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A schematic model of potential metapopulation dynamics in the study area. Potential connectivity between populations of a metapopulation in the study area of Landvikvannet and the connected fjords as hypothesized based on the results of the present study. The biological characteristics (VS = vertebral counts) of the different populations are given.
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Gillnet sampling and analyses of otolith shape, vertebral count and growth indicated the presence of three putative Atlantic herring (Clupea harengus L.) populations mixing together over the spawning season February–June inside and outside an inland brackish water lake (Landvikvannet) in southern Norway. Peak spawning of oceanic Norwegian spring sp...
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... Migration patterns and the utilization of spawning grounds have changed in response to population size over time and are considered highly plastic (Dragesund et al., 1997;Huse et al., 2010). In contrast, stationary spring spawning herring populations are geographically restricted populations which are adapted to local conditions, completing their entire life cycle within fjords (Aasen, 1952;Mikkelsen et al., 2018), semi-enclosed basins (Johannessen et al., 2009) or brackish lakes (Eggers et al., 2014). Differences in reproductive strategies in terms of reproductive investment can be linked to divergent life-history strategies of migratory and stationary herring populations. ...
... Furthermore, the variability of the identified clusters had been reduced, especially for cluster 2 which is now at levels of known populations ( Figure S8). For those populations with similar high variability ( Figure S8), it has been shown that they mix with migratory NSSH during spawning or at least parts of the local population display some sort of migratory behavior (Johannessen et al., 2009;Sørensen, 2012;Eggers et al., 2014;Eggers et al., 2015). ...
Atlantic herring (Clupea harengus) has a complex population structure and displays a variety of reproductive strategies. Differences in reproductive strategies among herring populations are linked to their time of spawning, as well as to their reproductive investment which can be an indicator for migratory vs. stationary behavior. These differences are reflected in the number of oocytes (fecundity) and the size of the oocytes prior spawning. We studied potential mixing of herring with different reproductive strategies during the spring spawning season on a coastal spawning ground. It has been hypothesized that both spring and autumn spawning herring co-occur on this specific spawning ground. Therefore, we investigated the reproductive traits oocyte size, fecundity, fertilization success as well as length of the hatching larvae during the spring spawning season from February to April. We used a set of 11 single nucleotide polymorphism markers (SNPs), which are associated with spawning season, to genetically identify autumn and spring spawning herring. Reproductive traits were investigated separately within these genetically distinct spawning types. Furthermore, we used multivariate analyses to identify groups with potentially different reproductive strategies within the genetic spring spawners. Our results indicate that mixing between ripe spring and autumn spawners occurs on the spawning ground during spring, with ripe autumn spawners being generally smaller but having larger oocytes than spring spawners. Within spring spawners, we found large variability in reproductive traits. A following multivariate cluster analysis indicated two groups with different reproductive investment. Comparisons with other herring populations along the Norwegian coastline suggest that the high variability can be explained by the co-occurrence of groups with different reproductive investments potentially resulting from stationary or migratory behavior. Fertilization success and the length of the hatching larvae decreased with progression of the spawning season, with strong inter-individual variation, supporting our findings. Incorporating such complex population dynamics into management strategies of this species will be essential to build its future population resilience.
... Sampling at the outer coast was carried out in the Grosfjord, where the water is polyhaline and low in oxygen (Vann-nett; https://www.vann-nett.no). Landvikvannet is an inland brackish water lake (1.85 km 2 , average depth = 10 m, and maximum depth = 25 m), where the upper layer has low salinity (<15 PSU) and is rich in oxygen, but under 10 m salinity is ∼20-25 PSU and conditions are anoxic (Eggers et al. 2014). This lake is connected to the ocean by a 3 km long canal (1-4 m deep) constructed in 1877. ...
Many anguillid eel species display facultative catadromy. Some eel spend their entire life cycle in marine coastal areas, but the geographical extent of this, especially at the extremes of their distributional ranges, is unknown. We analysed otolith Sr:Ca and Ba:Ca from yellow-stage European eel (Anguilla anguilla) sampled along the coast of Norway and in several freshwater lakes (58°N–63°N), to infer their initial settlement and later life movement patterns with regards to habitat salinity. Most eel (80%) sampled in marine habitats (n = 371) had settled and remained in marine water, but 20% had moved between marine and freshwater habitats and were hence classified as inter-habitat shifters. Among freshwater sampled eel (n = 99), 80% had settled and remained in fresh water, but 20% were classified as inter-habitat shifters. The average growth rates for marine water residents, inter-habitat shifters, and freshwater residents were 35, 27, and 17 mm·year–1, respectively. Northern European shallow marine habitats may serve as important yellow eel growth habitats and may be critical to buffer the European eel population against the general decrease in continental recruitment.
... Homing to natal spawning grounds has been evidenced through external tagging studies in the NW Atlantic (Stobo 1982;Stephenson et al. 2009;Wheeler and Winters 2011). However, tagging experiments on spawning grounds in the NE Atlantic revealed low recapture rates (Wood et al. 1954), or showed no clear evidence of homing to natal spawning grounds (Eggers et al. 2014). Biological markers, most notably otolith structure and microchemistry, have provided more empirical evidence that herring exhibit fidelity to spawning grounds and season in the Celtic Sea, west of the British Isles and Norway (Geffen et al. 2011;Deschepper et al. 2020;Berg et al. 2021). ...
... During the 1990s, the SSB of the NSS stock recovered to similar levels as in the early twentieth century (Toresen and Østvedt 2008), driving the recolonization of southern spawning grounds (Rottingen and Slotte 2001). In 2009, significant spawning in some southern spawning grounds led to recruitment of a strong year-class that continued using the former grounds (Eggers et al. 2014;ICES 2020c). However, a lack of recruitment success elsewhere in Norwegian waters has caused a substantial reduction in the overall SSB of the NSS stock (ICES 2020c;Tiedemann et al. 2020). ...
Scotland once had the largest herring fishery globally, generating local income, identity, and societal change. Following historic stock collapse, in spring 2018/2019 large herring shoals were observed on the west coast for the first time in decades, at a formerly important spawning ground. This highlights the urgency of maintaining historic (and contemporary) benthic spawning habitat, which these fish rely upon, in good condition. However, information on exact location, characteristics, and status of historic and contemporary spawning grounds, if existing, is not easily accessible. We searched over 1190 literature sources, dating back to 1884, using scientific databases and web-based searches, and ran a query for automated search of comprehensive historic reports. We present current knowledge on Scottish herring spawning grounds, retrieved through these searches and fisher interviews, maps showing historic and contemporary spawning grounds, and discuss challenges arising from the methods used to recognize these grounds. Knowledge gaps regarding location and environmental status of past and current spawning grounds, particularly relevant for Scotland’s west coast, are identified. Based on the importance of specific environmental and physical variables for herring reproductive success, we advocate the inclusion of essential spawning grounds into herring management plans. This will require additional data on spawning grounds, including historic local ecological knowledge rarely considered. An inclusive ecosystem-based approach to herring management would allow more targeted actions to conserve (and potentially restore) essential spawning habitat. More effective management strategies will also call for reversing the (global) issue of the disconnect between different stakeholder groups.
Graphical abstract
... After hatching, however, the larvae can be spread by the currents for 3 to 4 months (Holst and Slotte, 1998 Skagseth et al., 2015). Also for herring, local coastal spawning along the Norwegian Skagerrak coast has been observed during spring (Eggers et al., 2014), but the extent of larval drift, utilizing the coast as nursery area, from other populations in the North Sea and Skagerrak-Kattegat area remains unquantified. Due to well-defined spawning grounds and low connectivity, herring populations within the Skagerrak-Kattegat region are genetically distinct (Bekkevold et al., 2005;Pettersson et al., 2019;Han et al., 2020). ...
Coastal areas are important habitats for early life stages of many fish species. These habitats are used as nursery grounds and can provide a significant contribution to the recruitment of a fish population. In 1919, standardized sampling with a beach seine along the Norwegian Skagerrak coastline was established mainly to target 0-group fish. Here, we focus on Atlantic herring and European sprat to explore whether inter-annual variability in the abundance of these species is indicative of variability in recruitment. We investigated if the abundance of 0-group herring and sprat are affected by environmental factors. Further, the beach seine abundance indices were compared with recruitment estimates of neighboring stocks. There was a clear correlation between herring and sprat abundance in the beach seine samples. While sprat abundance was mainly affected by environmental factors such as temperature and current drift, herring abundance was positively affected by the recruitment of the neighboring stock of western Baltic spring spawners. One plausible explanation could be that sprat recruit to a more local component, while herring of the neighboring stock utilize the Skagerrak coastline as nursery grounds. This study demonstrates the importance of long time series and can provide new insight into the dynamics and structure of multiple fish species.
... There are exceptions to these generalizations; the local Norwegian herring stocks LPH, CSH, and LVH grew roughly in the same way as the oceanic stocks. Their large size has been attributed to cooccurrence with NSSH (Silva et al. 2013) and other coastal herring stocks (e.g., Skagerrak, and Kattegat herring) (Johannessen et al. 2009Eggers et al. 2014;Berg et al. 2017). More specifically, LPH have interacted with NSSH over prolonged periods, but mainly during the NSSH stock collapse in the late 1960s when its distribution was highly restricted over a couple of decades to near the Norwegian coast ). ...
Life-history traits of Pacific (Clupea pallasii) and Atlantic (Clupea harengus) herring, comprising both local and oceanic stocks subdivided into summer-autumn and spring spawners, were extensively reviewed. The main parameters investigated were body growth, condition, and reproductive investment. Body size of Pacific herring increased with increasing latitude. This pattern was inconsistent for Atlantic herring. Pacific and local Norwegian herring showed comparable body conditions, whereas oceanic Atlantic herring generally appeared stouter. Among Atlantic herring, summer and autumn spawners produced many small eggs compared to spring spawners, which had fewer but larger eggs—findings agreeing with statements given several decades ago. The 26 herring stocks we analysed, when combined across distant waters, showed clear evidence of a trade-off between fecundity and egg size. The size-specific individual variation, often ignored, was substantial. Additional information on biometrics clarified that oceanic stocks were generally larger and had longer life spans than local herring stocks, probably related to their longer feeding migrations. Body condition was only weakly, positively related to assumingly in situ annual temperatures (0–30 m depth). Contrarily, body growth (cm × y−1), taken as an integrator of ambient environmental conditions, closely reflected the extent of investment in reproduction. Overall, Pacific and local Norwegian herring tended to cluster based on morphometric and reproductive features, whereas oceanic Atlantic herring clustered separately. Our work underlines that herring stocks are uniquely adapted to their habitats in terms of trade-offs between fecundity and egg size whereas reproductive investment mimics the productivity of the water in question.
... The complex shape of herring otoliths makes it difficult to accurately describe the shape. Wavelet transform is reported to be more powerful than the commonly applied Fourier transform for shape analysis (Libungan and Pálsson, 2015), and it has been successfully applied to herring otoliths in the North Atlantic (Berg et al., 2018;Eggers et al., 2014;Libungan et al., 2015a, b). ...
... Otolith shape analysis has also become more widespread in stock discrimination studies of Atlantic herring than body morphometrics (e.g. Libungan et al., 2015;Eggers et al., 2014). In the current study, otolith and body morphometric analyses were analysed separately and the results examined to test their suitability to this dataset, before being combined in a joint analysis. ...
The aim of this study was to assess the identity of herring stocks in ICES Divisions 6.a, 7.b and 7.c, through genetic and morphometric analyses, in order to develop profiles of the northern (ICES Division 6.a North) and southern (ICES Divisions 6.a South, 7.b and 7.c) stocks, which could be used to discriminate the two stocks during times of mixing, such as, in the summer acoustic surveys. The study comprised an extensive review of the history of the existing stock delineations, comprehensive sampling for both the genetic and morphometric components of the project, genetic marker development, genetic screening of samples, the establishment of a genetic protocol for large scale sample screening, morphometric analyses and comparative analyses of both methods. The morphometric methods used in the current study indicated significant differences between the 6.a.S herring and the 6.a.N autumn spawning herring, however they did not show sufficient power to discriminate mixed survey catches. The genetic markers and assignment methods constitute a tool that can be used for the assignment of herring caught in mixed survey and commercial catches in Division 6.a into their population of origin with a high level of accuracy (>90%). This will enable separate survey indices and separate assessments to be developed for the stocks in this area, which will facilitate improved management.
... The relationship between genetic differentiation and shortest water distance revealed that samples from Landvik strongly departed from any geographically driven expectation (see Figure 3d), a situation also described Landvik is also inhabited by a taxonomically close species to sprat: the Atlantic herring. Landvik herring are considered as a self-sustaining and somewhat stationary population, characterized by slower growth, smaller length at maturity, lower vertebral count, shorter life span, higher relative fecundity, and divergent genetic profiles compared to the migratory oceanic herring in other parts of the Norwegian waters (Eggers, 2013;Eggers et al., 2014;Silva et al., 2013). Meristic trait vertebral count is often used as a population identifier in herring (e.g., Berg et al., 2017;Mosegaard & Madsen, 1996;Rosenberg & Palmén, 1981), and the observation that vertebral count in Landvik herring is similar to that in herring populations in the brackish Western Baltic Sea has led to the hypothesis that Landvik was colonized by low salinity adapted herring of Western Baltic Sea origin (Berg et al., 2019;Eggers et al., 2014). ...
... Landvik herring are considered as a self-sustaining and somewhat stationary population, characterized by slower growth, smaller length at maturity, lower vertebral count, shorter life span, higher relative fecundity, and divergent genetic profiles compared to the migratory oceanic herring in other parts of the Norwegian waters (Eggers, 2013;Eggers et al., 2014;Silva et al., 2013). Meristic trait vertebral count is often used as a population identifier in herring (e.g., Berg et al., 2017;Mosegaard & Madsen, 1996;Rosenberg & Palmén, 1981), and the observation that vertebral count in Landvik herring is similar to that in herring populations in the brackish Western Baltic Sea has led to the hypothesis that Landvik was colonized by low salinity adapted herring of Western Baltic Sea origin (Berg et al., 2019;Eggers et al., 2014). In addition, factorial crossing experiments performed at a range of salinities ranging from 6 to 35 revealed adaptation of herring populations to their native salinity conditions and also that adaption to salinity is transmitted to the offspring within the following generation (Berg et al., 2019). ...
Habitat changes represent one of the five most pervasive threats to biodiversity. However, anthropogenic activities also have the capacity to create novel niche spaces to which species respond differently. In 1880, one such habitat alterations occurred in Landvikvannet, a freshwater lake on the Norwegian coast of Skagerrak, which became brackish after being artificially connected to the sea. This lake is now home to the European sprat, a pelagic marine fish that managed to develop a self-recruiting population in barely few decades. Landvikvannet sprat proved to be genetically isolated from the three main populations described for this species; that is, Norwegian fjords, Baltic Sea, and the combination of North Sea, Kattegat, and Skagerrak. This distinctness was depicted by an accuracy self-assignment of 89% and a highly significant F ST between the lake sprat and each of the remaining samples (average of ≈0.105). The correlation between genetic and environmental variation indicated that salinity could be an important environmental driver of selection (3.3% of the 91 SNPs showed strong associations). Likewise, Isolation by Environment was detected for salinity, although not for temperature, in samples not adhering to an Isolation by Distance pattern. Neighbor-joining tree analysis suggested that the source of the lake sprat is in the Norwegian fjords, rather than in the Baltic Sea despite a similar salinity profile. Strongly drifted allele frequencies and lower genetic diversity in Landvikvannet compared with the Norwegian fjords concur with a founder effect potentially associated with local adaptation to low salinity. Genetic differentiation (F ST) between marine and brackish sprat is larger in the comparison Norway-Landvikvannet than in Norway-Baltic, which suggests that the observed divergence was achieved in Landvikvannet in some 65 generations, that is, 132 years, rather than gradually over thousands of years (the age of the Baltic Sea), thus highlighting the pace at which human-driven evolution can happen.
... Two population samples (#24 and 25) both from Landvikvannet in southern Norway clustered more closely with populations from the transition zone than with local fjord populations from western Norway. Landvikvannet is a unique brackish lake created in 1877 by opening a canal between the ocean and a freshwater lake, now harboring a local population of herring adapted to this brackish environment (Eggers et al., 2014). The populations around ...
Atlantic herring is widespread in North Atlantic and adjacent waters and is one of the most abundant vertebrates on earth. This species is well suited to explore genetic adaptation due to minute genetic differentiation at selectively neutral loci. Here we report hundreds of loci underlying ecological adaptation to different geographic areas and spawning conditions. Four of these represent megabase inversions confirmed by long read sequencing. The genetic architecture underlying ecological adaptation in herring deviates from expectation under a classical infinitesimal model for complex traits because of large shifts in allele frequencies at hundreds of loci under selection.
... The fact that the genetic structures observed in many pelagic fish are associated with oceanographic dynamics 4,5 means that the conclusions drawn from genetic diversity and heterogeneity among populations are often attributed to connectivity among populations. This approach is well suited to deriving detailed descriptions of spatial genetic structures and identifying sources of genetic variation; however, the oceanographic dynamics associated with pelagic fish populations tend to be cyclic (i.e., changing seasonally or annually) [6][7][8] . It is crucial therefore to base plans for the conservation of pelagic fish stocks on the dynamics of genetic structures within the context of oceanographic cycles. ...
Abstract Many fisheries management and conservation plans are based on the genetic structure of organisms in pelagic ecosystems; however, these structures tend to vary over time, particularly in cyclic ocean currents. We performed genetic analyses on the populations of the pelagic fish, Megalaspis cordyla (Osteichthyes: Carangidae) in the area surrounding Taiwan during 2000–2001. Genotyping was performed on M. cordyla collected seasonally around Taiwan as well as specimens collected from Singapore (Malacca strait) and Indonesia (Banda Sea). Gonadosomatic indices (GSI) revealed that M. cordyla does not spawn near Taiwan. Data related to the mitochondrial control region revealed that the samples from Singapore and Indonesia represented two distinct genetic cohorts. Genotyping revealed that during the summer (June–August 2000), the Indonesian variant was dominant in eastern Taiwan (presumably following the Kuroshio Current) and in the Penghu region (following the Kuroshio Branch Current). During the same period, the Singapore genotype was dominant along the western coast of Taiwan (presumably following the South China Sea Current); however, the number dropped during the winter (December–February 2001) under the effects of the China Coast Current. Divergence time estimates indicate that the two genetic cohorts split during the last glacial maximum. Despite the fact that these results are based on sampling from a single year, they demonstrate the importance of seasonal sampling in unravelling the genetic diversity in pelagic ecosystems.
... Since the days of Hjort (1914), the population structure and dynamics of herring have been investigated and are still debated (Reiss et al., 2009;Martinez Barrio et al., 2016). It has been documented that herring can consist of spatially discrete populations (Iles and Sinclair, 1982) or are comprised as metapopulations Eggers et al., 2014). One of the major life-history traits of herring is their fidelity to a specific spawning season, mainly autumn or spring (Husebø et al., 2005;Brophy et al., 2006), although spawning can be observed throughout the year at various locations. ...
Atlantic herring (Clupea harengus) has complex population structure and dynamics including diverse life histories and spawning times with spring and autumn spawning as the most common modes. Originally, spawning herring were phenotypically identified based on their maturity development or otolith microstructure by determining seasonal specific larval growth patterns. Recently, genetic markers have revealed clear genetic differentiation between spring- and autumn-spawning populations. All three methods were applied to herring caught at the same locations during spring and autumn to determine the coherence of methods. In a selected subset, most herring (∼77%) had an otolith microstructure and genetic assignment coinciding with the phenotypically assigned spawning season. Non-spawning herring (<5%) that were classified as belonging to the current spawning season using genotyping and otolith-typing were assigned as skipped spawners. For ∼8% of spawning herring, the genetic and otolith assignment contradicted the phenotypically assigned spawning season, characteristic of straying individuals. Otolith-typing contradicted the genetic and phenotypical assignment in ∼7% of the cases, potentially representing individuals reuniting back to the spawning season favoured by their genotype. Although the viability of offspring from these individuals remains undocumented, it is suggested that the observed switching of spawning season may contribute to gene flow between herring populations.
