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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.

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.

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.

International audience; 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.

Approximately 6% of the freshwater living northern straight-mouth nase Pseudochondrostoma duriense in two Spanish rivers had attached post-metamorphic sea lamprey Petromyzon marinus. Minimum prey size was 16·1 cm fork length and 56·3 g mass. The condition factor of attacked P. duriense was 16% lower than that of unattacked conspecifics.

19 pages, 7 figures, 6 tables, 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., M. Bao is supported by a PhD grant from the University of Aberdeen and also by financial support of the contract from the EU Project PARASITE (grant number 312068). A. Roura is supported by "Fundación Barrié de la Maza" postdoctoral fellowship and a Securing Food, Water and the Environment Research Focus Area grant (La Trobe University). This study was partially supported by a PhD grant from the Portuguese Foundation for Science and Technology (FCT) (SFRH/BD/44892/2008) and partially supported by the European Regional Development Fund (ERDF) through the COMPETE—Operational Competitiveness Programme and national funds through FCT—Foundation for Science and Technology, under the project BPEst-C/MAR/LA0015/2013. This work has been partially supported by the project 10PXIB2111059PR of the Xunta de Galicia and the project MIGRANET of the Interreg IV B SUDOE (South-West Europe) Territorial Cooperation Programme (SOE2/P2/E288). D.J. Nachón is supported by a PhD grant from the Xunta de Galicia (PRE/2011/198).

cited By 4; International audience; Understanding dispersal capacities for migratory species is crucial for their management. By coupling otolith microchemistry and microsatellite genetic analyses, we provided information on snapshot and long-term dispersal capacity of Allis shad (Alosa alosa), an anadromous clupeid in decline throughout its distribution range. The allocation of natal origin was obtained from water chemistry and signatures in otoliths of juveniles and spawners within a Bayesian model. The majority of adults were assigned to a source river with high degrees of confidence; only 4% were undetermined. Otolith natal origins were used to define a population baseline by grouping individuals from the same natal river and not from the same sampling location as usually done. While A. alosa exhibited a high level of natal site fidelity, this species showed weak genetic structure, which supported the evidence of a substantial flow of strayers between river basins in the vicinity or at longer distances. However, long distance straying was probable but not frequent. In a context of global change, straying would be a key mechanism to drive dispersal and allow resilience of Allis shad populations.

12 páginas, 4 figuras, 3 tablas, 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, M. Bao is supported by a PhD grant from the University of Aberdeen and also by financial support of the contract from the EU Project PARASITE (grant number 312068). This study was partially supported by a PhD grant from the Portuguese Foundation for Science and Technology (FCT) (SFRH/BD/44892/2008) and partially supported by the European Regional Development Fund (ERDF) through the COMPETE — Operational Competitiveness Programme and national funds through Foundation for Science and Technology (FCT), under the project B PEst-C/MAR/ LA0015/2013. The authors thank the staff of the Station of Hydrobiology of the USC "Encoro do Con" due their participation in the surveys. This work has been partially supported by the project 10PXIB2111059PR of the Xunta de Galicia and the project MIGRANET of the Interreg IV B SUDOE (South-West Europe) Territorial Cooperation Programme (SOE2/P2/E288). D.J. Nachón is supported by a PhD grant from the Xunta de Galicia (PRE/2011/198).

Knowledge regarding the marine phase of twaite shad (Alosa fallax (Lacépède, 1803)) and allis shad (Alosa alosa (Linnaeus, 1758)) remains limited. To understand the coastal and continental distribution and dynamic of migration of these species, in the present study we collected official data records of marine landings (kg), separated according to species, for all fish markets in Galicia (north-west of the Iberian Peninsula) and undertook freshwater field sampling campaigns. The findings indicate that both species of shad exhibit a coastal distribution near the rivers where they spawn, namely the Minho and Ulla rivers. Catches at sea declined from January, a phenomenon that can be understood as the onset of migration to the rivers. Bycatch reports are important because both A. fallax and A. alosa are listed in the Annex V of the European Union Habitats Directive, which lists animal species whose exploitation and taking in the wild may be subject to management measures. However, for a better management of these species it is necessary to understand the magnitude of displacements and connection of shad populations at sea.

In order to contribute to a better understanding of feeding behaviour of adult twaite shad, Alosa fallax ,i n freshwater, we studied the stomach content composition of 147 A. fallax during their upstream spawning migration in the River Ulla (Galicia, NW Spain). The diet composition included 34 types of prey, aquatic invertebrates being the most important food. The present study is, to the best of our knowledge, the first record of A. fallax feeding on potamodromous Pseudochondrostoma duriense. Additional keywords: Alosa fallax, diet, Iberian Peninsula, piscivorous, Pseudochondrostoma duriense.