulting from different functional categories were ultimately combined into a final set of seven assemblage types using an ensemble clustering algorithm. Clustering patterns found were linked to depth, substrate type and latitudinal variation. Biomass-based metrics performed better in comparison to abundance-based metrics, but the combination of both achieved the best results in the definition of a final set of seven assemblage types along the coast.