In considering the use of estuarine habitats by fish assemblages, alternative views of the assemblage are increasingly being explored, based on functional rather than taxonomic aspects. The allocation of all taxa to a number of functional guilds allows a description of fish assemblages in terms of vertical zonation, habitat preferences, including the substratum preference of benthic/demersal species, and dietary preferences. This paper presents the first comparison of the structure of the fish assemblages in the tidal marshes of 6 European estuaries (Bay of Cadiz, Spain; Mira, Portugal; Loire, France; Westerschelde, The Netherlands; Humber, UK; Forth, UK). Cluster analysis was used to compare the assemblages recorded in 29 tidal marsh and associated estuary reference sites. Distinct differences between estuaries were apparent in comparing the taxonomic structure (species, family) of the assemblages, indicating a division between assemblages of northern and southern Atlantic European tidal marshes. The use of ecological guilds successfully separated marsh assemblages dominated by freshwater adventitious/diadromous taxa (Loire) from those dominated by marine juvenile taxa (Mira, Forth). Comparisons of vertical distribution guilds indicated a major division between pelagic-dominated (Forth, upper Loire) and benthic-dominated (Cadiz, Westerschelde, Mira, Loire) assemblages. This division was also partly reflected by the dietary preference guilds, with a strong separation between assemblages dominated by planktivores (Forth) and those in which most taxa are either invertebrate feeders (Humber, Cadiz, Westerschelde) or invertebrate/fish feeders (most Mira sites, lower Loire). The very low representation of a primarily piscivorous guild in all sites (in contrast to the fish assemblages of the main estuaries) supports the view that European tidal marshes contain fish assemblages with high proportions of juvenile individuals and may provide significant refugia for life stages vulnerable to predation. Although the comparability of datasets is limited by differences in sampling regimes and methodologies, the use of functional guilds, reflecting different aspects of the use of the marshes, reveals that functional similarities exist between the fish assemblages of European tidal marshes, despite considerable taxonomic, physical and chemical differences between the sites. Further modifications to the use of functional guilds are discussed, and recommendations made to refine their value for studying fish assemblages in tidal marsh environments. |
