Fish composition and assemblage structure in three Eastern English Channel macrotidal estuaries: A comparison with other French estuaries

This study has analysed for the first time fish composition and assemblage structures of three small macrotidal estuaries of the Eastern English Channel (EEC) and has explored the influences of 19 biotic and abiotic variables on the fish assemblages. Fish from Canche, Authie and Somme estuaries were collected during spring (June 2006 and May 2007) and autumn (September 2006) along the estuarine gradients using a 1.5 m beam trawl. Using identical sampling protocols, the study also analysed and compared for the first time taxonomic and functional aspects of the fish assemblages in 15 estuaries located along the Atlantic and English Channel coasts. SIMPER analysis showed high similarities in fish assemblages in the three EEC estuaries and during either spring or autumn periods. However, intra-estuary similarities were relatively low, indicating that fish assemblage structures (species richnesses or abundances) were more variable within the estuary (salinity gradient) than between estuaries and/or seasons (spring vs autumn). Although numerous environmental variables were included in the study, only 47% of the variability observed in the fish distribution was explained. Fish spatial variations in the EEC estuaries are mostly driven by abiotic variables as opposed to biological interactions. As indicated by CCA, salinity and muddy sediments were the two most important factors structuring the fish assemblages. The macrobenthos being very abundant in the EEC estuaries (580–1121 ind. m⁻²), the availability of potential prey is probably not a limiting factor in the utilization of estuaries by fish. Contrary to the majority of French estuaries dominated by estuarine species (ES), the fish assemblages of the EEC estuaries are clearly dominated by marine migrant (MM) species (65% on average) with high abundance of juveniles (mostly young-of-the-year). Cluster and SIMPROF's analyses distinguished the functional structure of the 15 estuarine fish assemblages into different clusters. The three EEC estuaries form a similar group with the largest French estuaries (Seine, Loire and Gironde). The latters were characterized by higher MM/ES ratio compared to the other French estuaries. Freshwater flow and tidal range were the main factors associated with this group of estuaries. The results of the present study do not indicate that anthropogenic impacts (chemical contamination) on estuaries affect their ecological functioning as described by the fish ecological guilds. It is suggested that the guild approach may not be useful to provide valuable information on the ecological status of estuaries.     

Anthropogenic disturbance on nursery function of estuarine areas for marine species

Estuaries serve as nursery grounds for many marine fish species. However increasing human activities within estuaries and surrounding areas lead to significant habitat loss for the juveniles and decrease the quality of the remaining habitats. This study is based on the data of 470 beam trawls from surveys that were conducted in 13 French estuaries for the purpose of the European Water Framework Directive. It aimed at testing the effects of anthropogenic disturbances on the nursery function of estuaries. With a multispecific approach based on ecological guilds, two fish metrics, abundance and species richness of Marine Juvenile migrant fishes, were used as proxies for the estuarine nursery function. Indices of heavy metal and organic contaminations were used to estimate anthropogenic disturbances impacting these estuaries. Fish metrics were described with statistical models that took into account: (a) sampling protocol, (b) estuarine features and (c) contamination. The results of these models showed that the fish metrics highly depend on the sampling protocol, and especially type of gear, depth and salinity, which highlights the necessity of considering such metrics at the sampling (trawl haul) scale. Densities and species richness of Marine Juvenile fishes appeared to be strongly and negatively correlated to contamination indices. These results are consistent with the hypothesis that human disturbances impact the nursery function of estuaries. Finally, the densities of Marine Juvenile migrant species appeared as a potential robust and useful fish indicator for the assessment of the ecological status of estuaries within the Water Framework Directive.