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This study investigates stock mixing of genetically distinct Atlantic cod (Gadus morhua) stocks in the Kattegat, an area geographically located between the North Sea and the Baltic Sea, by combining genetic population identification with habitat assignments from hatch to capture from otolith microchemistry. Cod captured in Kattegat were genetically assigned to either the North Sea or the endemic Kattegat population. Otolith chemical fingerprints differed significantly between populations during the larval and pelagic juvenile stage with higher strontium and lower barium and manganese concentrations in the North Sea population than the Kattegat population, indicating that North Sea cod are spawned in the North Sea or Skagerrak and drift into the Kattegat during the early life stages. Individual cod of both populations undertook frequent, but predominantly short-term, migrations to other areas than the Kattegat, with La presente etude se penche sur le melange de stocks de morues (Gadus morhua) genetiquement distincts dans le Kattegat, une region situee entre la mer du Nord et la mer Baltique, en combinant l'identification genetique a la population et l'affectation d'habitats de l'eclosion a la capture, a la lumiere de la microchimie des otolites. Les morues capturees dans le Kattegat sont affectees genetiquement soit a la population de la mer du Nord ou a la population endemique au Kattegat. Les empreintes digitales chimiques des otolites des differentes populations different significativement aux etapes larvaire et pelagique juvenile, alors que les individus de la population de la mer du Nord presentent des concentrations plus elevees de strontium et plus faibles de baryum et de manganese que ceux de la population du Kattegat, ce qui indique que les morues de la mer du Nord sont originaires de la mer du Nord ou du Skagerrak et derivent jusque dans le Kattegat durant les premieres etapes de la vie. Des individus des deux populations entreprennent des migrations frequentes, mais la plupart du temps de courte duree, vers d'autres regions que le Kattegat, moins de 25 % des individus residant toute leur vie dans le Kattegat. D'une saison et d'une classe d'age a l'autre, les individus des deux populations se retrouvent le plus souvent dans le Kattegat (67 %), environ 25 % des individus des deux populations se trouvant dans l'ouest de la mer Baltique et moins de 10 %, dans le Skagerrak-la mer du Nord. L'etude demontre l'utilite de cette approche pour inferer la connectivite de populations et les trajectoires de migration des poissons individuels, ainsi que ses applications possibles dans des domaines fondamentaux et appliques de l'ecologie des poissons et des sciences halieutiques. [Traduit par la Redaction], Introduction Fish migrations and stock mixing are major issues for assessments of stock status and for fisheries management, which are usually constrained to geographical management areas that do not always [...]

Understanding migration patterns and habitat use is of great importance for management and conservation of marine living resources. The chemical composition of otoliths is influenced by the surrounding environment; therefore, they are indispensable data archives. To extract migration patterns and historical habitat use of individual fish, we analyse otolith chemical compositions obtained by laser ablation inductively coupled plasma mass spectrometry by a regimeswitching state-space model. The state-space model filters the measurement noise from the environmental signal. In turn, the filtered signal is converted to geographical positions through a calibration of strontium to salinity. The method is validated by a simulation study and applied to 404 Atlantic cod (Gadus morhua) otoliths. La comprehension des habitudes migratoires et de l'utilisation de l'habitat est d'une grande importance pour la gestion et la conservation des ressources marines vivantes. Parce que leur composition chimique est influencee par le milieu environnant, les otolites constituent d'indispensables archives de donnees. Afin d'inferer les habitudes migratoires et d'utilisation de l'habitat passees de poissons individuels, nous analysons la composition chimique d'otolites obtenue par spectrometrie de masse avec plasma a couplage inductif jumelee a l'ablation au laser (LA-ICP-MS) a l'aide d'un modele d'espace d'etats des changements de regime. Ce modele filtre le bruit de mesure du signal du milieu ambiant. Le signal filtre est ensuite converti en positions geographiques par l'etalonnage du strontium en fonction de la salinite. La methode est validee par une etude de simulation et appliquee a 404 otolites de morue (Gadus morhua). [Traduit par la Redaction], 1. Introduction Understanding life history, habitat use and migration patterns is vital for management and conservation of marine resources. Knowledge of migration patterns are needed to evaluate the potential effects [...]

Preserving and maximizing genetic diversity in conservation programs, including for restocking, are of high importance. The threatened European flat oyster (Ostrea edulis) is currently the subject of several applied conservation and restocking programs, but concerns have been raised over potential negative side effects of these programs, for example due to our limited knowledge about the genetic effects in natural populations of releasing offspring of hatchery origin. Here, we developed an effective, easily applicable and highly reliable method to assess the genetic diversity and parental contributions in flat oyster hatchery production based on analyses of 17 microsatellite loci. We analysed four broodstocks and their hatchery-reared spat (total n = 354) and compared diversity to that in wild samples of adults and spat from the broodstock source in the Limfjorden (total n = 138). Based on four hatchery tank experiments with fully resolved parentage assignments, we found that five swarming events (larval releases) were characterized by a single maternal and multiple paternal contributions, and that the number of contributing parents varied greatly both among individual tanks, and between swarming events within tank. On average, the effective number of breeders was only one third of the actual broodstock size. Although the broodstock exhibited high genetic variation, the high reproductive skew resulted in produced offspring representing only a relatively small subset of this variation. The work demonstrates potential impact of hatchery reared offspring on decreasing genetic diversity in wild populations, but also that genetic monitoring can be integrated in conservation programs to minimize negative effects on restoration and supplementary restocking programs that utilize hatchery reared spats to support natural populations.

Abstract Background Marine fishes have been shown to display low levels of genetic structuring and associated high levels of gene flow, suggesting shallow evolutionary trajectories and, possibly, limited or lacking adaptive divergence among local populations. We investigated variation in 98 gene-associated single nucleotide polymorphisms (SNPs) for evidence of selection in local populations of Atlantic cod (Gadus morhua L.) across the species distribution. Results Our global genome scan analysis identified eight outlier gene loci with very high statistical support, likely to be subject to directional selection in local demes, or closely linked to loci under selection. Likewise, on a regional south/north transect of central and eastern Atlantic populations, seven loci displayed strongly elevated levels of genetic differentiation. Selection patterns among populations appeared to be relatively widespread and complex, i.e. outlier loci were generally not only associated with one of a few divergent local populations. Even on a limited geographical scale between the proximate North Sea and Baltic Sea populations four loci displayed evidence of adaptive evolution. Temporal genome scan analysis applied to DNA from archived otoliths from a Faeroese population demonstrated stability of the intra-population variation over 24 years. An exploratory landscape genetic analysis was used to elucidate potential effects of the most likely environmental factors responsible for the signatures of local adaptation. We found that genetic variation at several of the outlier loci was better correlated with temperature and/or salinity conditions at spawning grounds at spawning time than with geographic distance per se. Conclusion These findings illustrate that adaptive population divergence may indeed be prevalent despite seemingly high levels of gene flow, as found in most marine fishes. Thus, results have important implications for our understanding of the interplay of evolutionary forces in general, and for the conservation of marine biodiversity under rapidly increasing evolutionary pressure from climate and fisheries induced changes in local environments.

Long time series of reliable individual growth estimates are crucial for understanding the status of a fish stock and deciding upon appropriate management. Tagging data provide valuable information about fish growth, and are especially useful when age‐based growth estimates and stock assessments are compromised by age‐determination uncertainties. However, in the literature there is a lack of studies assessing possible changes in growth over time using tagging data. Here, data from tagging experiments performed in the Baltic Sea between 1971 and 2019 were added to those previously analysed for 1955–1970 to build the most extensive tagging dataset available for Eastern Baltic cod (Gadus morhua, Gadidae), a threatened stock with severe age‐determination problems. Two length‐based methods, the GROTAG model (based on the von Bertalanffy growth function) and a Generalized Additive Model, were used to assess for the first time the potential long‐term changes in cod growth using age‐independent data. Both methods showed strong changes in growth with an increase until the end of the 1980s (8.6–10.6 cm/year for a 40 cm cod depending on the model) followed by a sharp decline. This study also revealed that the current growth of cod is the lowest observed in the past 7 decades (4.3–5.1 cm/year for a 40 cm cod depending on the model), indicating very low productivity. This study provides the first example of the use of tagging data to estimate multidecadal changes in growth rates in wild fish. This methodology can also be applied to other species, especially in those cases where severe age‐determination problems exist. [ABSTRACT FROM AUTHOR]

Sandeels are an ecologically important group of fishes; they are a key part of the food chain serving as food for marine mammals, seabirds and fish. Sandeels are further targeted by a large industrial fishery, which has led to concern about ecosystem effects. In the North Sea, the lesser sandeel Ammodytes marinus is by far the most prevalent species of sandeel in the fishery. Management of sandeel in the North Sea plus the Kattegat is currently divided into seven geographical areas, based on subtle differences in demography, population dynamics and results from simulations of larval dispersal. However, little is known about the underlying genetic population structure. In this study, we used 2,522 SNPs derived from restriction site‐associated DNA sequencing (RADseq) typed in 429 fish representing four main sandeel management areas. Our main results showed (a) a lack of a clear spatially defined genetic structure across the majority of genetic markers and (b) the existence of a group of at least 13 SNPs under strong linkage disequilibrium which together separate North Sea sandeel into three haplotype clusters, suggestive of one or more structural variants in the genome. Analyses of the spatial distribution of these putative structural variants suggest at least partial reproductive isolation of sandeel in the western management area along the Scottish coast, supporting a separate management. Our results highlight the importance of the application of a large number of markers to be able to detect weak patterns of differentiation. This study contributes to increasing the genetic knowledge of this important exploited species, and results can be used to improve our understanding of population dynamics and stock structure. [ABSTRACT FROM AUTHOR]

Climate changes in the Arctic are predicted to alter distributions of marine species. However, such changes are difficult to quantify because information on present species distribution and the genetic variation within species is lacking or poorly examined. Blue mussels, Mytilus spp., are ecosystem engineers in the coastal zone globally. To improve knowledge of distribution and genetic structure of the Mytilus edulis complex in the Arctic, we analyzed 81 SNPs in 534 Mytilus spp. individuals sampled at 13 sites to provide baseline data for distribution and genetic variation of Mytilus mussels in the European Arctic. Mytilus edulis was the most abundant species found with a clear genetic split between populations in Greenland and the Eastern Atlantic. Surprisingly, analyses revealed the presence of Mytilus trossulus in high Arctic NW Greenland (77°N) and Mytilus galloprovincialis or their hybrids in SW Greenland, Svalbard, and the Pechora Sea. Furthermore, a high degree of hybridization and introgression between species was observed. Our study highlights the importance of distinguishing between congener species, which can display local adaptation and suggests that information on dispersal routes and barriers is essential for accurate predictions of regional susceptibility to range expansions or invasions of boreal species in the Arctic. [ABSTRACT FROM AUTHOR]