Non-invasive fish monitoring is essential in Mediterranean reservoirs where drought and drawdown demand rapid, whole water-column assessments. We combined hydroacoustics and standardized netting to quantify spatial and seasonal variation in fish density and biomass, size structure, and species composition in Zahara-El Gastor Reservoir (SW Spain) and to evaluate an integrated monitoring approach. We conducted four seasonal surveys combining split-beam horizontal (0–4 m) and vertical echosounding with CEN standard multi-mesh gillnets and trammel nets across dam, central, and riverine zones of the reservoir. Acoustic data, analysed at 300 m Elementary Sampling Distance Units (ESDUs), revealed strong spatial and seasonal patterns: approximately 70% of acoustic detections occurred in the upper 4 m, where fish density peaked in summer at 122 ± 179 ind·10 −3 m −3 , with consistently higher values near the surface. Catches were numerically dominated by invasive bleak Alburnus alburnus (≈96.6% of individuals), while Andalusian barbel Luciobarbus sclateri accounted for ≈40% of biomass. Overall density was about five times higher than early post-impoundment surveys, consistent with bleak proliferation and reduced storage volume during drought. Size-class distributions showed no coherent seasonal shifts, indicating a lack of clear seasonal changes in community structure. Integrating horizontal and vertical hydroacoustics with minimal netting enables rapid, fish-friendly, and cost-effective monitoring under climate-driven water-level fluctuations, providing a practical basis for adaptive reservoir monitoring. • Fish density varied strongly by season, depth, and reservoir zone, but community composition and size structure were broadly stable. • The reservoir is now dominated by non-native bleak, while native cyprinids are scarce. • Drought-driven drawdown and stratification concentrated fish into the oxygenated surface layer, inflating summer densities. • Horizontal hydroacoustics is critical in Mediterranean reservoirs under summer stratification and drought to avoid underestimating fish density and to represent shallow/surface layers. • Integrated vertical and horizontal hydroacoustic monitoring supports fish-friendly, cost-effective assessments and improves management readiness under drought extremes.
Encina et al. (Sun,) studied this question.