Historical distribution of freshwater fishes and the reference conditions concept in a large Mediterranean basin

Abstract

The European Water Framework Directive 2000/60/EC (WFD) defines the ecological status of aquatic systems as the deviation between their present biological state and that which would be expected in the absence of any major human disturbance, referred to by the WFD as the ‘reference condition’ (RC). It assumes that their biotic composition should remain balanced and constant over time. This study tested both assumptions against an analysis of the historical distribution of the fish fauna in a large and highly disturbed Mediterranean basin. If fish communities change substantially over time, it will mean that the validity of the RC concept comes into question. Using presence/absence data for historical native fish fauna from the Guadalquivir Basin, distribution changes among sub‐basins were quantified by mapping between the nineteenth century and today. The range of two native species (Anguilla anguilla and Salmo trutta) has changed significantly. In addition, the diadromous species assemblage has almost become locally extinct, with the exception of the eel. Finally, most Guadalquivir sub‐basins (94.7%) have suffered major changes in the composition of their fish communities, either by losing native species or by adding new non‐native species. These results render the definition of any RC unlikely. In Mediterranean areas, the WFD objective of ‘good ecological status’ recovery based on the integrity of aquatic communities is a theoretical rather than a real goal. Nonetheless, the WFD provides an ecological guiding principle that can also be transferred to the conservation of freshwater ecosystems. As an alternative to the RC concept in Mediterranean lotic ecosystems, specific multimetric indices can be used, based on expert criteria, the metrics of which can also relate to the conservation value of water bodies, and not only to their ecological status.

Shedding light on the decline of Iberian freshwater fish species over the period 1980–2020

“…In contrast, the number of alien species notably increased. This trend affecting the Iberian Peninsula has already been documented in other studies (Aparicio et al, 2000;Ramos-Merchante et al, 2021). A similar pattern was also observed in other rivers with Mediterranean-type climate (Clavero et al, 2010;Habit et al, 2010;Haubrock et al, 2021), and some authors identified a global trend (He et al, 2019;Seebens et al, 2017;Su et al, 2021).…”

Macrophyte communities as indicators of the ecological status of drainage canals and regulated rivers (Eastern Poland)

“…Infrastructure development in river valleys causes severe modification of river channel form and processes. The immediate hydrological alterations caused by dams and flow regulation alter the form of river channels and thus the composition and extent of aquatic vegetation (Bejarano et al, 2018 ; Jansson et al, 2000 ; Jones et al, 2020 ; Merritt & Wohl, 2002 ; Ramos-Merchante et al, 2021 ; Riis & Sand-Jensen, 2001 ; Tombolini et al, 2014 ). Regulated rivers usually have significantly lower richness (Nilsson et al, 1997 ) and different floristic composition, with a higher number of sporadic species and annuals (Bejarano et al, 2020 ; Netten et al, 2011 ).…”

Historical cartographic and topo-bathymetric database on the French Rhône River (17th–20th century)

“…It provides data on (i) the morphological pattern of the Rhône and its former branches, (ii) the type of land cover on the floodplain, and (iii) topo-bathymetric information (Bravard, 2010). Most diachronic studies have used this 1860s map series as a starting point for detecting geomorphic changes by making comparisons with later maps surveyed at the end of the 19th century and mid-20th century and aerial images taken since the 1930s (Bravard et al, 1986;Bravard, 2010;Provansal et al, 2014;Parrot, 2015;Dépret et al, 2017;Räpple, 2018;Thorel et al, 2018;Tena et al, 2020). Therefore, the post-1860 planform adjustments are fairly well understood and are combined with vertical adjustments measured on reaches where topo-bathymetric information has been available since the late 19th century.…”