Diadromous fish have exhibited a dramatic decline since the end of the 20th century. The allis shad (Alosa alosa) population in the Gironde-Garonne-Dordogne (GGD) system, once considered as a reference in Europe, remains low despite a fishing ban in 2008. One hypothesis to explain this decline is that the downstream migration and growth dynamics of young stages have changed due to environmental modifications in the rivers and estuary. We retrospectively analysed juvenile growth and migration patterns using otoliths from adults caught in the GGD system 30 years apart during their spawning migration, in 1987 and 2016. We coupled otolith daily growth increments and laser ablation inductively-coupled plasma mass spectrometry measurements of Sr:Ca, Ba:Ca, and Mn:Ca ratios along the longest growth axis from hatching to an age of 100 days (i.e., during the juvenile stage). A back-calculation allowed us to estimate the size of juveniles at the entrance into the brackish estuary. Based on the geochemistry data, we distinguished four different zones that juveniles encountered during their downstream migration: freshwater, fluvial estuary, brackish estuary, and lower estuary. We identified three migration patterns during the first 100 days of their life: (a) Individuals that reached the lower estuary zone, (b) individuals that reached the brackish estuary zone, and (c) individuals that reached the fluvial estuary zone. On average, juveniles from the 1987 subsample stayed slightly longer in freshwater than juveniles from the 2016 subsample. In addition, juveniles from the 2016 subsample entered the brackish estuary at a smaller size. This result suggests that juveniles from the 2016 subsample might have encountered more difficult conditions during their downstream migration, which we attribute to a longer exposure to the turbid maximum zone. This assumption is supported by the microchemical analyses of the otoliths, which suggests based on wider Mn:Ca peaks that juveniles in 2010s experienced a longer period of physiological stress during their downstream migration than juveniles in 1980s. Finally, juveniles from the 2016 subsample took longer than 100 days to exit the lower estuary than we would have expected from previous studies. Adding a new marker (i.e., Ba:Ca) helped us refine the interpretation of the downstream migration for each individual.

Historical and existing environmental and human induced pressures have negatively impacted diadromous species, as well as benefits to humans, derived from the ecosystem services these species contribute to. As species move across national boundaries, successful management is rendered a complex process. Future climate change scenarios are likely to change species distributions, further impacting management and the benefits available to people across each species range. To provide an evidence base on the contribution of eleven diadromous fish populations to ecosystem services, and so gains and losses of ecosystem service benefits in the European Atlantic Area, we completed an evidence review of available literature identified within the search strategy. We also gathered expert opinion of diadromous species contributions to ecosystem services for individual catchments across Europe, to gather the full extent of contributions. Evidence was arranged in a matrix, documenting each species ecosystem service contribution to aid communication of results and application to policy and decision makers. The evidence base provided by the literature review identified that each diadromous species contributed to provision of multiple ecosystem services. Volume of evidence available to compile the matrix was far greater for Atlantic salmon Salmo salar and brown trout Salmo trutta. All species had historically contributed to provisioning ecosystem services. However, in recent decades, declines in certain species populations has reduced opportunity for commercial fisheries, leading to contributions to cultural services such as recreational activities and cultural heritage becoming comparatively greater. Evidence of contribution to regulating ecosystem services is growing in recent years, especially transfer of nutrients from marine to freshwater environments. Inclusion of local expert knowledge enabled verification of evidence from literature, in addition to identification of important contributions of lesser studied species, to culturally important commercial fisheries and cultural heritage in European regions. Collecting expert knowledge also aided communication of ecosystem service approaches.

The Allis shad, Alosa alosa (Linnaeus, 1758) and the Twaite shad, Alosa fallax (Lacépède, 1803) are two anadromous and congeneric Clupeidae species, i.e., closely related, even hybridising with each other, which makes it very difficult to differentiate between them. The marine phase represents the major part of the life cycle of these species, yet there are still large gaps in the knowledge of the biology, ecology and distribution during this phase. To overcome this lack of data, this study analysed the biometric characteristics, as well as different tissues and organs, of specimens of the genus Alosa, caught as bycatch by inshore fisheries on the continental shelf of the coastal region of Galicia, NW Iberian Peninsula. Specimens of the genus Alosa (N = 345) were acquired on daily first sale notes between January and March 2021 at the fish markets of A Guarda, Malpica and A Coruña, three of the most important landing sites for bycatches of these species in Galicia, whose fleet operates mainly on the continental shelf. Analysis of gill rakers revealed a slightly higher percentage of A. fallax than A. alosa, as well as the presence of probable F1 hybrids. At the demographic level, although there were significant differences, with A. alosa having the highest mean age, the results showed a typical age class distribution of the adult contingent, at the age of sexual maturity, for the three taxa. Condition index results clearly reflected the size differences between the parental species and the intermediate character of the hybrids, with A. alosa having the highest value for this index, the hybrids intermediate values and A. fallax the lowest values. Trophic spectrum was common to all three taxa, including fish, plankton, benthos and cephalopods; however, A. fallax showed more pronounced ichthyophagous behaviour than A. alosa and the hybrids. Both species and hybrids share habitat, simultaneously occupying both shallow coastal areas and deeper, more distant areas. Part of the occupied habitat is in Natura 2000 areas. Two new areas are proposed to increase the protection of these species, while fulfilling the EU’s obligations towards them.

Deciphering the effects of historical and recent demographic processes responsible for the spatial patterns of genetic diversity and structure is a key objective in evolutionary and conservation biology. Using population genetic analyses, we investigated the demographic history, the contemporary genetic diversity and structure, and the occurrence of hybridization and introgression of two species of anadromous fish with contrasting life history strategies and which have undergone recent demographic declines, the allis shad ( Alosa alosa ) and the twaite shad ( Alosa fallax ). We genotyped 706 individuals from 20 rivers and 5 sites at sea in Southern Europe at thirteen microsatellite markers. Genetic structure between populations was lower for the nearly semelparous species A. alosa , which disperses greater distances compared to the iteroparous species, A. fallax . Individuals caught at sea were assigned at the river level for A. fallax and at the region level for A. alosa . Using an approximate Bayesian computation framework, we inferred that the most likely long term historical divergence scenario between both species and lineages involved historical separation followed by secondary contact accompanied by strong population size decline. Accordingly, we found evidence for contemporary hybridization and bidirectional introgression due to gene flow between both species and lineages. Moreover, our results support the existence of at least one distinct species in the Mediterrannean sea: A. agone in Golfe du Lion area, and another divergent lineage in Corsica. Overall, our results shed light on the interplay between historical and recent demographic processes and life history strategies in shaping population genetic diversity and structure of closely related species. The recent demographic decline of these species’ populations and their hybridization should be carefully considered while implementing conservation programs.

Allis shad, Alosa alosa (Linnaeus, 1758), and twaite shad, Alosa fallax (Lacépède, 1803), are two anadromous fish species belonging to the family Clupeidae. Like all other diadromous species, their populations in European rivers are in decline and their scarcity has become an emerging problem. Anthropogenic activities, mainly large dams and other river obstacles have restricted their range in the eastern Atlantic and western Mediterranean. The studies carried out in Europe in the last 20 years provide a good overview of the conservation status of the shads, however, data on many biological characteristics of their life cycle, especially those corresponding to their development in the marine and coastal environment, are still very scarce. In the Iberian Peninsula, scientific data on their populations are scarcely available. In Galicia, there are studies on the populations of these species in the Ulla and Miño rivers; nevertheless, it is not known whether these species can reside in other Galician rivers suitable for their development. The microchemistry of the otoliths of diadromous fish is strongly correlated with the microchemistry of the water of their river of birth. In the present study, the microchemical composition of the otoliths of 95 adults, distributed between both species, caught at sea and in the Ulla and Miño rivers was analyzed and compared with that of the otoliths of 135 juveniles, distributed between both species, from seven Galician and French rivers. In this way, through this correlation it was possible to quantify the degree of philopatry of the specimens, specify their river of birth and establish their dispersal range during the marine phase. Thus, through the application of Machine Learning methodologies, we provide new information on the dispersal range of these species and the interactions between populations.

Allis shad, Alosa alosa (Linnaeus, 1758) and Twaite shad, Alosa fallax (Lacépède, 1803) are two anadromous Clupeidae species that spend most of their life cycle in the marine environment, where they have a coastal distribution, entering the rivers only during the spawning migration. There is no commercial shad fishery at sea; however, due to their gregarious, shoal-forming behaviour close to the shore, they are frequently caught as “bycatch” along with the “intentional catches”. In fact, both species have been arriving at Galician fish markets (NW Iberian Peninsula) for several decades. Hence, it was necessary to conduct a study in order to gain a deeper understanding of the extent of this phenomenon. To this end, we analysed the statistics on first-sale fishing in the Galician Fishing Platform. In addition, specimens of the genus Alosa (N = 345, sold in first sales not as A. alosa) were acquired at the A Guarda, Malpica and Coruña fish markets, from January to March 2021, in order to determine their specific identity. Finally, additional information regarding the capture of the specimens (precise location of capture, bathymetry, fishing method, target species, etc.) was requested from fishermen. There are records of catches of specimens of the genus Alosa in 14 Galician fish markets, from 1997 to 2020. Catches of A. alosa were regular throughout the period 1997–2020, amounting to a total of 23,956 kg. Regarding A. fallax, it is surprising to note the virtual absence of records throughout the period studied and the high pulses in certain years, with truly exceptional catches (5950 kilos in 1998 or 7320 kilos in 2018). Examination of the number of gill rakers, showed that 59% (N = 203) of the acquired specimens were not A. alosa. Of these 203 misidentified specimens, 81% (N = 164) were A. fallax and 19% (N = 39) were hybrids. Most of the shads were caught in the midst of flatfish, hake or sea bass fisheries, with driftnets being the main fishing gear, and the fishing vessels involved were of different sizes (from large vessels to traditional boats).

The increase of non-native species in rivers is of great concern. To assess the potential impact of a species it is necessary to consider the ecological state of the receiving ecosystem and the biology of the potentially invasive species. In this work we characterize two populations of Gobio lozanoi Doadrio & Madeira, 2004 (bio-metric and demographic parameter) and the invasibility (as a function of habitat quality, competing species, food supply, etc.) of the two rivers that they inhabit. In addition, biomarkers of energy reserve level are analyzed to investigate their role in the invasiveness of the species. The results show differences in energy reserve levels and invasiveness between the two fish populations, and differences in the potential invasibility of the two tributaries. In the river with lower resistance to invasion, the G. lozanoi population is well-structured, and specimens have higher lipid values. On the contrary, in the river with better ecological status (and therefore greater resistance to invasion) we found lower lipid values, higher protein values and low juvenile survival rate. The lipid level is revealed as a good indicator of invasiveness in populations of alien species, under favorable conditions for invasion.

O sabalo e a saboga son especies anadromas moi parecidas, con reproducion fluvial e fase de crecemento marino. Ainda que non son obxecto de pesca comercial, o seu comportamento gregario, formando cardumes, fai que sexan capturadas frecuentemente no medio marino de maneira “incidental” entre as “capturas intencionadas”; feito que leva comprobandose dende hai varias decadas nas lonxas galegas. O proxecto 1MARDEALOSAS xorde coma unha oportunidade para avaliar o impacto desta actividade sobre as especies indicadas e ampliar o conecemento da sua ecoloxia e distribucion. Desenvolvido no medio marino galego, os resultados permitiran establecer politicas e plans de xestion eficaces para a sua proteccion e conservacion.

The specific stock composition and dispersion of anadromous fish species aggregations in the marine environment are poorly known, while they can play a major role in the metapopulation dynamics. Otolith microchemistry has proven to be a powerful tool to address natal origins of anadromous fish. We used archived otolith microchemistry to investigate the population-specific composition of subadult European shads (Alosa alosa and Alosa fallax) in the ocean during the 1980s. The allocation of natal origin was addressed relying on contemporary water and juveniles’ signatures within a Bayesian model. A great discrimination of natal origin was obtained at the Biscay Gulf scale. However, the discrimination of 1980s natal origin for the southern rivers with similar geology based on 2013 water and juveniles’ baselines was doubtful. Our results showed that the most abundant southern populations were dominant, suggesting that population-specific composition was related to population relative abundance. The dispersion in the marine environment was plastic; alternatively, shads were found large distances away from their natal rivers, while others remained in the vicinity of their natal river plume.

As poboacions de peixes migradores atopanse en franco declive a nivel global. As causas subxacentes desta diminucion son a perda do habitat, a sobrepesca, a contaminacion e o cambio climatico. Este acusado descenso, xunto cos intereses economico, cientifico e patrimonial que xeran estas especies, motivou o deseno dun proxecto multidisciplinario desenvolvido ao longo do suroeste europeo, o proxecto DiadES. DiadES avaliara e contribuira a mellora dos servizos ecosistemicos achegados polos peixes migradores, tendo en conta os impactos do cambio climatico na sua distribucion e creara unha plataforma transnacional de medidas de xestion innovadoras.

Complete List of Authors: Nachón, David; IRSTEA EABX, Aquatic Ecosystems and Global Changes research unit, 50 avenue de Verdun 33612 ; Estación de Hidrobioloxía ‘Encoro do Con’, Universidade de Santiago de Compostela, Castroagudín, s/n 36617 Bareille, Gilles; CNRS/ Univ Pau & Pays Adour/ e2s UPPA, Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Materiaux – MIRA*, UMR 5254, 64000, Drouineau, Hilaire; IRSTEA EABX, Aquatic Ecosystems and Global Changes research unit, 50 avenue de Verdun 33612 Tabouret, Hélène; CNRS/ Univ Pau & Pays Adour/ e2s UPPA, Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Materiaux – MIRA*, UMR 5254, 64000, Taverny, Catherine; F.D.A.A.P.P.M.A. 33, 10 ZA du Lapin – 33750 Boisneau, Catherine; UMR CNRS CITERES 7324, Université de Tours, 33 Allée F de Lesseps, 37200 Berail, Sylvain; CNRS/ Univ Pau & Pays Adour/ e2s UPPA, Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Materiaux – MIRA*, UMR 5254, 64000, Pécheyran, Christophe; CNRS/ Univ Pau & Pays Adour/ e2s UPPA, Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Materiaux – MIRA*, UMR 5254, 64000, Claverie, Fanny; CNRS/ Univ Pau & Pays Adour/ e2s UPPA, Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Materiaux – MIRA*, UMR 5254, 64000, Daverat, Françoise; IRSTEA EABX, Aquatic Ecosystems and Global Changes research unit, 50 avenue de Verdun 33612

All around the world, diadromous fish are facing multiple anthropogenic pressures resulting in a global decline in these migratory species. In Gironde, the allis shad populations which were the most abundant in Europe are no exception and have dramatically declined since 2000. Failure in reproduction success is one of the proposed explanations for this collapse. In light of this, we studied the reproductive strategy of the allis shad populations in the Gironde watershed. We used an original methodology to define the thermal behavior of allis shad during reproduction using an electivity index for 14 years of field monitoring in two rivers, composed of daily reproduction activity and temperature. For the first time, this study deeply explores and defines the thermal preference of a shad species during the reproduction. A temperature preference between 14.5 °C and 23 °C by spawners during the reproduction was observed. Despite annual fluctuations, an overall similarity of temperature ranges between the two rivers was observed. The thermal preference matched with the thermal tolerance of early stages (16.2 °C to 24.8 °C) and confirms that allis shad spawners adopt behavioral rules of reproduction in order to maximize the survival of their offspring. The similarity of shad thermal behavior may indicate the phylogenetic basis of thermal preference for fish species.

Hybridization dynamics between co‐occurring species in environments where human‐mediated changes take place are important to quantify for furthering our understanding of human impacts on species evolution and for informing management. The allis shad Alosa alosa (Linnaeus, 1758) and twaite shad Alosa fallax (Lacépède, 1803), two clupeids sister species, have been severely impacted by human activities across Europe. The shrinkage of A. alosa distribution range along with the decline of the remaining populations' abundance threatens its persistence. The main objective was to evaluate the extent of hybridization and introgression between those interacting species. We developed a set of 77 species‐specific SNP loci that allowed a better resolution than morphological traits as they enabled the detection of hybrids up to the third generation. Variable rates of contemporary hybridization and introgression patterns were detected in 12 studied sites across the French Atlantic coast. Mitochondrial markers revealed a cyto‐nuclear discordance almost invariably involving A. alosa individuals with an A. fallax mitochondrial DNA and provided evidence of historical asymmetric introgression. Overall, contemporary and historical introgression revealed by nuclear and mitochondrial markers strongly suggests that a transfer of genes occurs from A. fallax toward A. alosa genome since at least four generations. Moreover, the outcomes of introgression greatly depend on the catchments where local processes are thought to occur. Undoubtedly, interspecific interaction and gene flow should not be overlooked when considering the management of those species.

The non-native signal crayfish Pacifastacus leniusculus (Dana, 1852) has successfully been established in many catchments of Europe, causing detrimental impacts on both native flora and fauna. The aim of this paper was to report the anthropogenic introduction of P. leniusculus and its ectosymbionts (family Branchiobdellidae) in the River Tambre (NW Spain). Seventy one individuals of P. leniusculus were captured in the Barrie de la Maza reservoir in November 2016. The presence of several age classes, including young of the year individuals, and ovigerous females suggests that the population is already established and viable. Most individuals (96%) presented branchiobdellidans and 4% exhibited white spots probably caused by a virus. Melanised spots caused by the oomycete Aphanomyces astaci (Schikora, 1906) (crayfish plague) were not found in this sample. This population is expected to disperse through the River Tambre basin. The Tambre population may also be a source of individuals for new introductions in the region, where currently no P. leniusculus populations are known at a minimum radio of 150 km in straight line. Therefore, immediate management and preventive actions should be pursued to control the dispersion and growth of this population and to avoid new introductions (e.g. ban of crayfish fishing).

Spawning individuals of allis shad, Alosa alosa (Linnaeus, 1758), and twaite shad, Alosa fallax (Lacépède, 1803), were sampled from three rivers on the Atlantic coast of the Iberian Peninsula (Ulla, Minho, Mondego) during 2008 to 2013 to assess the presence of the zoonotic marine parasite Anisakis spp. larvae. The results revealed that both shad species were infected by third-larval stage Anisakis simplex s.s. and Anisakis pegreffii. The latter is reported in mixed infections in both shad species of Western Iberian Peninsula for the first time. In A. alosa, the prevalence of Anisakis infection can reach 100%, while in A. fallax, prevalence was up to 83%. Infected individuals of the former species also often contain much higher number of parasites in theirs internal organs and flesh: from 1 to 1138 Anisakis spp. larvae as compared to 1 to 121 larvae, respectively. In general, numbers of A. pegreffii were higher than those of A. simplex s.s. Our results suggest that in the marine environment of the Western Iberian Peninsula, both anadromous shad species act as paratenic hosts for A. simplex s.s. and A. pegreffii, thus widening the distribution of the infective nematode larvae from the marine to the freshwater ecosystem. This finding is of great epidemiological relevance for wildlife managers and consumers, considering the zoonotic and gastroallergic threats posed of these parasites.

Samples of anadromous Alosa alosa (Clupeidae) (n = 163) and Alosa fallax (Clupeidae) (n = 223), caught in Western Iberian Peninsula Rivers from 2008 to 2013, were examined for buccal, branchial and internal macroparasites, which were identified using morphological and molecular methods. Alosa alosa were infected with Anisakis simplex s.s., Anisakis pegreffii, Hysterothylacium aduncum, Rhadinorhynchus pristis, Mazocraes alosae, Hemiurus appendiculatus, Ceratothoa italica and an unidentified ergasilid copepod. Ceratothoa italica represents a new host record for A. alosa. Alosa fallax were infected with A. simplex s.s., A. pegreffii, H. aduncum, H. appendiculatus, Clavellisa emarginata and an unidentified cymothoid isopod. This is the first report of C. italica, C. emarginata and M. alosae in the Iberian Peninsula. The phylogenetic positions of M. alosae, H. appendiculatus and C. emarginata were assessed using 18S and 28S ribosomal RNA (rRNA); our contributions provide a better understanding of the phylogenetic relationships within their groups. Qualitative and quantitative differences in the parasite faunas of these two shad species are consistent with different feeding strategies. The results provide information about host migration behaviour and transmission pathways through diet during the marine trophic phase of the shad's life cycle and their roles as paratenic or final hosts and transporters of parasites between seawater and freshwater environments. The zoonotic parasites A. simplex s.s. and A. pegreffii pose a risk for consumers or riverine mammals (e.g. European otter). The use of parasites as biological tags for shad stocks in Western Iberian Rivers could be a useful approach in multidisciplinary studies concerning fish stock delimitation and characterization.