Esmeralda Pereira

Fish Aggregating Devices (FADs), traditionally used to attract and concentrate fish, can also serve as effective environmental enrichment tools in reservoirs, particularly in those with homogeneous characteristics and scarce refuge habitat, enhancing structural complexity and promoting recreational fishing opportunities. This study aimed to evaluate patterns in the use of prototype fish aggregation devices (FADs) in small size reservoirs. It was conducted at the Nascentes Reservoir (Crato), a small Mediterranean reservoir (ca. 10 ha) located in southern Portugal. These FADs were installed to enhance refuge habitat for fish species of interest to recreational fisheries, particularly largemouth bass (Micropterus salmoides Lacepède, 1802), thereby promoting the occurrence of trophy specimens. Two types of FADs were deployed and tested: (1) bank FADs (TREES), used in shallow waters near the margins; and (2) pelagic FADs (DAPs), suspended in the water column in deeper areas at the center of the reservoir. To monitor movement patterns and habitat use, an acoustic telemetry receiver array was deployed with a design to secure a three-dimensional fine-scale positioning with high accuracy. A total of 20 largemouth bass were tagged with acoustic transmitters equipped with pressure (i.e., depth) sensors. A before–after approach was used with 10 fish tracked before FAD deployment and 10 after. Results of fish behavior analysis provide strong evidence of fish using DAPs, but not TREES. In the presence of FADs, fish reduced their home ranges and movement amplitudes, becoming closely associated with these artificial habitats. Several environmental predictors explained fish behavior in the presence of artificial refuges, namely, diel period, moonlight intensity, and fish depth. The findings of this study are expected to contribute to the development of guidelines for refuge habitat enhancement in small- to medium-sized Mediterranean reservoirs, thereby increasing their recreational fishing attractiveness.

LIFE REVIVE aims to restore ecological status and ecosystem services in the Lima and Vouga river basins (NW Iberian Peninsula), where hydromorphological alteration and hydropower-driven flow regulation are major causes of water bodies failing to reach Good Ecological Status under the EU WFD. The project targets key pressures such as longitudinal fragmentation by weirs and dams, artificial flow regimes, degradation of spawning substrates, and the spread of invasive aquatic plants, which strongly affect fish communities, including sea lamprey, salmonids, and other diadromous species. Technically, the project combines barrier removal or eco-adaptation, nature-like fish passes, and spawning-habitat renaturalisation with optimized environmental flow regimes (EFR) downstream of important hydropower systems, explicitly accounting for present and future hydroclimatic scenarios. Multi-scale ecohydrological modelling (species distribution models, habitat suitability models, GLM/GAM approaches) will quantify fish–flow–habitat relationships and support the definition of operational EFR guidelines that balance ecological requirements with hydropower and agricultural constraints through joint work with the main Portuguese hydropower operator, EDP. Impact evaluation is structured around a rigorous BACI monitoring design in intervention and control tributaries, using standard WFD biological indices for fish and aquatic/riparian vegetation, hydromorphological indices (HQA, HMS, RHS), and project-specific Key Performance Indicators for water quality, biodiversity, and habitat. Expected outcomes include the restoration of at least 51 km of rivers towards free-flowing conditions, reduced hydromorphological pressure in more than 20 km of heavily modified river stretches, and measurable increases in the distribution and abundance of fish species and native vegetation. A strong communication and capacity-building programme underpins public engagement, while a decision matrix for barrier prioritization, technical workshops, and pilot replications in additional basins (e.g., Alva, Mouro, Deva, and Tea in Galicia) are designed to maximize transferability, policy uptake, and long-term sustainability of the solutions beyond the project lifetime.

The catadromous thinlip grey mullet ( Chelon ramada (Risso, 1827)) undertakes extensive upstream migrations into freshwater, but knowledge of its movement strategies and fidelity to these habitats remains limited. This study used long-term passive integrated transponder (PIT) and acoustic telemetry in the Mondego River basin (Portugal) to examine fine-scale spatial and temporal movement. Between 2017 and 2018, 1067 individuals were PIT-tagged, and monitored 16 km upstream from the tidal limit. In the first year, 17.43% were detected; after 5 years, 4% still returned (annual decline rate 2.7%/year), indicating strong freshwater fidelity in some individuals, independent of size. Acoustic tracking of 30 adults (2019–2020) revealed predominant use of Tidal/Freshwater (60%) and Midstream Freshwater habitats (23%), with rapid movements between these habitats and the Lower Estuary (9%). Three movement profiles emerged: longer residents within a single habitat type, directional migrants transitioning between zones, and irregular commuters. Several individuals remained in estuarine zones year-round, providing the first direct evidence that a fraction of the adult population forgoes annual spawning migrations, revealing greater life-history flexibility than previously understood.

The influence of an in‐situ acclimatization period of five weeks, seeking the settlement and fidelity of hatchery‐reared dusky groupers (Epinephelus marginatus) to the release sites in restocking actions, was tested using acoustic biotelemetry. Eight tagged adult groupers were submitted to the wild conditions in a fish farm cage, while seven remained in the hatchery and tagging location until they were all released. Ten months later, half of the acclimatized groupers were still present in the release site, while all the non‐acclimatized rapidly dispersed. These results show that an in‐situ acclimatization period of five weeks promotes the establishment and site fidelity of hatchery‐reared dusky groupers, contributing to successful restocking.

The European catfish (Silurus glanis L.) is a large, non‐native, invasive freshwater species known for its high fecundity, long lifespan and significant predatory potential, possibly threatening native fish communities. While some studies exist on its spatial use, most focus on its native range and lack detailed circadian and annual activity patterns. This study uses acoustic biotelemetry to track 10 adult fish in the Tagus River's Belver reservoir, employing transmitters with 3D accelerometer and pressure sensors to record activity and depth over a year. Generalised additive models identify environmental variables influencing activity and depth patterns. The European catfish is active year‐round, with peak activity in summer and minimal in autumn. It occupies shallower depths in spring and summer (median depths of 2.6 and 4.8 m, respectively) and deeper waters in autumn and winter (median depths of 9.8 and 10.8 m, respectively). Circadian vertical movement patterns exhibit high seasonal and individual variability. Significant environmental predictors of the European catfish activity include river flow, water temperature and the photoperiod, while the predictors of depth use are river flow and the photoperiod. These insights can aid in developing effective control measures, such as optimising fishing efforts spatially and temporally to enhance the efficiency of removing this invasive fish.

To investigate the potential benefits of the catadromous thinlip grey mullet (Chelon ramada Risso, 1827) migration to freshwater, the total lipid content and fatty acid (FA) profile of female’s muscle and gonads caught in both the estuary and river were analyzed. The freshwater contingent presented a higher body condition, greater muscle gross energy, and larger gonads with higher lipid reserves. These animals showed a muscle profile rich in C16:1n-7 and lower LC-PUFA that contrast with the higher relative amount of C18:1n-9, n-3 FA, and unsaturated LC-PUFA, such as C18 and C20 FA found in the estuarine contingent. The gonads of both contingents showed a constant and high relative amount of polyunsaturated FAs (PUFA, 37%). However, in terms of essential fatty acids (EFAs), the estuarine contingent had a higher relative amount of C18:2n-6, C20:4n-6, C20:5n-3, and C22:6n-3. On the other hand, the freshwater contingent showed a higher relative amount of n-3 FA precursors, namely C18:3n-3, and a still low relative amount of C22:6n-3. This suggests a mismatch between the metabolic omega-3 pathway and the physiological maturity stages, similar to a phenomenon of dormancy. In this sense, not all these individuals may reproduce annually, and the later stages of gonad development will require supplementary energy derived from feeding at the estuary. Thus, freshwater migration may promote a reproductive strategy enabling adults to take advantage of the warm and food-rich summer/autumn period, adjust spawning and juvenile appearance, and reduce the population’s exposure to habitat changes and/or stochastic events.

The upstream movement of juvenile eels (median total length (TL) = 138 mm) in the Mondego River, Portugal, was analysed between January 2017 and August 2019. A total of 12,019 individuals (TL ranging from 60 to 287 mm) were counted and measured on an eel ladder at Coimbra weir, 44 km upstream from the river mouth, during their upstream movements. Although eels were counted throughout the year, a clear seasonal peak was detected in spring and late summer, with 93% of total movements occurring between April and September. No differences were found in the total length of individuals moving upstream between years. Generalised additive models, with an explained deviance of 83% and an adjusted R² of 0.88, identified minimum water temperature as the main environmental variable explaining these movements, with a threshold of 13 °C triggering the onset of movements, and a peak between 15 and 19 °C, which corresponds to the spring and summer periods. Other predictors such as photoperiod, precipitation and river flow also contributed as explanatory variables in the model but less importantly. The present results may contribute to the management of this endangered species in the southern region of its distribution range by defining adequate temporal windows to monitor and promote the movement of juvenile European eels through obstacles to migration.