Enric Aparicio

Brown trout (Salmo trutta) populations at the southern edge of the species’ distribution are increasingly exposed to interacting climatic, biotic, genetic, and anthropogenic pressures. This study provides an integrative assessment of the drivers of variation in brown trout density, biomass, and size structure across six Mediterranean river basins in Catalonia (NE Iberian Peninsula), based on long-term standardized electrofishing surveys (2016–2025; 88 sites). Generalized linear mixed models revealed that mean summer temperature, genetic introgression from hatchery-derived Atlantic lineages, and the density of introduced fish species were the most consistent negative predictors of total density, biomass, and juvenile abundance (220 mm FL), with lower values in harvest-allowed sections, whereas total density was less responsive to regulation. These findings indicate that Mediterranean brown trout populations are primarily constrained by thermal conditions, genetic integrity, and biological invasions, with implications for conservation and fishery management under ongoing climate warming.

Freshwater ecosystems in the Mediterranean region are highly vulnerable to biological invasions, yet invasive macroinvertebrates remain less studied than other taxonomic groups. In Catalonia (NE Spain), non-native freshwater molluscs are increasingly widespread and may substantially alter ecosystem functioning. This study analyses the spatiotemporal expansion and environmental drivers of four invasive freshwater molluscs (Physella acuta, Potamopyrgus antipodarum, Corbicula fluminea and Dreissena polymorpha) across the Catalan River Basin District. Species occurrence data were compiled from regional and global biodiversity databases and long-term bio-monitoring programmes, harmonised and aggregated into a 10 x 10 km grid. Temporal trends in cumulative occupancy were used to quantify expansion rates, while Generalized Linear Models were applied to identify key environmental and anthropogenic correlates of species occurrence. Gastropods (P. acuta and P. antipodarum) exhibited rapid and extensive range expansion since the mid-2000s, whereas bivalves showed limited or negligible spread. Expansion patterns were partly influenced by increased sampling effort following the implementation of the EU Water Framework Directive. Species distributions were consistently associated with low-altitude, low-slope river reaches subject to high anthropogenic pressure. Physella acuta was strongly linked to degraded lowland habitats, C. fluminea to large, hydrologically stable river sections, and P. antipodarum was primarily constrained by channel slope. These results reveal a longitudinal zonation of invasion risk in Mediterranean rivers and highlight the combined role of hydromorphology and human pressure in shaping invasion dynamics. Maintaining long-term monitoring and protecting headwater and mid-mountain reaches are essential to limit further spread and safeguard native freshwater biodiversity.

The Mediterranean brown trout ( Salmo trutta species complex) comprises genetically and ecologically distinct lineages primarily distributed across the Western Mediterranean region. Conservation of the Mediterranean brown trout is complicated by unresolved taxonomy. Although some populations have recently been recognised as distinct species, most—including those in the Iberian Peninsula—remain classified under the nominal S. trutta , which is currently listed as Least Concern by the IUCN. This taxonomic ambiguity masks severe conservation threats, particularly the widespread genetic introgression from stocked Atlantic‐origin trout. In this study, we assessed introgression patterns, environmental drivers and conservation implications for Mediterranean brown trout populations in Catalonia (northeastern Iberian Peninsula). Genetic data from 134 sites were analysed using microsatellite markers and LDH‐C1* allele frequencies, and the influence of historical stocking intensity and climatic variables on introgression was evaluated using hierarchical partitioning and regression models. Our results revealed extensive introgression, with only 10.4% of the populations retaining genetic purity. Stocking intensity was the primary predictor of introgression, followed by temperature and precipitation seasonality. Although the overall distribution of brown trout in the region remains stable, genetically pure Mediterranean populations have experienced severe range contraction, with a 61.5% reduction in extent of occurrence (EOO) and an 89.6% reduction in area of occupancy (AOO). This decline might meet IUCN criterion B for Endangered, although a formal assessment is currently impeded by taxonomic uncertainty and data limitations. The recognition of Mediterranean brown trout as a distinct species, alongside urgent measures to halt ongoing introgression and preserve remaining pure populations, is essential for its long‐term conservation.

Fish secondary production integrates multiple demographic parameters, including population density, growth, mortality, and recruitment, and thereby provides a comprehensive measure of ecological performance. It is also a valuable tool for assessing the ecological integrity of stream ecosystems and the responses of fish populations to habitat alteration, climatic variability, and anthropogenic pressures. Despite its relevance, empirical estimates of fish production remain limited due to methodological constraints. In this study, we quantified secondary production and production-to-biomass (P/B) ratios for Mediterranean brown trout (Salmo trutta complex) across six headwater stream reaches in the northeastern Iberian Peninsula, characterized by contrasting hydrological regimes, channel morphology, and water chemistry. Field sampling was conducted over two consecutive annual cycles (2008/2009 and 2009/2010) at all sites, with extended monitoring at two reaches until 2017 to assess long-term variability. Annual trout production, over the two consecutive annual cycles, ranged from 30.9 to 167.8 kg ha⁻¹ year⁻¹ (mean = 82.2 kg ha⁻¹ year⁻¹), and mean P/B ratios ranged from 0.61 to 1.13 (mean = 0.80). These values fall within the intermediate range reported for brown trout globally and reflect the constrained energy dynamics of Mediterranean streams. Higher production was generally associated with strong age-1 recruitment, elevated standing biomass, and greater water alkalinity. Long-term analyses revealed that interannual variation in trout production was significantly correlated with discharge variability, with higher production occurring under more stable flow conditions. However, in addition to flow variability other factors, such as habitat complexity, may modulate local productivity. Consequently, interannual fluctuations at the long-term sites revealed substantial demographic variability influenced by site-specific environmental conditions. These findings offer reference baselines for Mediterranean trout populations and contribute to the ecological basis for their conservation and management.

Understanding individual movement patterns in stream-resident salmonids is critical for conservation and river management, particularly in Mediterranean streams characterized by high environmental variability. We tagged 997 Mediterranean brown trout (Salmo trutta complex) and conducted an 11-month mark–recapture study using Passive Integrated Transponder (PIT) technology to assess movement behavior in the Flamisell River (Ebro Basin, northeastern Iberian Peninsula). Movements followed a leptokurtic distribution, with 81.8% of the individuals classified as sedentary (median movement = 24.9 m) and 18.2% as mobile (median movement = 376.2 m). Generalized linear models revealed distinct drivers of fish movement for each group. In sedentary trout, movement was mainly influenced by mesohabitat type, season, sex, and body size, with males and larger individuals moving farther. In mobile trout, mesohabitat type, density, and body size were key predictors. Movement patterns were repeatable over time, indicating consistent behavioral tendencies. These results support a bimodal movement strategy and highlight the importance of individual variation. Conservation planning should account for both sedentary and mobile groups to preserve functional and genetic connectivity and improve resilience of Mediterranean streams.

The European Fish Index EFI+ is the only fish-based multimetric index for the assessment of the ecological status of running waters that is validated and thus applicable across most countries of the European Union. Metrics of the index rely on several attributes of the species present in the fish assemblage, irrespective of their native/alien status. The abundance of alien fish, together with other anthropogenic impacts, is one of the most important threats to the conservation of native fish and ecosystem health and is also an indicator of degraded stream conditions. Therefore, to improve the performance of the EFI+ in regions with high incidence of alien species, the EFI+ was adapted to include alien fish pressure as a new metric that reflects the number of alien species as well as the proportional abundance of alien individuals. The application of the adapted index (A-EFI+) is illustrated with data from several Iberian Mediterranean basins and showed similar or stronger correlations than the original EFI+ with anthropogenic pressure (land-use variables and alterations in hydrology and river morphology) and with other regional fish indices. EFI+ has been invaluable to intercalibrate fish indices across Europe, and A-EFI+ is similar but explicitly includes alien pressure, thus helping to provide a more comprehensive assessment of ecosystem health and to communicate it to society.

The European Fish Index EFI+ is the only fish-based multimetric index for the assessment of the ecological status of running waters validated and thus applicable across the European Union. Metrics of the index rely on several attributes of the species present in the fish assemblage, irrespective of their native/alien status. The abundance of alien fish, together with other anthropogenic impacts, is one of the most important threats to the conservation of native fish and ecosystem health and is also an indicator of degraded stream conditions. Therefore, to improve the performance and utility of the EFI+ in regions with high incidence of alien species, the EFI+ was adapted to include the alien fish pressure by averaging the two original metrics with a new metric that reflects the number of alien species as well as the proportional abundance of alien individuals. The application of the adapted index (A-EFI+) is illustrated with data from several Iberian Mediterranean basins and showed similar or stronger correlations than the original EFI+ with anthropogenic pressure (land-use variables) and with other regional fish indices. EFI+ has been invaluable to intercalibrate fish indices across Europe and A-EFI+ is similar but explicitly includes the alien pressure, thus helping to provide a more comprehensive assessment of ecosystem health and to communicate it to society.

In recent years, a remarkable effort has been made to recover the connectivity for fish in the rivers of Catalonia, mainly through the construction of new fish ladders, or river connectors. However, evaluating the effectiveness of new fish ladders is not yet a common practice, although knowing how it works is a key aspect of improving the design of new river connectors in other infrastructures. For this reason, the Catalan Water Agency has launched a study to monitor the effectiveness of several recently built river connectors. A total of five fish ladders were selected in the rivers Anoia, Ripoll, Brugent, Llémena and Borró, placed in four different watersheds (Llobregat, Besós, Ter and Fluvià, respectively). These rivers have a similar hydrology (average river discharge inferior to 1 m³/s), and a similar potential fish assemblage (Barbus haasi or B. meridionalis, Squaliuslaietanus, Anguilla anguilla, and in some cases Salaria fluviatilis). Several exotic species are also present in some of these rivers: Gambusia holbrooki, Lepomis gibbosus, Phoxinus sp., Barbatula sp. During 2021, between two and three monitoring campaigns (spring, summer, fall) were carried out. The methodology used consisted first in the installation of permanent traps at the top exit of ladders to obtain direct estimates of pass rate by species, and on comparative fish sampling on each side of the barrier. Additionally, regular monitoring of several hydraulic variables (water velocity, draft, and elevation) was performed at a selection of internal points on each ladder. Finally, the expected mobility per species was also estimated using the package Fishmove (Radinger, 2013), to compare it with experimental estimations and determine the efficiency of ladders. The results demonstrate the effectiveness of the evaluated ladders, at least for some of the species present. Among native species, barbels (B. haasi and B. meridionalis) showed the highest rates of passage. Some exotic species were also able to occasionally use the ladders. The efficiency of the ladders is mostly high but decreases when there are internal points with excessive speeds, or other failures with respect to the optimal design.

The genus Phoxinus is comprised of at least 15 currently recognized species inhabiting Eurasia. Morphological traits have been traditionally used to delineate species in Phoxinus; however, the high level of phenotypic plasticity observed in the genus has confounded this process. Molecular genetic analyses have revealed a higher than expected genetic structure within Phoxinus. Here, we analyzed both nuclear and mitochondrial molecular genetic markers to infer the phylogeography and divergence times of Phoxinus in the Iberian Peninsula. Our results show that the Iberian lineages of Phoxinus were polyphyletic. They also support the co-existence of three species in the Iberian Peninsula, two corresponding to two previously recognized species (Phoxinus bigerri and Phoxinus septimaniae) and a third undescribed species (Phoxinus sp.). Phoxinus bigerri is structured into western Cantabrian, eastern Cantabrian, and Artibai basins. We hypothesize that this structure is a consequence of glaciation–deglaciation cycles during the Pleistocene. While the presence of P. septimaniae in the Iberian Peninsula is possibly the result of human translocation, that of Phoxinus sp. in lower Ebro rivers may be attributed to past fluvial captures. Our study represents the first report to show a relationship among Phoxinus populations from central Pyrenean rivers of Spain and France. Furthermore, we found genetic hybridization between Phoxinus sp. and P. septimaniae in the shared localities, a likely consequence of anthropogenic activities. Overall, our findings provide insight into the genetic structure of Iberian Phoxinus populations, including the presence of an undescribed species and the putative introduction of some species that may have implications for conservation.