Freshwater fish in a highly regulated Mediterranean basinbiological responses to environmental stressors and to fishways implementation

Abstract

Rivers are considered among the most impacted ecosystems worldwide due to the intensification of the demand of water and resources associated with freshwater ecosystems. Artificial barriers created to make use of water resources are responsible of habitat fragmentation, thus modifying the natural dynamics of rivers and communities. The flow regime alteration as a consequence of flow regulation infrastructures modifies the environmental conditions, causing severe degradation of the habitat and water quality and favouring the establishment and spread of exotic species, which cause changes in biological communities. In semi-arid Mediterranean regions such as the Iberian Peninsula, the effect of human impacts on freshwater systems is more intense. This thesis has been conducted in the context of the Segura River basin, placed in a geographical area considered among the most arid Mediterranean regions, and where human impacts on rivers entail multiple environmental stress factors on their fish assemblages, mainly derived from flow regulation. The contents of this PhD thesis are especially relevant since they deal with scarcely studied topics in Mediterranean rivers, also very rarely evaluated in the field and considering fish assemblages dominated by non-native species. The aim of this thesis is to increase knowledge on the response of river fish assemblage, and also its sentinel species, to multiple environmental stressors along the longitudinal gradient of the Segura River basin. In addition, the implementation of fish passage facilities in the context of LIFE+ Segura Riverlink project to improve longitudinal connectivity, also allows us to evaluate the use and ecological relevance of different types of fishways in a highly fragmented river sector. The results confirm that the magnitude and interaction of the environmental stress factors present in the Segura River basin determine the composition and structure of the fish assemblage. The flow regime alteration is confirmed as a key factor structuring the fish assemblage in river reaches and, jointly to other stressors such as habitat alteration, fragmentation and non-native species, it causes the fish assemblage homogenization by losing originality and specialization. Furthermore, these stress factors conditioned population variability of Luciobarbus sclateri despite the high plasticity found in this endemic species, considered as a sentinel fish in Mediterranean rivers. On the other hand, as a result of fishways evaluation, the use of all the infrastructures by the fish assemblage is confirmed, mainly by potamodromous species during the movement period, which confirms their functionality as migration corridor. But the fishways were also used by other species as new compensatory habitats where they can find refuge and food resources. Even so, the results of this study point to nature-like fishways, whose design better reproduces natural conditions of river reaches, as the best option to implement in Mediterranean rivers with strong flow fluctuations, since they also limit the passage of invasive species. Additionally, technical types are also confirmed as a successful restoration tool, which can also be used as a control trap for non-native species. In short, the results of this thesis improve knowledge on biological responses of fish species inhabiting a strongly impacted river area, which may be useful for the implementation of successful conservation and restoration measures.