Mysids have a great potential as the new high nutritional, sustainable and functional natural resources aquaculture require globally. Despite being an excellent source of proteins and essential fatty acids, their potential in aquaculture has barely been evaluated. We have characterized the nutritional value of Mesopodopsis slabberi in the Guadalquivir estuary, where constitutes a key prey for fish juveniles. Seasonal and ontogenic biochemical composition variation was studied. M. slabberi shows high protein per dry mass (60%) with a slight a significant (p < 0.05) decline in summer. Glutamic +glutamine, lysine and aspartic +asparagine were the dominant aminoacids, with relatively high levels of leucine, alanine, glycine, methionine, phenylalanine and tryptophane, all of great relevance for aquaculture feeds. Total lipids content is high in M. slabberi, representing PUFAs 44.15 ± 1.12% of the total fatty acids. The highest concentrations of EPA and DHA fatty acids were found in spring. Most important the ratio DHA/EPA was high and similar in all season with an average value of 0.83 ± 0.03. DHA/AA and EPA/AA ratio were 4.92 ± 0.38 and 6.10 ± 0.48, respectively. Highest n3/n6 ratio (p < 0.05) were detected in spring 7.83 ± 0.77. M. slabberi show an adequate antioxidant balance with TAC/LP ratio greater than 90 in spring and summer reaching the highest levels (132) in autumn. In Guadalquivir estuary, M. slabberi presents high zinc and selenium levels. Our results demonstrate how M. slabberi has high nutritional profile and potential to be used as a valuable functional ingredient in aquaculture, both inert and as a living resource. More research in relation to culture technology and functional assess is necessary to open new ways to reach aquaculture long-term sustainability.

Phytoplankton community composition expresses estuarine functionality and its assessment can be improved by implementing novel quantitative fatty acid based procedures. Fatty acids have similar potential to pigments for quantifying phytoplankton functional groups but have been far less applied. A recently created dataset containing vast information on fatty acids of phytoplankton taxonomic groups was used as reference to quantify phytoplankton functional groups in the yet undescribed Guadalquivir River Estuary. Twelve phytoplankton groups were quantitatively distinguished by iterative matrix factor analysis of seston fatty acid signatures in this turbid estuary. Those phytoplankton groups including species unfeasible for microscopy identification (coccoid or microflagellated cells) could be quantified when using fatty acids. Conducting monthly matrix factor analyses over a period of two years and the full salinity range of the estuary indicated that diatoms dominated about half of the phytoplankton community spatiotemporally. The abundance of Cyanobacteria and Chlorophytes was inversely related to salinity and little affected by seasonality. Euglenophytes were also more abundant at lower salinity, increasing their presence in autumn-winter. Coccolithophores and Dinophytes contributed more to phytoplankton community at higher salinity and remained little affected by seasonality. Multivariate canonical analysis indicated that the structure of the estuarine phytoplankton community was most influenced by salinity, secondly influenced by water temperature, irradiance and river flow, and unaffected by nutrients. Fatty acids are especially suited for phytoplankton community research in high turbid estuaries and generate outcomes in synergy with those derived from classical pigment assessments.

Weakfish (Cynoscion regalis) has been present in Iberian waters since at least 2011, when it was first recorded in the Guadalquivir estuary. Little is known about the preferences and feeding strategies of weakfish outside of its native range; therefore, in this work, we carried out a comprehensive study between March 2021 and September 2021 to elucidate these matters. In total, the stomach contents of 300 fish were examined. The fish were collected in spring and summer in the Gulf of Cadiz (Spain), with individuals caught ranging from 185 to 590 mm in total length. Due to the sampling period and size range of individuals, ontogenic and seasonal (spring–summer) variations in the diet were also explored. Overall, fish and crustaceans were the dominant groups consumed by weakfish. The European anchovy (Engraulis encrasicolus) and caramote prawn (Penaeus kerathurus) were the most abundant prey in each group. While no differences were found in the percentage of occurrence of fish in the non-empty stomachs analyzed in spring and summer (83%), a small increase was found in the percentage of occurrence of crustaceans from spring (20%) to summer (29%). In addition, the analysis of the results also suggested that weakfish of smaller sizes feed more on crustaceans, while bigger individuals feed mainly on fish. Our findings indicate that weakfish is a fully carnivorous species with a preference for fish. All this is in line with the diet and strategy that weakfish exhibits in its native area and with the first assessment made with the existing population in the Sado Estuary (Portugal). In recent years, due to the appearance of non-native species, there is increasing concern among local fishermen and the local authorities about the reduction in captures of certain species such as Penaeus kerathurus; therefore, this study could help researchers to understand the role of weakfish concerning this matter. The present study contributes to the ecological knowledge about this species in the Iberian Peninsula and the Gulf of Cadiz, helping the local authorities to establish, if needed, an appropriate management program to cope with this recently introduced species.

Introduction: The Strait of Gibraltar, a narrow gateway between the Atlantic Ocean and the Mediterranean Sea, concentrates intense biotic exchange and functions as a global “ecological gate” for marine species. Despite its ecological relevance, sustained acoustic telemetry monitoring has long been hindered by oceanographic complexity, strong currents, intense maritime traffic, and geopolitical constraints. As its Atlantic approach, the Gulf of Cádiz is a highly productive area supporting diverse habitats, key spawning and nursery grounds, and major migration pathways, while underpinning important commercial fisheries. Objective: to deploy a system to enable continuous monitoring of animal movements across this key biogeographic boundary. Methodology: The EU-funded STRAITS project, a multidisciplinary consortium integrating physical oceanography and marine biotelemetry, has deployed an acoustic telemetry curtain across the Strait of Gibraltar, complemented by a coastal acoustic receiver array in the Gulf of Cádiz. Results: Over the past two years, a consistent pattern of frequent acoustic detections has demonstrated the system’s potential. Data reveal spatio-temporal migration patterns of commercially important species such as Atlantic bluefin tuna (Thunnus thynnus) and meagre (Argyrosomus regius), alongside conservation-relevant species like ocean sunfish (Mola mola). Conclusions: These results highlight the Strait of Gibraltar as a biodiversity hotspot and a strategic location for quantifying connectivity, migration timing, and species fluxes, supporting ecosystem-based management and blue policy frameworks.

Introduction: Climate change is reducing freshwater availability worldwide, making it essential to understand how freshwater inflow influences estuarine ecosystem functioning and marine fisheries productivity. In the Gulf of Cádiz (SW Spain), one of the most important fishing areas in Spain, the Guadalquivir Estuary serves as a key nursery habitat for commercially important fish and crustacean species. Objective: The aim of this study is to evaluate the effects of droughts and floods on estuarine functioning and coastal fisheries. Methodology: We analyzed 25 years of monthly data (1997–2022) from the Guadalquivir Long-Term Ecological Research Program (GUADALQUIVIR-LTER), using time-series analyses and dynamic structural equation modelling. Environmental variables, zooplankton and mysid biomass, and juvenile biomass of anchovy, sardine, and meagre were examined to assess trophic relationships and recruitment dynamics. Results: Our findings show that positive North Atlantic Oscillation (NAO) phases, associated with drought conditions in southern Europe, reduced freshwater inflow from the Alcalá del Río Dam into the estuary. Freshwater input increased organic matter and turbidity, which positively affected the mysid Rhopalophthalmus tartessicus, an important prey species for anchovy recruits. The mysid Mesopodopsis slabberi showed the strongest positive effect on anchovy recruitment (0.39). Although turbidity initially had a negative effect on M. slabberi, a significant positive effect appeared after monthly lag = 4. Conclusions: These findings demonstrate that spring freshwater inflow is essential for maintaining estuarine productivity, enhancing mysid abundance, and supporting anchovy recruitment, ultimately benefiting adult fish stocks after juveniles migrate from the estuary to coastal waters.