water Article Variability in Population Traits of a Sentinel Iberian Fish in a Highly Modified Mediterranean-Type River Ana Sánchez-Pérez 1, * , Francisco J. Oliva-Paterna 1 ,Fátima Amat-Trigo 2 and Mar Torralva 1   Citation: Sánchez-Pérez, A.; Oliva-Paterna, F.J.; Amat-Trigo,F.; Torralva, M. Variability in Population Traits of a Sentinel Iberian Fish in a Highly Modified Mediterranean-Type River. Water 2021, 13, 747. https:// doi.org/10.3390/w13060747 Academic Editor: Heiko L. Schoenfuss Received: 9 February 2021 Accepted: 3 March 2021 Published: 10 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Departamento de Zoología y AntropologíaFísica, Facultad de Biología, Campus de Espinardo, Universidad de Murcia, 30100 Murcia, Spain; fjoliva@um.es (F.J.O.-P.); torralva@um.es (M.T.) 2 Department of Life and Environmental Sciences, Faculty of Science and Technology, Bournemouth University, Poole, Dorset BH12 5BB, UK; fatima.amat@um.es * Correspondence: ana.sanchez15@um.es Abstract: Human pressures on water resources have been suggested as a driver of biological traits that induce changes in native fish populations. This study highlighted the interplay between envi- ronmental stress factors, mostly related to flow regulation, and the longitudinal river gradient in biological traits such as the growth, size structure and somatic condition of a sentinel fish, Luciobarbus sclateri. We found an increase in size-related metrics and somatic condition at population levels associated with downstream reaches, although fragmentation and habitat alteration, flow regime alteration and the abundance of non-native fish were also significantly involved in their variability. Age-related parameters and growth were only explained by flow regime alterations and the abun- dance of non-native fish species. The high plasticity observed in L. sclateri population traits suggests that this is a key factor in the species adaptability to resist in a strongly altered Mediterranean river basin. However, the interplay of multiple stressors plays an important role in fish population dynamics and could induce complex responses that may be essential for long-term monitoring in sentinel species. Keywords: sentinel species; longitudinal gradient; human impacts; flow regime alteration; non- native fish; fragmentation; habitat alteration; Mediterranean rivers; Segura River basin 1. Introduction Freshwater ecosystems are considered among the most altered as a consequence of the historical pressure of human activities [1,2]. Hydraulic management to take advantage of water resources and the effects of climate change are inducing quantitative and qualitative changes in river systems [3,4]. These changes imply hydro-morphological, chemical and biological alterations which affect the freshwater fauna [2,5]. Hydraulic management is especially intense in Mediterranean regions where water resources are scarce [6]. Rivers in semi-arid regions, such as the Iberian Peninsula, are heavily impacted by the construction of a large number of dams and weirs [7,8]. Mediterranean regions are characterized by marked seasonality and inter-annual variability with severe periods of floods and droughts [9,10]. The native freshwater fauna is adapted to such natural variability and displays great resistance and resilience [11–13]; however, it is considered especially sensitive to human impacts [14,15]. Human pressures are particularly severe in Mediterranean regions and they coincide with high natural variability, causing severe alterations to fluvial ecosystems [8,16,17]. Despite the high adaptability of freshwater fauna, the expected increase in human impacts under future sce- narios of global climate change could increase its vulnerability to such pressure, especially in Mediterranean regions [11,18,19]. The flow regime is considered one of the main driving forces of freshwater ecosystems, determining the structure and ecological dynamic of rivers [19,20]. In the Iberian Peninsula, the alteration of the natural flow regime by dam regulation is one of the most important Water 2021, 13, 747. https://doi.org/10.3390/w13060747 https://www.mdpi.com/journal/water