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Memoria de tesis doctoral presentada por Marta Albo-Puigserver para obtener el título de Doctora en Ciencias del Mar por la Universitat Politècnica de Catalunya (UPC), realizada bajo la dirección de la Dra. Marta Coll Montón y del Dr Joan Navarro Bernabé del Institut de Ciències del Mar (ICM-CSIC).-- 209 pages ; [EN] Mediterranean marine ecosystems are fundamentally driven by their pelagic communities and small and medium-size pelagic fish (SMPF) play crucial ecological roles. Although fluctuations of the populations of these organisms have been mainly attributed to environmental variability and overfishing, there is still a lack of understanding on specific mechanisms that drive their population changes. In recent decades a decline in landings, biomass and body condition of sardine and anchovy in the northwestern Mediterranean Sea has been observed and several hypotheses to explain these changes have been formulated. Therefore, accurate scientific data at species and community level is needed in order to understand the most likely causes of these declines. Within this context, this PhD thesis aims to improve the knowledge of the seasonal dynamics and functioning of the pelagic compartment focusing in the trophic relationships and energy dynamics of SMPF in the northwestern Mediterranean Sea. I used different methods to advance on the knowledge about the ecological and functional role of several pelagic fish species in the western Mediterranean Sea, including clupeoids, horse mackerels and mackerels. Specifically, stable isotope and stomach content analyses in combination with direct and indirect body condition indices, were used to study seasonal variation in energy content in relation to their breeding strategies, feeding preferences and trophic role in the pelagic food-web. Results showed stability in the trophic structure through the year with a trophic segregation of small pelagic fish depending on the ontogenetic stage. Seasonal differences in energy density were highly related with the spawning period and food availability. To integrate the new findings and to test the role of potential pressures on main pelagic species, I developed qualitative food-web models. Results showed that an increase of sea surface temperature in combination with an increase of explotation of sardine and anchovy and changes in zooplankton composition were the most plausible pressures to explain the observed changes in small pelagic fish populations. Overall, these results provide pivotal information to assist the adaptive management of pelagic fish species in the region ; [ES] Los ecosistemas marinos mediterráneos están fundamentalmente condicionados por sus comunidades pelágicas donde los peces pelágicos pequeños y medianos (SMPF) desempeñan funciones ecológicas cruciales. Si bien las fluctuaciones de sus poblaciones se han atribuido principalmente a la variabilidad ambiental y la sobrepesca, todavía hay una falta de comprensión sobre los mecanismos específicos que impulsan los cambios en las poblaciones pelágicas. En las últimas décadas se ha observado una disminución en la biomasa, en los desembarques y en la condición de la sardina y la anchoa del noroeste del mar Mediterráneo, y se han formulado varias hipótesis para explicar estos cambios. Actualmente, se necesitan datos científicos precisos a nivel de especies y comunidades para comprender las causas más probables de estas disminuciones. En este contexto, esta tesis doctoral pretende mejorar el conocimiento de la dinámica estacional y el funcionamiento del compartimento pelágico, centrándose en las relaciones tróficas y la dinámica energética de los SMPF en el noroeste del mar Mediterráneo. Se usaron diferentes técnicas para avanzar en el conocimiento del papel ecológico y funcional de varias especies de peces pelágicos en el mar Mediterráneo occidental, incluyendo clupeoides, jureles y caballas. En concreto, se utilizó el análisis de isótopos estables y de contenido estomacal en combinación con índices directos e indirectos de condición corporal, para estudiar la variación estacional del contenido de energía en relación con las estrategias de reproducción de los SMPF, las preferencias tróficas y el papel trófico en la comunidad pelágica. Los resultados mostraron estabilidad en la estructura trófica a lo largo del año con una segregación trófica de peces pelágicos pequeños en función del estadío ontogenético. Las diferencias estacionales en la densidad energética estuvieron altamente relacionadas con el periodo de reproducción y la disponibilidad de alimentos. Para integrar toda esta información y testar el rol de las presiones potenciales en las principales especies pelágicas, se desarrolló un modelo cualitativo. Los resultados mostraron que un aumento de la temperatura de la superficie del mar en combinación con el aumento de la explotación de la sardina y la anchoa y cambios en la composición del zooplancton fueron las presiones más plausibles para explicar los cambios observados en las poblaciones de peces pelágicos pequeños. En general, estos resultados proporcionan información fundamental para contribuir al manejo adaptativo de las especies de peces pelágicos en la región ; [CAT] Els ecosistemes marins mediterranis estan fonamentalment condicionats per les seves comunitats pelàgiques on els peixos pelàgics petits i mitjans (SMPF) exerceixen funcions ecològiques crucials. Si bé les fluctuacions de les seves poblacions s'han atribuït principalment a la variabilitat ambiental i la sobrepesca, encara hi ha una manca de comprensió sobre els mecanismes específics que impulsen els canvis en la població. En les últimes dècades s'ha observat una disminució en la biomassa, en els desembarcaments i en la condició de la sardina i el seitó al nord-oest del mar Mediterrani, i s'han formulat diverses hipòtesis per explicar aquests canvis. Actualment, es necessiten dades científiques precises a nivell d'espècies i comunitats per comprendre les causes més probables d'aquestes disminucions. En aquest context, aquesta tesi doctoral pretén millorar el coneixement de la dinàmica estacional i el funcionament del compartiment pelàgic, centrant-se en les relacions tròfiques i la dinàmica energètica dels SMPF al nord-oest del mar Mediterrani. Es varen fer servir diferents tècniques per avançar en el coneixement del paper ecològic i funcional de diverses espècies de peixos pelàgics al mar Mediterrani occidental, incloent clupeoids, sorells i verats. En concret, es varen utilitzar anàlisi d'isòtops estables i de contingut estomacal en combinació amb índexs directes i indirectes de condició corporal, per estudiar la variació estacional del contingut d'energia en relació amb les seves estratègies de reproducció, preferències tròfiques i paper tròfic en la comunitat pelàgica. Els resultats van mostrar estabilitat en l'estructura tròfica al llarg de l'any amb una segregació tròfica dels peixos pelàgics petits en funció de l'etapa ontogenètica. Les diferències estacionals en la densitat energètica van estar altament relacionades amb la temporada de reproducció i la disponibilitat d'aliments. Per integrar tota aquesta informació i testar el paper de les pressions potencials en les principals espècies pelàgiques, es va desenvolupar un model qualitatiu. Els resultats van demostrar que l'augment de la temperatura superficial del mar en combinació amb un augment d'explotació de la sardina i el seitó i canvis en la composició del zooplàncton, eren les pressions més pausibles per explicar els canvis observats en les poblacions de peixos pelàgics petits. En general, els resultats obtinguts proporcionen informació fonamental per contribuir al maneig adaptatiu de les espècies de peixos pelàgics a la regió ; The present thesis was conducted in the Institut de Ciències del Mar (ICM-CSIC; Barcelona, Spain) within the framework of the ECOTRANS Project (Dynamics and Ecological role of small pelagic fishes in the North Western Mediterranean: Energy Transfer from planktonic organisms and top predators; CTM2011-26333 - Funded by the Spanish Ministry of Economy and Competitiveness, Spanish Government) and the associated predoctoral fellowship BES-2012-054267 and short-stay research grants EEBB-I-14-08310; EEBB-I-15-09324; EEBB-I-16-10882 ; Peer reviewed

16 pages, 7 figures, 3 tables, supplementary material https://doi.org/10.3354/meps12962 ; The recent northward expansion of the round sardinella Sardinella aurita in the Mediterranean Sea has been documented as a consequence of rising sea temperature. At the same time, declines in sardine and anchovy biomass have been observed in the NW Mediterranean Sea, necessitating an assessment of whether the expansion of round sardinella may affect sardine and anchovy populations. Here, we combined stomach content and isotopic analyses to describe the trophic habits of round sardinella in the NW Mediterranean Sea and its trophic relationships with 2 sympatric small pelagic fish, European anchovy Engraulis encrasicolus and European pilchard Sardina pilchardus. Results revealed changes in the diet of round sardinella during the year. In summer, the most important prey were copepods (Acartia spp.) and cladocerans (Penilia avirostris). During winter, the diet was composed mainly of copepods and tunicates (mainly appendicularians), but microplankton was also numerically important in adult diets. In contrast to previous studies, during spring, round sardinella principally fed on salps (Thaliacea). To our knowledge, this is the first time that salps have been identified as an important prey for round sardinella. When compared to coexisting small pelagic fish, we found that round sardinella adults had a different trophic niche than anchovy and sardine. In contrast, round sardinella juveniles partially overlapped the trophic niche with the juveniles of the other 2 species. Therefore, the range expansion of round sardinella probably would not affect sardine and anchovy populations. Only in a situation of food limitation could juveniles of round sardinella compete with and affect both sympatric species. Our results provide new insights into the ecological role of this range-expanding species in the NW Mediterranean Sea, and highlight the importance of gelatinous zooplankton as prey ; We also thank the General Fisheries Commission for the Mediterranean Sea (GFCM) for their financial support to present this work at the International Symposium on drivers of dynamics o small pelagic fish resources. This study was funded by ECO-TRANS (CTM2011-26333) and PELWEB (ES-PN-2017-CTM2017-88939-R) projects. M.A.P. was funded by an FPI grant and a Short-Stay FPI grant (BES-2012-054267; Spanish Government). J.N. was funded by a Ramon y Cajal contract (RYC-2015-17809) of the Spanish Government ; Peer reviewed

11 pages, 7 figures, supplementary data https://doi.org/10.1016/j.scitotenv.2020.144002 ; Sustainable fishing practices must ensure human wellbeing by safeguarding the integrity of marine life-supporting systems. Unfortunately, a significant challenge to fisheries management is that sustainable fishing levels can decline, often synergistically, by co-occurring with climate-driven environmental stressors. Within one of the most impacted marine areas in the world, and encompassing a number of highly targeted commercial species, the small pelagic fish community of the western Mediterranean Sea has recently shown signs of collapse. In this study, we identify a worrying coincidence where fishing hotspots for the commercially valuable European sardine Sardina pilchardus and anchovy Engraulis encrasicolus occur in marine areas mostly affected by climate change. To identify these areas, we overlayed detailed, spatially explicit measurements of fishing pressure with the finest-scale maps of cumulative climate change impacts onto these species. According to our results, doubly impacted marine areas largely occur in the north-western Mediterranean Sea, with climate and fisheries mostly affecting European sardine. Reducing local stressors (i.e., fishing pressure) in highly impacted areas may contribute to maintain these communities within a "safe operating space" (SOS), where they remain resilient to climate change. Accordingly, the redistribution and/or reduction of fishing intensity may alleviate pressure in those areas already affected by climate change. Sustainable fishing strategies may benefit, therefore, from the SOS concept and the spatial assessments provided in this study ; This study was carried out within the Spanish Research project PELWEB (CTM2017-88939-R) funded by Spanish Ministry of Science, Innovation and Universities, the Catalonian Government PELCAT projects (CAT 152CAT00013, TAIS ARP059/19/00005), and the study received funding from European Union's Horizon 2020 research and innovation programme under grant agreement No 817578 (TRIATLAS project) ; Peer reviewed

10 pages, 4 figures, 6 tables, supplementary data https://doi.org/10.1016/j.ecss.2021.107699 ; Sardine and anchovy have shown important changes in landings, biomass, abundance and body condition with time in the Western Mediterranean Sea. Several hypotheses have been proposed to explain these changes, including the negative interaction with jellyfish blooms. Increases in jellyfish blooms may be one of the reasons for a decline in these fish stocks because several jellyfish species have been shown to feed on fish larvae and juveniles. The main aim of the present study was to test the plausible relationship between jellyfish blooms and stock dynamics (abundance, biomass, and fitness) of anchovy and sardine, and its fisheries within an ecological context of the western Mediterranean Sea. Our main hypothesis was that jellyfish blooms, in combination with other environmental drivers, could have negative effects due to their predation on early stages of small pelagic fish (direct mortality) or due to predation on zooplankton, which is also prey of the small pelagic fish at different ontogenetic stages (direct competition). To test our hypothesis, we developed Bayesian Generalized Linear Mixed Models to compare landings, biomass, abundance, and Kn condition factor of both species with several climatic indices, oceanographic variables, and the occurrences of jellyfish blooms. Our results revealed that the jellyfish bloom occurrence had a high probability of negatively and broadly affecting both species in addition to changes in environmental conditions. This suggests that jellyfish blooms should be added to the likely causes of change when analyzing small pelagic fish change ; This study was carried out within the Spanish Research project PELWEB (CTM2017-88939-R) funded by Spanish Ministry of Science, Innovation and Universities, and the Catalonian Government PELCAT projects (CAT 152CAT00013, TAIS ARP059/19/00005). [.] MC acknowledges the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S) to the Institute of Marine Science (ICM-CSIC). ; Peer reviewed

10 pages, 6 figures, 1 table, supplementary data https://doi.org/10.1016/j.marpolbul.2020.111399 ; We quantified the incidence of microplastics in the gut contents of the European sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) in the Northwestern Mediterranean Sea and tested which variables influence this abundance, including the prevalence of parasites (i.e., trematoda larvae and nematodes). We detected a 58% occurrence of microplastics ingestion in sardines and a 60% in anchovies. With respect to sardines, the individuals with lower body conditions were found to have the highest microplastics ingestion probabilities, whereas in anchovies such probabilities were observed in individuals with higher gonadosomatic indices and smaller size. The areas with the highest microplastics ingestion probabilities were the Gulf of Alicante for sardines and the Gulf of Lion - Ebro Delta for anchovies. Both species showed a positive relationship between parasites and microplastics ingestion. These results highlight that both parasitism and ingestion of microplastics are concerns for the health of marine stocks and human consumers ; This study was carried out within PELWEB (CTM2017-88939-R) and PELCAT (CAT 152CAT00013, TAIS ARP059/19/00005) projects. [.] MEDITS data collection has been co-funded by the EU through the European Maritime and Fisheries Fund (EMFF) within the National Program of collection, management and use of data in the fisheries sector and support for scientific advice regarding the Common Fisheries Policy ; Peer reviewed

20 pages, 15 figures, supplementary material https://www.frontiersin.org/articles/10.3389/fmars.2020.00622/full#supplementary-material ; In the Northwestern Mediterranean Sea, the European sardine (Sardina pilchardus) and the European anchovy (Engraulis encrasicolus) are the most important small pelagic fish in terms of biomass and commercial interest. During the last years, these species have experimented changes in their abundance and biomass trends in the Northwestern Mediterranean Sea, in addition to changes in growth, reproduction and body condition. These species are particularly sensitive to environmental fluctuations with possible cascading effects as they play a key role in connecting the lower and upper trophic levels of marine food webs. It is therefore essential to understand the factors that most profoundly affect sardine and anchovy dynamics. This study used a two-step approach to understand how the environment influences the adult stages of these species in the Northwestern Mediterranean Sea. First, we explored the effects of environmental change over time using Random Forests and available datasets of species occurrence, abundance, biomass and landings. We then applied species distribution models to test the impact of the extreme pessimistic and optimistic Intergovernmental Panel on Climate Change (IPCC) pathway scenarios, and to identify possible climate refuges: areas where these species may be able to persist under future environmental change. Findings from the temporal modeling showed mixed effects between environmental variables and for anchovy and sardine datasets. Future pathway projections highlight that both anchovy and sardine will undergo a reduction in their spatial distributions due to future climate conditions. The future climate refuges are the waters around the Rhone River (France) and the Ebro River (Spain) for both species. This study also highlights important knowledge gaps in our understanding of the dynamics of small pelagic fish in the region, which is needed to progress towards an ecosystem approach to fisheries management ; This study was carried out within the Spanish Research project PELWEB (CTM2017-88939-R) funded by the Spanish Ministry of Science, Innovation and Universities, the European Research Contract SPELMED (EASME/EMFF/2016/032) funded by the ECEASME and the Catalonian Government PELCAT projects (CAT 152CAT00013 and TAIS ARP059/19/00005) ; Peer reviewed

17 pages, 7 figures, 3 tables, supporting Information https://doi.org/10.1002/ece3.8293.-- Data availability statement: All the DNA metabarcoding raw reads of prey species obtained from the stomach contents were deposited in the BioProject database (Gen-Bank, NCBI, 1988) as BioProject PRJNA653773. All other data used in the manuscript are included in the manuscript or as Supporting Information ; The northward expansion of round sardinella (Sardinella aurita) in the Mediterranean Sea, together with declines and fluctuations in biomass and landings of European sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) observed in recent decades, may suggest potential inter-specific competition in the pelagic domain. The coexistence of sympatric zooplanktivorous fish species might therefore be exposed in part to trophic niche overlap and competition for food. Combining visual diet characterization under the microscope with DNA metabarcoding from stomach contents of fish collected in spring results show that predation on relatively large krill is equally important for sardinella than for the other two niche overlapping species. Furthermore, an important overlap is found in their isotopic niche, especially with anchovy, using nitrogen (δ15N) and carbon (δ13C) stable isotopes in muscle tissue. In fact, the three fish species are able to feed effectively in the whole prey size spectrum available during the sampled season, from the smallest diatoms and copepods to the larger prey (i.e., decapods and euphausiids), including fish larvae. Moreover, effective predation upon other large prey like siphonophores, which is observed only when multi-proxy analyses in stomach contents are applied, might also be relevant in the diet of sardinella. The overlapping diet composition in spring, together with the effective use of food resource by sardinella, can be of special interest in potential future scenarios with warmer water temperature leading to lower zooplankton and/or higher jellyfish availability, where sardinella may take advantage over other species due to its feeding plasticity ; This study was partially funded by the projects PELWEB (ES-PN-2017-CTM 2017-88939-R) and PELCAT (CAT 152CAT00013, TAIS ARP059/19/00005). MEDITS data collection has been co-funded by the EU through the European Maritime and Fisheries Fund (EMFF) within the National Program of collection, management, and use of data in the fisheries sector and support for scientific advice regarding the Common Fisheries Policy. This work acknowledges the "Severo Ochoa Centre of Excellence" accreditation (CEX2019-000928-S) to the Institute of Marine Science (ICM-CSIC) ; Peer reviewed

21 pages, 8 figures, 3 tables, supplementary material https://www.frontiersin.org/articles/10.3389/fmars.2021.570354/full#supplementary-material.-- Data Availability Statement: The datasets presented in this article are not readily available because the dataset is available by request to the Spanish Institute of Oceanography. Requests to access the datasets should be directed to webmaster@ieo.es ; Small pelagic fish (SPF) in the western Mediterranean Sea are key elements of the marine food web and are important in terms of biomass and fisheries catches. Significant declines in biomass, landings, and changes in the age/size structure of sardine Sardina pilchardus and anchovy Engraulis encrasicolus have been observed in recent decades, particularly in the northern area of the western Mediterranean Sea. To understand the different patterns observed in SPF populations, we analyzed key life history traits [total length at age, length at maturity, gonadosomatic index (GSI), and body condition (Kn)] of sardine and anchovy collected between 2003 and 2017, from different fishing harbors distributed along a latitudinal gradient from northern to southern Spain. We used Generalized Linear Models (GLM) to estimate the length at maturity and Generalized Additive Models (GAMs) to test the relationship with environmental variables (seawater temperature, water currents, and net primary productivity). The life history traits of both species presented seasonal, interannual and latitudinal differences with a clear decline in length at age, length at first maturity, and body condition, for both species in the northern part of the study area. In the southern part, on the contrary, life history traits did not present a clear temporal trend. The environmental conditions partially explained the long-term changes in life history traits, but the selected variables differed between areas, highlighting the importance of regional oceanographic conditions to understand the dynamics of small pelagic fish. The truncated length-at-age pattern for both species with the disappearance of the larger individuals of the population could have contributed to the poor condition of small pelagic fish populations in the northern part of the western Mediterranean Sea in recent years. In the south area, recent declines in body condition for sardine and anchovy were observed and could be a possible first sign for future population declines. This study highlights the importance of understanding the trade-off between the energy invested in reproduction, maintenance and growth at seasonal and interannual level to advance our knowledge on how environmental and human pressures influence population dynamics of small pelagic fish at local and regional scales ; This study was carried out within the Spanish Research project PELWEB (CTM2017-88939-R) funded by Spanish Ministry of Science, Innovation and Universities and the European Research Contract SPELMED (SC NR 02-TENDER EASME/EMFF/2016/032XXX) funded by EC EASME. Fisheries data collection has been co-funded by the EU through the European Maritime and Fisheries Fund (EMFF) within the National Program of collection, management and use of data in the fisheries sector and support for scientific advice regarding the Common Fisheries Policy (Regulation, EU 2017/1004) ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S) ; Peer reviewed