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  • 1.
    Berggren, Hanna
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nordahl, Oscar
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Tibblin, Petter
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Larsson, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Forsman, Anders
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Testing for local adaptation to spawning habitat in sympatric subpopulations of northern pike by reciprocal translocation of embryos2016In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 11, no 5, article id e0154488Article in journal (Refereed)
    Abstract [en]

    We tested for local adaption in early life-history traits by performing a reciprocal translocation experiment with approximately 2500 embryos of pike (Esox lucius) divided in paired split-family batches. The experiment indicated local adaptation in one of the two subpopulations manifested as enhanced hatching success of eggs in the native habitat, both when compared to siblings transferred to a non-native habitat, and when compared to immigrant genotypes from the other subpopulation. Gene-by-environment effects on viability of eggs and larvae were evident in both subpopulations, showing that there existed genetic variation allowing for evolutionary responses to divergent selection, and indicating a capacity for plastic responses to environmental change. Next, we tested for differences in female life-history traits. Results uncovered that females from one population invested more resources into reproduction and also produced more (but smaller) eggs in relation to their body size compared to females from the other population. We suggest that these females have adjusted their reproductive strategies as a counter-adaptation because a high amount of sedimentation on the eggs in that subpopulations spawning habitat might benefit smaller eggs. Collectively, our findings point to adaptive divergence among sympatric subpopulations that are physically separated only for a short period during reproduction and early development – which is rare. These results illustrate how combinations of translocation experiments and field studies of life-history traits might infer about local adaptation and evolutionary divergence among populations. Local adaptations in subdivided populations are important to consider in management and conservation of biodiversity, because they may otherwise be negatively affected by harvesting, supplementation, and reintroduction efforts targeted at endangered populations.

  • 2.
    Forsman, Anders
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Tibblin, Petter
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Berggren, Hanna
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nordahl, Oscar
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Koch-Schmidt, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Larsson, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Pike Esox lucius as an emerging model organism for studies in ecology and evolutionary biology: a review.2015In: Journal of Fish Biology, ISSN 0022-1112, E-ISSN 1095-8649, Vol. 87, no 2, p. 472-479Article, review/survey (Refereed)
    Abstract [en]

    The pikeEsox luciusis a large, long-lived, iteroparous, top- predator fish species with a circumpolardistribution that occupies a broad range of aquatic environments. This study reports on a literaturesearch and demonstrates that the publication rate ofE. luciusresearch increases both in absolute termsand relative to total scientific output, and that the focus of investigation has changed over time frombeing dominated by studies on physiology and disease to being gradually replaced by studies on ecol-ogy and evolution.Esox luciuscan be exploited as a model in future research for identifying causes andconsequences of phenotypic and genetic variation at the levels of individuals, populations and speciesas well as for investigating community processes.

  • 3.
    Larsson, Per
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Tibblin, Petter
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Koch-Schmidt, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Engstedt, Olof
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nilsson, Jonas
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nordahl, Oscar
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Forsman, Anders
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Ecology, evolution, and management strategies of northern pike populations in the Baltic Sea2015In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 44, no Supplement 3, p. S451-S461Article, review/survey (Refereed)
    Abstract [en]

    Baltic Sea populations of the northern pike (Esox lucius) have declined since the 1990s, and they face additional challenges due to ongoing climate change. Pike in the Baltic Sea spawn either in coastal bays or in freshwater streams and wetlands. Pike recruited in freshwater have been found to make up about 50 % of coastal pike stocks and to show natal homing, thus limiting gene flow among closely located spawning sites. Due to natal homing, sub-populations appear to be locally adapted to their freshwater recruitment environments. Management actions should therefore not involve mixing of individuals originating from different sub-populations. We offer two suggestions complying with this advice: (i) productivity of extant freshwater spawning populations can be boosted by modifying wetlands such that they promote spawning and recruitment; and (ii) new sub-populations that spawn in brackish water can potentially be created by transferring fry and imprinting them on seemingly suitable spawning environments.

  • 4.
    Nordahl, Oscar
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Intraspecific diversity of pike (Esox lucius) in the Baltic Sea and new insights on thermoregulation in fish2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fish display a fascinating variation in behavior, morphology and physiology among species, among individuals within species, and within individuals over time. A central quest in ecology and evolution is to understand causes and consequences of such variation. This thesis aims to contribute to this knowledge by: (1) investigating the evolutionary processes that shape intraspecific variation among sympatric subpopulations of pike in coastal areas of the Baltic Sea; and (2) exploring whether fish can utilize sun-basking to regulate body temperature and whether this has any consequences for fitness.

    Identifying barriers that delineate populations is a first step towards evaluating the evolutionary origin of intraspecific variation. This thesis reports on genetic population structures among co-existing pike in the coastal Baltic Sea separated by homing behavior, different spawning strategies and geographic barriers. Field studies revealed that these subpopulations also show phenotypic divergence in reproductive and meristic traits. Experimental studies suggested that differentiation among subpopulations likely was a result of divergent selection and local adaptations to spawning grounds. These adaptations that may further reinforce barriers among subpopulations due to a reduced success of immigrant genotypes.

    For the second aim of the thesis, we first studied seasonal and diel patterns of activity and vertical migration among the Baltic Sea pike. The results suggested that pike exposed themselves to sunlight during spring and summer, and that body temperatures were positively correlated with sun exposure during these basking events. This was followed by experimental studies on inanimate physical models and a field study on carp which demonstrated that fish can become warmer than ambient water when exposed to sun light, a previously overlooked mechanism for fish thermoregulation, and that the heat gain was positively correlated with growth.

    This thesis contributes to our understanding of the origin and maintenance of intraspecific variation among coexisting populations with direct implications for management of pike. It also establishes sun-basking as a novel mechanism for fish to obtain body temperatures in excess of ambient water which could motivate adaptations, both evolutionary and plastic, that optimize heat gain, affect spatiotemporal distributions and biotic interaction within and among species.

  • 5.
    Nordahl, Oscar
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Koch-Schmidt, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Sunde, Johanna
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Yildirim, Yeserin
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Tibblin, Petter
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Forsman, Anders
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Larsson, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Genetic differentiation between and within ecotypes of pike (Esox lucius) in the Baltic Sea2019In: Aquatic conservation, ISSN 1052-7613, E-ISSN 1099-0755Article in journal (Refereed)
    Abstract [en]

    Aquatic systems often lack physical boundaries for gene flow, but ecological and behavioural barriers can form surprisingly fine spatial scale genetic patterns that challenge traditional, large scale management. To detect fine spatial scale structures, understand sources of intraspecific diversity, and design appropriate management plans requires identification of reproductively isolated units. This study reports on genetic differentiation in pike (Esox lucius) within a coastal area stretching 55 km from south to north in the central Baltic Sea. Pike is here an economically and ecologically important top predator that has declined in abundance. However, population structures have mostly been studied on large spatial scales, and without considering the potential for genetic divergence between the sympatric anadromous fresh water and the resident brackish water spawning ecotypes. To this end, 487 individuals from the east coast of Sweden and the island of oland, representing sympatric anadromous and resident spawning individuals, categorized to ecotype based on spawning location or otolith microchemistry, were genotyped for 10 microsatellites and used to test for divergence between ecotypes. Furthermore, divergence between regions (island/mainland), neighbouring spawning locations (n = 13) and isolation by distance within and between regions were evaluated for the anadromous ecotype. The results revealed strong genetic differences between regions, between spawning locations separated by as little as 5 km and the first evidence of genetic differentiation between resident and anadromous ecotypes; despite a high dispersal capacity of pike and a high connectivity within the study area. The signatures of isolation by distance indicated that connectivity among populations differed between regions, probably reflecting availability of spawning habitats. To safeguard against the challenges and uncertainties associated with environmental change, adaptive conservation management should aim to promote high intra-population functional genetic diversity without compromising the continued integrity and coexistence of the different ecotypes and of locally adapted sub-populations.

  • 6.
    Nordahl, Oscar
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Tibblin, Petter
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Koch-Schmidt, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Berggren, Hanna
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Larsson, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Forsman, Anders
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Sun-basking fish benefit from body temperatures that are higher than ambient water2018In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 285, no 1879, article id 20180639Article in journal (Refereed)
    Abstract [en]

    In terrestrial environments, cold-blooded animals can attain higher bodytemperatures by sun basking, and thereby potentially benefit from broaderniches, improved performance and higher fitness. The higher heat capacityand thermal conductivity of water compared with air have been universallyassumed to render heat gain from sun basking impossible for aquaticectotherms, such that their opportunities to behaviourally regulate body temperatureare largely limited to choosing warmer or colder habitats. Here wechallenge this paradigm. Using physical modelswe first showthat submergedobjects exposed to natural sunlight attain temperatures in excess of ambientwater. We next demonstrate that free-ranging carp (Cyprinus carpio) canincrease their body temperature during aquatic sun basking close to thesurface. The temperature excess gained by basking was larger in dark thanin pale individuals, increased with behavioural boldness, and was associatedwith faster growth. Overall, our results establish aquatic sun basking as a novelecologically significant mechanism for thermoregulation in fish. The discoveryof this previously overlooked process has practical implications for aquaculture,offers alternative explanations for behavioural and phenotypicadaptations, will spur future research in fish ecology, and calls for modificationsof models concerning climate change impacts on biodiversity inmarine and freshwater environments.

  • 7.
    Tibblin, Petter
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Berggren, Hanna
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nordahl, Oscar
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Larsson, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Forsman, Anders
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Causes and consequences of intra-specific variation in vertebral number2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 26372Article in journal (Refereed)
    Abstract [en]

    Intraspecific variation in vertebral number is taxonomically widespread. Much scientific attention hasbeen directed towards understanding patterns of variation in vertebral number among individualsand between populations, particularly across large spatial scales and in structured environments.However, the relative role of genes, plasticity, selection, and drift as drivers of individual variation andpopulation differentiation remains unknown for most systems. Here, we report on patterns, causesand consequences of variation in vertebral number among and within sympatric subpopulations ofpike (Esox lucius). Vertebral number differed among subpopulations, and common garden experimentsindicated that this reflected genetic differences. A QST-FST comparison suggested that populationdifferences represented local adaptations driven by divergent selection. Associations with fitness traitsfurther indicated that vertebral counts were influenced both by stabilizing and directional selectionwithin populations. Overall, our study enhances the understanding of adaptive variation, which iscritical for the maintenance of intraspecific diversity and species conservation.

  • 8.
    Tibblin, Petter
    et al.
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Forsman, Anders
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Koch-Schmidt, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nordahl, Oscar
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Johannessen, Peter
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Nilsson, Jonas
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Larsson, Per
    Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.
    Evolutionary divergence of adult body size and juvenile growth in sympatric subpopulations of a top predator in aquatic ecosystems2015In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 186, no 1, p. 98-110Article in journal (Refereed)
    Abstract [en]

    Evolutionary theory predicts that different selective regimes may contribute to divergent evolution of body size and growth rate among populations, but most studies have focused on allopatric populations. Here, we studied five sympatric subpopulations of anadromous northern pike (Esox lucius) in the Baltic Sea subjected to allopatric habitats for a short period of their life cycle due to homing behavior. We report differences in adult body size among subpopulations that were in part due to variation in growth rate. Body size of emigrating juveniles also differed among subpopulations, and differences remained when individuals were reared in a common environment, thus indicating evolutionary divergence among subpopulations. Furthermore, a QST-FST comparison indicated that differences had evolved due to divergent selection rather than genetic drift, possibly in response to differences in selective mortality among spawning habitats during the allopatric life stage. Adult and juvenile size were negatively correlated across subpopulations, and reconstruction of growth trajectories of adult fishes suggested that body size differences developed gradually and became accentuated throughout the first years of life. These results represent rare evidence that sympatric subpopulations can evolve differences in key life-history traits despite being subjected to allopatric habitats during only a very short fraction of their life.

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