River connectivity and its implications for freshwater fish conservationMethods of study in the Iberian Peninsula

Résumé

One of the main threats to riverine ecosystems is fragmentation due to the presence of man-made obstacles, such as dams and weirs. These structures completely isolate contiguous river segments and interrupt the natural movement of water, nutrients, sediment and organisms through the river network. During the 20th century dam construction surged to meet the growing demands of society. Nowadays, there are more than 45 000 large dams around the world. The effects of dams in freshwater ecosystems are not fully understood. Studies of barrier impacts in riverine ecosystems are usually local in scale or are focused on certain organisms. The Iberian Peninsula has more than a thousand large dams and countless weirs and small barriers. It also holds a large number of endemic freshwater fish species, many of them restricted to a single river basin. However, river connectivity studies in the Iberian Peninsula are scarce and the extent of fragmentation damages is not known. Knowledge and new methods are needed to reach efficient river management and conservation. The main objective of this thesis is to develop new tools to better analyse river fragmentation at the basin scale and for different species. These methods are applied to different Iberian river basins to improve the knowledge of river connectivity in the Peninsula. Moreover, the consequences of river fragmentation for fish species are studied using these new tools. In Chapter 1 I provide a general view of river fragmentation due to barriers, the conservation status of rivers globally and briefly describe the main impacts caused by dams. Chapter 2 provides a deep review of river fragmentation effects, the main methods used to study longitudinal river connectivity and the importance of river basin connectivity for conservation. The chapter then focuses on river connectivity studies in the Iberian Peninsula. Then, Chapters 3 and 4 are dedicated to the study of river connectivity for diadromous fish species. In Chapter 3 a new connectivity index is developed for Atlantic salmon (Salmo salar L.) in the Bidasoa River basin. This new index considers the quality of river segments for salmon spawning and fry development to adjust management and conservation planning. Then, Chapter 4 analyses the relationship between river connectivity improvements and the adult population of the Bidasoa basin using the connectivity index developed in Chapter 3. Chapter 5 describes a set of more than 100 000 Iberian freshwater fish records collected by the Department of Environmental Biology of the University of Navarra (previously Department of Zoology and Ecology) over the last 20 years. The set was uploaded to the Global Biodiversity Information Facility (GBIF) and analysed in the final chapters of this thesis. Chapters 6 and 7 are focused on developing connectivity indices for the study of potamodromous fish species. In Chapter 6 a potamodromous freshwater fish conservation value is added to a connectivity index. The new connectivity index highlights important segments for conservation and adds a new dimension for river connectivity management and restoration studies. Chapter 7 develops a simple method to study river connectivity for freshwater fish metapopulations. A river connectivity index is adjusted to study the connectivity of fish metapopulations. The index could be used to assess natural and artificial meta-population fragmentation. In the following chapter (Chapter 8) the improvement in river connectivity was assessed after more than 150 weirs were removed in Spain. Changes in fish community composition could not be studied as there was not enough fish data. The dam removals did not improve river connectivity significantly and neither prior nor subsequent ecological monitoring was performed. Finally, Chapter 9 contains a general discussion of this thesis and Chapter 10 gathers its main conclusions.