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Species management advice

Here, we provide genetic knowledge of some marine species in the Baltic Sea. Based on our studies, we discuss species potential to adapt to new environmental conditions, and highlight threats to the genetic diversity. We also present some advice for informed management of species and Baltic Sea biodiversity.

Baltic isopod

Baltic isopod, Idotea balthica, is a key species in shallow-water Baltic Sea habitats. It is an important food source for coastal fish, and also the most important grazer on Fucus bladder wrack and narrow wrack). Sometimes this isopod becomes extremely abundant, resulting in the complete removal of Fucus in that area.

  • Maintain current distribution of Fucus and Zostera spp. This is because the abuncance of Idotea balthica depends primarily on the availability of Fucus and, to some extent, of Zostera.
  • Do not to transport Idotea individuals across the Baltic. Idoteashows a genetic population structure in the Baltic Sea, with most geographic areas hosting unique genetic components, see map. This pattern could allow the populations to adapt to local environmental conditions. Care should thus be taken not to destroy these possibilites through "genetic mixing".
  • In management plans, consider that Idotea populations on the Swedish Bothnian Sea coast seem to be small and fragmented.

Idotea shows a strong genetic population structure in the Baltic Sea, with most geographic areas hosting unique genetic components, see map. This pattern could allow the populations to adapt to local environmental conditions. The Estonian coast seems to be a hotspot of genetic diversity, while especially the Swedish Bothnian Sea coast is poor in diversity, harboring isolated and small populations.

Genetic diversity is strongly correlated with distance from the entrance of the Baltic Sea. This is likely a result of the demographic history of Idotea balthicaIdotea colonized the Baltic from the North Sea stepwise through several multiple bottleneck events, with each event further reducing genetic diversity.

Tolerance to climate change & potential to adapt
An experimental study in BONUS BAMBI simulating future salinity and temperature conditions of the Baltic Sea reduced the survival of Idotea from all geograpic regions. The negative effect was strongest for populations from the marginal region, indicating that the population sizes here will strongly decrease in the future Baltic Sea, and even that entire populations can be wiped out.

However, there were differences in the survival among populations from the same region, suggesting that there is a geographic variation in the tolerance to climate change conditions among populations. Results also show that there are differences in the tolerance between individuals from the same population. This could indicate it may be possible for the isopods to adapt to the future conditions.

Future salinity reduction in the Baltic Sea might affect the Idoteapopulations either directly or indirectly through the loss of Fucus (habitat loss).

Since Idotea lacks a long-distance dispersal life stage, fragmentation of the habitat (Fucus) will reduce connectivity among geographic areas. Thus, habitat fragmentation will reduce genetic diversity and might threaten local and isolated populations, especially on the Swedish Bothian Sea coast. 

A molecular phylogeny of the north-east Atlantic species of the genus Idotea (Isopoda) with focus on the Baltic Sea. Panova M, Nygren A, Jonsson PR & Leidenburger S. Zoologica Scripta, 2016.

Mapping present and future potential distribution patterns for a meso-grazer guild in the Baltic Sea. Leidenberger S, De Giovanni R, Kulawik R, Williams AR, Bourlat SJ & Maggs C. J of Biogeography, 2015.

Physiological plasticity is key to the presence of the isopod Idotea baltica(Pallas) in the Baltic Sea. Wood HL, Nylund G & Eriksson SP. J of Sea Research, 2014.

Ecology and distribution of the isopod genus Idotea in the Baltic Sea: key species in a changing environment. Leidenberger S, Harding K & Jonsson PR. J Crust. Biol, 2012.

Baltic isopod.
Baltic isopod.
Photo: Joakim Hansen/Azote

Contact

Pierre DeWit, Department of Marine Sciences – Tjärnö

Bladder wrack

Bladder wrack, Fucus vesiculosus, is the most dominant and ecologically important macro alga in the Baltic Sea. It provides essential ecosystem services as maintainers of local biodiversity, but also as nurseries for commercially important fish species. Recent genetic data show that bladder wrack and narrow wrack (see below) are very closely related. In several cases, analyses fail to separate them from each other. Ecologically, they both provide important ecosystem functions in the Baltic Sea particularly as habitat forming organisms.

  • Maintain large population sizes and connectivity at present levels.
  • Identify and protect populations with high sexual activity, i.e. populations with many genotypes and thus, a capacity for future adaptations. This is particularly important in areas where cloning is otherwise predominant, and at range margins in Gulf of Bothnia and Gulf of Finland, see map.
  • Provide management plans for marginal populations, i.e. in Gulf of Bothnia, Gulf of Finland and Gulf of Riga.
  • Consider genetic monitoring.
  • Consider restoration of earlier lost populations in Gulf of Gdansk, using individuals from areas of similar salinity.

The population structure in bladder wrack is strong in the Baltic Sea, with limited gene flow (connectivity) even among populations as little as one kilometers apart. Between some areas the genetic differences are large: Populations along the Finnish coast, from Turku to Wasa and in Gulf of Finland, are genetically very different from the rest of the Baltic. See map.

Clones in some areas
In the Baltic Sea, both bladder wrack and the closely related narrow wrack reproduce sexually but also asexually, by cloning. Females and males are separate individuals and can form new individuals by dropping adventitious branches (a few centimeter long vegetative branches) that grow to new thalli.

In the Gulf of Bothnia and especially on the Swedish side, most Fucus populations are dominated by individuals reproducing asexually. In other areas, there is a mixture of populations with individuals reproducing both clonally and sexually. In the Baltic proper, cloning is rare, and at the Swedish west coast and further out in the Atlantic, cloning has never been found. See map.

If cloning is common, diversity of genotypes is low and recombination that produce new genotypes is rare, making these populations especially vulnerable to environmental change. In contrast, populations with sexual activity have a potential for adapting to environmental change. Consequently, it is important to identify and protect sexually active populations in areas otherwise dominated by clones.

Tolerance to climate change & potential to adapt
Experimental studies in the BONUS BAMBI-project clearly showed that predicted future combinations of salinity and temperature represent serious challenges for the Fucus populations, particularly in the central and marginal regions of the Baltic Sea. This may lead to contractions in the species range, and a shift southwards, towards the Baltic Sea entrance.

We did find evidence for genetic variation in tolerance to future salinity and temperature, especially in the central and entrance populations. Such genetic variation may provide the raw material necessary for adaptation to a changing environment. However, our genotypic data just documents the potential for adaptation. We cannot foresee whether the speed or the magnitude of the adaptive response can match the rate of climate change.

See also results from our modelling of Fucus distribution in the future Baltic Sea.

The warming and salinity decrease predicted for the Baltic Sea over the coming 50 to 100 years may result in loss of populations in marginal areas (the gulfs of Bothnia, Finland and Riga). See future distribution.

In areas with clonal reproduction (= low levels of sexual reproduction) the potential for adaptation to new environmental conditions is low. Here, populations are particularly vulnerable, and environment sweeps could lead to potential extinction, see map.

Eutrophication, increased turbidity or cascading effects of overfishing have historically eradicated local populations of bladder wrack. Loss of this type is likely to lead to loss of genetic variation including local adaptations. In some areas, populations have recovered.

References

Living on the edge: Gamete release and subsequent fertilisation in Fucus vesiculosus. Rothäusler E, Uebermuth C, Haavisto F & Jormalainen V. Phycologia, January 2019.

High climate velocity and population fragmentation may constrain climate-driven range shift of the key habitat former Fucus vesiculosus in the Baltic Sea. Jonsson PR, Kotta J, Andersson HC, Herkul K, Virtanen E, Nyström Sandman A & Johannesson K. Diversity and Distribution, 2018.

Reciprocal transplants support a plasticity-first scenario during colonisation of a large hyposaline basin by a marina macro alga. Johansson D, Pereyra RT, Rafajlovic M, Johannesson K. BMC Ecology, 2017.

Genetic variation of a foundation rockweed species affects associated communities. Jormalainen V, Danelli M, Gagnon K, Hillebrand H, Rothäusler E, Salminen J-P and Sjöroos J. Ecology, 2017.

Neutral processes forming large clones during colonisation of new areas. Rafajlović M, Kleinhans D, Gulliksson C, Fries J, Johansson D, Ardehed A, Sundqvist L, Pereyra RT, Mehlig B., Jonsson PR & Johannesson K. Journal of Evolutionary Biology, 2017.

Divergence within and among seaweed siblings (Fucus vesiculosus and F. radicans) in the Baltic SeaArdehed A, Johansson D, Sundqvist L, Schagerström E, Zagrodzka Z, Kovaltchouk NA, Bergström L, Kautsky L, Rafajlovic M, Pereyra RT, Johannesson K. PLoS ONE, 2016.

Complex spatial clonal structure in the macroalgae Fucus radicans with both sexual and asexual recruitmentArdehed A, Johansson D, Schagerström E, Kautsky L, Johannesson K & Pereyra RT. Ecology and Evolution, 2015.

Genetic biodiversity in the Baltic Sea: Species‐specific patterns challenge management. Wennerström, L, Linda Laikre L, Ryman N, Utter FM, Ab Ghani NI, André C, DeFaveri J, Johansson D, Kautsky L, Merilä J, Mikhailova N, Pereyra R, Sandström A, Teacher AGF, Wenne R, Vasemägi A, Zbawicka M, Johannesson K & Primmer CR. Biodiversity and Conservation, 2013.

Frequent clonality in fucoids (Fucus radicans and F. vesiculosus; Fucales Phaeophyceae) in the Baltic Sea. Johannesson K, Johansson D, Larsson KH, Huenchunir CJ, Perus J, Forslund AH, Kautsky L & Pereyra RT. Journal of Phycology, 2011.

Genetic structure in populations of Fucus vesiculosus (Phaeophyceae) over spatial scales from 10 m to 800 km. Tatarenkov A, Jönsson RB, Kautsky L & Johannesson K. Journal of Phycology, 2007.

Intriguing asexual life in marginal populations of the brown seaweed Fucus vesiculosusTatarenkov A, Bergström L, Jönsson RB, Serrao EA, Kautsky L and Johannesson K. Molecular Ecology, 2005.

Bladder wrack
Bladder wrack, Fucus vesiculosus.

Contact

Kerstin Johannesson, Department of Marine Sciences – Tjärnö

Ricardo Pereyra, Department of Marine Sciences – Tjärnö

Brown trout

Brown trout, Salmo trutta, is an ecologically and culturally important species in many freshwater habitats. It is a host of the threatened freshwater pearl mussle Margaritifera margaritifera.

  • Avoid stocking except for strict conservation purposes. If stocking is needed, individuals that are closely related to the naturally occurring local population should be used.
  • Protect and restore spawning grounds.
  • Manage water systems as meta populations. That is, allow for genetic exchange among populations (connectivity), and make sure that larger areas covering several spawning grounds are managed jointly.
  • Ecological importance and good genetic knowledge make this species suitable for environmental monitoring of genetic diversity.

Brown trout is one of the most well studied species within population genetics. The species has a strong genetic substructure with distinct populations; at least one population exists per river/creek.

Many populations have been genetically analyzed. This basic information on the genetic characteristics can be used in management, for instance to analyze which spawning populations that are being fished during mixed stock fishery. This is important in order to assure that weak populations are not over-harvested. Genetic information can also be used to identify populations suitable for releases for conservation purposes.

There is a meta population structure (multiple populations connected via gene flow) in restricted areas of the Baltic Sea, e.g. around the islands Bornholm and Gotland. Genetic exchange between populations that spawn in separate small creeks around these islands appears important for maintaining the genetic diversity of the meta population as a whole.

Mixed stock fisheries at sea show a high proportion of long distance migrants in the catch, indicating fishing of several populations. Such mixed stock fisheries risk the depletion of weak populations and will, as a consequence, reduce genetic biodiversity.

Loss of spawning grounds due to habitat modifications.

Large scale stocking will reduce genetic biodiversity, and might erase unique genetic elements needed for local adaptations.

REFERENCES
Baltic Sea genetic biodiversity: Current knowledge relating to conservation management. Wennerström L, Jansson E and Laikre L. Aquatic Conservation: Marine and Freshwater Ecosystems, 2017.

Genetic baseline for conservation and management of sea trout in the nothern Baltic SeaÖstergren J, Nilsson J, Lundqvist H, Dannewitz J and Palm, S. Conservation Genetics, 2016.

Recent genetic changes in enhanced populations of sea trout (Salmo trutta m. trutta) in the southern Baltic rivers revealed with SNP analysis. Wenne R, Bernas R, Pocwierz-Kotus A, Drywa A and Was A. Aquatic living resources, 2016.

Report of the Baltic Fisheries Assessment Working Group (WGBFAS). ICES 2015. OBS 35 MB pdf

Wild Estonian and Russian sea trout (Salmo trutta) in Finnish coastal sea trout catches: Results of genetic mixed-stock analysis. Koljonen ML, Gross R and Koskiniemi J. Hereditas, 2014.

Genetic diversity within sea trout population from an intensively stocked southern Baltic river, based on microsatellite DNA analysisBernaś R, Burzyński A, Dębowski P, Poćwierz-Kotus A and Wenne, R. In Fisheries Management and Ecology, 2014.

Long‐term temporal changes of genetic composition in brown trout (Salmo trutta L.) populations inhabiting an unstable environment. Østergaard S, Hansen MM, Loeschcke V & Nielsen EE. Molecular Ecology, 2003.

Spatial and temporal population structure of sea trout at the Island of Gotland, Sweden, delineated from mitochondrial DNA. Laikre L, Järvi T, Johansson L, Palm S, Rubin JF, Glimsäter CE, Landergren P and Ryman N. Journal of Fish Biology, 2002.

Conservation genetic management of brown trout (Salmo trutta) in EuropeReport by the concerted action on identification, management and exploitation of genetic resources in the brown trout (Salmo trutta), TROUTCONCERT; EU FAIR CT97-3882. Edited by Linda Laikre. Danmarks Fiskeriundersøgelser, Afd for Ferskvandsfiskeri, 1999.

Brown trout
Brown trout, Salmo trutta
Photo: Anders Tedeholm/Azote

Contact

Linda Laikre, Department of Zoology, University of Stockholm

Lovisa Wennerström, Department of Zoology, University of Stockholm

Baltic cod

Baltic cod, Gadus morhua, is one of the major cod stocks globally. It has a key role in the Baltic Sea with impact on the whole ecosystem, from the main pelagic fish  herring to the zooplankton and phytoplankton.

Analyses of ancient cod DNA indicate that the eastern cod population has been isolated for a long time, possibly already from the time of the Baltic colonisation 6 000 to 8 000 years BP.

  • Manage the eastern and western Baltic cod as separate units.
  • Monitor mixing proportions of eastern and western cod in the Arkona basin.
  • The western Baltic cod should potentially be divided into more than one management unit, since it is unclear whether there are several spawning populations within this area.

One large cod population spawns east of Bornholm, and one or several populations spawn west of Bornholm, see map. Eastern Baltic cod is genetically distinct and adapted to the brackish environment. Adaptations include differences in hemoglobin type, osmoregulatory capacity, egg bouyancy, sperm swimming characteristics and spawning period.

In the western Baltic, there are spawning aggregations in Öresund, Kiel and Mecklenburg Bay which are genetically distinct from both the eastern Baltic cod, and from the North Sea cod. In the Arkona basin, spawning and migrating cod from the eastern and western Baltic cod stocks intermingle in proportions that vary seasonally.

Individuals in spawning condition have been observed in the Åland deep, but it is unclear if fertilisation actually takes place and if the offspring survive. Historically spawning occured also in the Gdansk deep and off Gotland, but these populations seem to have gone extinct.

Map of spawning areas and relative abundance of the eastern and western cod stocks in the Baltic Sea. Abundance is approximated from catches and surveys in ICES Subdivisions in 2016, and indicated by colour saturation.

The eastern population is adapted to spawning in low salinity and may not be replaced by cod from elsewhere if depleted. 

Immigration of eastern cod into the western Baltic management unit may mask a poor state of the populations in the western unit. 

Cod is subjected to large scale fishing, which is expected to affect the genetic composition in the populations.

Genetic analyses reveal complex dynamics within a marine fish management area. Hemmer‐Hansen et al. Evolutionary Applications, 2018.

Report of the Baltic Fisheries Assessment Working Group (WGBFAS), 19-26 April 2017, Copenhagen, Denmark. ICES 2017.

Connectivity of larval cod in the transition area between North Sea and Baltic Sea and potential implications for fisheries managementHuwer B, Hinrichsen HH, Hüssy K & Eero M. ICES Journal of Marine Science, 2016.

Spatio-temporal trends in stock mixing of eastern and western Baltic cod in the Arkona Basin and the implications for recruitment. Hüssy K, Hinrichsen H-H, Eero M, Mosegaard H, Hemmer-Hansen J, Lehmann A & Lundgaard L S. ICES Journal of Marine Science, 2016.

Adaptation to low salinity promotes genomic divergence in Atlantic cod (Gadus morhua L.)Berg P, Jentoft S, Staar B, Ring KH, Knutsen H, Lien S, Jakobsen K, Andre C. Genome Biology and Evolution, 2015.

Implications of stock recovery for a neighbouring management unit: Experience from the Baltic cod. Eero, M., Hemmer-Hansen, J., and Hüssy, K. ICES Journal of Marine Science, 2014.

Genetic differentiation of brackish water populations of cod Gadus morhua in the southern Baltic, inferred from genotyping using SNP-arrays. Poćwierz-Kotus A et al. 2014. Marine Genomics, 2014.

Differences in salinity tolerance and gene expression between two populations of Atlantic Cod (Gadus morhua) in response to salinity stress.Larsen PF, Nielsen EE, Meier K, Olsvik PA, Hansen MM, Loeschcke V. Biochem Genet, 2012.

Review of western Baltic cod (Gadus morhua) recruitment dynamics
K. Hüssy. ICES Journal of Marine Science, 2011.

Genomic signatures of local directional selection in a high gene flow marine organism; the Atlantic cod (Gadus morhua). Nielsen, EE, Hemmer-Hansen J, Poulsen NA, Loeschcke V, Moen T, Johansen T, Mittelholzer C, Taranger G-L, Ogden R and Carvalho GR. BMC Evolutionary Biology, 2009.

Haemoglobin polymorphisms affect the oxygen binding properties in Atlantic cod populationsAndersen Ø, Wetten OF, De Rosa MC, André C et al 2009. Proc Royal Soc B, 2009.

Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis. Nielsen EE et al. Mol Ecol, 2003.

Salinity requirements for successful spawning of Baltic and Belt Sea cod and the potential for cod stock interactions in the Baltic Sea. Nissling A, Westin L. Mar Ecol Progr Ser, 1997.

The Baltic cod (pdf). Bagge O, Thurow F, Steffensen E and Bay J. Dana, 1994.

Haemoglobin polymorphism of cod in the Baltic and the Danish Belt SeaSick K. Hereditas, 1965.

Baltic cod
Baltic cod, Gadus morhua
Photo: Patrick Gijsber

Contact

Carl Andre, Department of Marine Scieces – Tjärnö

Herring

The Baltic Sea herring, Cluepa harengus, is an important prey species for many predators including cod and seals. The species includes populations of autumn-spawning herrings, mainly in the western and southern Baltic Sea; and spring-spawning herring, mainly in the eastern and northern parts of the sea basin. There is wide overlap in the distribution of the different forms.

  • Spring and autumn spawning herring should be managed as separate units.
  • Assure large populations of both spawning types, to maintain potential for adaptations.

Genetic adaptation of Baltic populations to the Baltic Sea environment has been confirmed. Thus, the Baltic herring represents a marine species that has evolved to survive in the brackish environment. To maintain the potential to adapt to future environmental change, large population sizes are needed.

Adaptive differences have been documented between spring and autumn spawning herring in the Baltic Sea. Thus, it is important to retain large populations of both spawning types.

Strong fishing pressure will reduce population size and thus negatively affect genetic biodiversity. This is because large populations maintain genetic diversity while small populations lose genetic diversity.

Local adaptations might be lost if Baltic herring populations as a whole are over harvested. Strong fishing pressure on either spring or autumn spawners could have the same effect.

The genetic basis for ecological adaptation of the Atlantic herring revealed by genome sequencing. Barrio AM, Lamichhaney S, Fan G, Rafati N, Pettersson M, Zhang H, Dainat J, Ekman D, Höppner M, Jern P, Martin M, Nystedt M, Liu X, Chen W, Liang X, Shi C, Fu Y, Ma K, Zhan X, Feng C, Gustafson U, Rubin CJ, Sällman Almén M, Blass M, Casini M, Folkvord A, Laikre L, Ryman N, Ming-Yuen Lee S, Xu X & Andersson L. eLife, 2016.

Population-scale sequencing reveals genetic differentiation due to local adaptation in Atlantic herring. Lamichhaneya S, Barrio AM, Rafatia N, Sundström G, Rubina CJ, Gilberta ER, Berglund J, Wetterbom A, Laikre L, Webster MT, Grabherr M, Ryman N, and Andersson L. PNAS, 2012.

Detecting population structure in a high gene-flow species, Atlantic herring (Clupea harengus): direct, simultaneous evaluation of neutral vs putatively selected loci. André C, Larsson LC, Laikre L, Bekkevold D, Brigham J, Carvalho GR, Dahlgren TG, Hutchinson WF, Mariani S, Mudde K, Ruzzante DE and Ryman N. Heredity, 2011.

Temporally stable genetic structure of heavily exploited Atlantic herring (Clupea harengus) in Swedish waters. Larsson LC, Laikre L, André C, Dahlgren TG, Ryman N. Heredity, 2010.

Concordance of allozyme and microsatellite differentiation in a marine fish, but evidence of selection at a microsatellite locus. Larsson LC, Laikre L, Palm S, André C, Carbalho GR, Ryman N. Molecular Ecology, 2007.

Environmental correlates of population differentiation in Altantic herring.Bekkevold D, André C, Dahlgren TG, Clausen LAW, Tortensen E, Mosegaard H, Carvalho GR, Christensen TB, Norlinder E, Ruzzante DE. Evolution, 2005.

Marine landscapes and population genetic structure of herring (Clupea harengus L.) in the Baltic Sea. Jørgensen HBH, Hansen MM, Bekkevold D, Ruzzante DE, Loeschcke V. Molecular Ecology, 2005.

Lack of correspondence between genetic and morphologic variability patterns in Atlantic herring. Ryman N, Lagercrantz U, Andersson L, Chakraborty R, Rosenberg R. Heredity, 1984. 

Baltic herring
Herring, Cluepa harengus
Photo: Gunnar Aner/Azote

Contact

Linda Laikre, Department of Zoology, University of Stockholm

Narrow wrack

Narrow wrack, Fucus radicans, formed less than 8 000 years ago from bladder wrack inside the Baltic Sea. It has since spread and established along the coasts of the Bothnian Sea, and along the coast of Estonia in the Baltic Proper. Narrow wrack and bladder wrack are very closely related and are difficult to separate. Work is ongoing to resolve their taxonomic relationship. Ecologically, they both provide important ecosystem functions in the Baltic Sea particularly as habitat forming organisms.

  • Maintain large population sizes and connectivity at present levels.
  • Protect populations with any sexual activity, see map. The sexually reproducing Estonian populations should be highly prioritised, as should local populations with sexual reproduction in the Gulfs of Bothnia and Finland.
  • The Estonian populations are genetically different from the Gulf of Bothnian populations, and should thus not be used to replace lost populations in Gulf of Bothnia.
  • Provide management plans for populations in Gulf of Bothnia and Estonian waters, and in Gulf of Finland – if present.

The warming and salinity decrease predicted for the Baltic Sea over the coming 50 to 100 years could risk the loss of populations or the whole species. Due to large areas with no or very little sexual reproduction (see map) this species has a low potential for adapting to new environmental conditions. This is particularily true for the populations in Gulf of Bothnia, see below.

Narrow wrack is endemic to Gulf of Bothnia and Estonian waters. It may also be present in Gulf of Finland. Recent genetic data show that narrow wrack and bladder wrack (see above) are very closely related and sometimes difficult to separate.

The population structure in Fucus wracks is strong in the Baltic Sea, with limited gene flow (connectivity) even among populations as little as one kilometers apart. There are large genetic differences between populations in Gulf of Bothnia and in Estonia. A population in the Russian part of Gulf of Finland may represent a genetically distant narrow wrack. Map with population structure is found here.

Clones are vulnerable
Both wracks can reproduce asexually by cloning. Females and males are separate individuals who can form new individuals by dropping adventitious branches (a few centimeter long vegetative branches) that grow to new thalli. The ratio of sexual to asexual recruitment varies among populations, from no clones to nearly all individuals being one same clone, see map.

If cloning is common, diversity of genotypes is low and recombination that produce new genotypes is rare, making these populations especially vulnerable to environmental change. In contrast, populations with sexual activity have a potential for adapting to environmental change. Consequently, it is important to identify and protect sexually active populations in areas otherwise dominated by clones, see map.

In the Gulf of Bothnia and especially on the Swedish side, most Fucus populations are dominated by individuals reproducing clonally. In other areas, there is a mixture of populations with individuals reproducing both clonally and sexually. In the Baltic proper, cloning is rare, and at the Swedish west coast and further out in the Atlantic, cloning has never been found. 

Super-female
Along the Swedish coast of Gulf of Bothnia, asexually reproducing clones of narrow wrack are very common. Most clones are locally distributed but two clones – one female and one male – have extensive geographic distributions.

The "superfemale" is found along a 550 km coastline in Gulf of Bothnia, making up 20–95 percent of individuals of local populations in this area, see map above. Due to this dominant clone, the genetic structure in narrow wrack is less fine-scaled than in bladder wrack here.

In Estonian waters, asexually reproducing clones seem to be nearly absent.

 

Neutral processes forming large clones during colonisation of new areas. Rafajlović M, Kleinhans D, Gulliksson C, Fries J, Johansson D, Ardehed A, Sundqvist L, Pereyra RT, Mehlig B., Jonsson PR & Johannesson K. Journal of Evolutionary Biology, 2017.

Divergence within and among seaweed siblings (Fucus vesiculosus and F. radicans) in the Baltic Sea. Ardehed A, Johansson D, Sundqvist L, Schagerström E, Zagrodzka Z, Kovaltchouk NA, Bergström L, Kautsky L, Rafajlovic M, Pereyra RT & Johannesson K. PLoS ONE, 2016.

Complex spatial clonal structure in the macroalgae Fucus radicans with both sexual and asexual recruitment. Ardehed A, Johansson D, Schagerström E, Kautsky L, Johannesson K & Pereyra RT. Ecology and Evolution, 2015

Frequent clonality in fucoids (Fucus radicans and F. vesiculosus; Fucales Phaeophyceae) in the Baltic Sea. Johannesson K, Johansson D, Larsson KH, Huenchunir CJ, Perus J, Forslund AH, Kautsky L & Pereyra RT. Journal of Phycology, 2011.

Narrow wrack
Narrow wrack, Fucus radicans
Photo: Lena Bergström

Contact

Kerstin Johannesson, Department of Marine Sciences – Tjärnö

Ricardo Pereyra, Department of Marine Sciences – Tjärnö

Nine-spined stickleback

The nine-spined stickleback, Pungitius pungitius, and the three-spined stickleback are taxonomically related and have a similar ecology. The two species can occur sympatrically in the Baltic Sea. However, the nine-spined seems to be less common in marine and brackish habitats than the three-spined stickleback. Also, the levels of genetic differentiation and diversity of the nine-spine sticklebacks differ from that of the three-spined sticklebacks.

  • There is no particular genetic management advice communicated or warranted.

There are no immediate threats to populations or genetic variation.

The degree of genetic differentiation between nine-spine stickleback populations is relatively high, in contrast to the three-spined sticklebacks. This suggests limited dispersal ability and thus reduced connectivity among different populations. In accordance, the levels of genetic diversity within local nine-spined populations are relatively high, but lower than in the closely related three-spine sticklebacks.

Two historically divergent lineages
Besides ecological differences between the two species (e.g. migration ability and habitat preferences), differences in their postglacial colonization history of the Baltic Sea also affect the genetic layout of the two species. While the three-spined sticklebacks origin from one single marine colonization, the nine-spined sticklebacks origin from several colonization events. Thus the nine-spined sticklebacks show two historically divergent lineages in the Baltic Sea – one eastern and one western, which meet at the Danish Straights.

The relatively high degree of genetic differentiation in neutral marker genes among different Baltic Sea populations suggests ample opportunities for local adaptation in nine-spined sticklebacks.

Nine-spined stickleback
Nine-spined stickleback, Pungitius pungitius

Contact

Britta MeyerMax Planck Insitute for Evoultionary Biology, Germany

Pike 

Pike, Esox lucius, is one of several freshwater fishes inhabiting the Baltic Sea. The species naturally prefer the less saline areas and thus colonize mostly the coastal areas. The pike is subject to commercial and sport fishery in coastal areas all around the Baltic. Data is currently missing on whether this fishery is affecting the genetic diversity of the species.

  • Genetically healthy pike populations in the Baltic Sea require the maintenance of spawning areas along the coastline as well as in adjacent freshwater bodies.
  • In order to maintain, support or restore local pike populations it is important to restaurate spawning grounds in some areas. This will also help maintain the possibilities for genetic exchange between areas, and thus reduce loss of genetic variation.
  • The possibility for genetic exchange among spawning areas is important. Thus, both local scale and large scale management are needed for pike.
  • For the brackish spawning pike, particular attention should be paid to areas with clear genetic distinction, see “Knowledge on genetics in the Baltic” below.
  • Stocking should be avoided except to conserve weak populations. Such conservation releases should use genetically similar populations, primarily located close to the restocking site. Releases should be documented carefully and be monitored genetically.

The genetic pattern differ between pike spawning in coastal brackish water as compared to pike that migrate up into in adjacent freshwater areas to spawn. Freshwater spawners show much stronger genetic substructuring with larger genetic differences between populations.

The brackish spawning pike show relatively weak genetic substructuring. Obviously, genetic exchange occur over large geographic distances including, potentially across the Baltic via the Islands of Åland. However, genetic core areas with particularly distinct and genetically variable brackish populations occur in the Estonian and Stockholm Archipelagos, in the Bothnian Bay and as well as around the Swedish Quark.

The species has declined locally in some areas around the Baltic, for instance the Kalmar Sund region. This can result in loss of genetic variation, but since genetic diversity is not monitored little is known of such potential effects.

Decline and/or loss of local populations could risk the loss of genetic variation, and break the genetic exchange between spawning grounds. The stocking activities that occur in some areas risk transferring non-native genetic material to natural populations, which can reduce their adaptive potential. Such stocking activities are typically poorly documented and not monitored genetically. This prohibits finding out their potential effects.

Population genetics of pike. Wennerström L, Bekkevold D, Laikre L. Chapter 3.1 in Biology and Ecology of Pike, eds Nilsson A and Skov C. Taylor and Francis, 2018.

Temporally stable, weak genetic structuring in brackish water northern pike (Esox lucius) in the Baltic Sea indicates a contrasting divergence pattern relative to freshwater populations. Wennerström L, Olsson J, Ryman N, Laikre L. Canadian Journal of Fisheries and Aquatic Sciences, 2016.

From regionally predictable to locally complex population structure in a freshwater top predator: river systems are not always the unit of connectivity in Northern Pike Esox lucius. Bekkevold D, Jacobsen L, Hemmer-Hansen J, Berg S, Skov C. Ecology of Freshwater Fish, 2015.

Ecology, evolution, and management strategies of northern pike populations in the Baltic Sea. Larsson, P., Tibblin, P., Koch-Schmidt, P., Engstedt, O., Nilsson, J., Nordahl, O., and Forsman, A. Ambio, 2015.

Evolutionary divergence of adult body size and juvenile growth in sympatric subpopulations of a top predator in aquatic ecosystems. Tibblin P, Forsman A, Koch-Schmidt P, Nordahl O, Johannessen P, Nilsson J, and Larsson, P. Am. Nat., 2015.

Genetic biodiversity in the Baltic Sea: species-specific patterns challenge management. Wennerström L, Laikre L, Ryman N, Utter FM, Ab Ghani NI, Andre C, DeFaveri J, Johansson D, Kautsky L, Merilä J, Mikhailova N, Pereyra R, Sandström A, Teacher A.G.F., Wenne R, Vasemägi A, Zbawicka N, Johannesson K, and Primmer CR. Biodiversity and Conservation, 2013.

Spatial genetic structure of northern pike (Esox lucius) in the Baltic Sea. Laikre L, Miller LM, Palmé A, Palm S, Kapuscinski AR, Thoresson G, Ryman N. Molecular Ecology, 2005.

Pike
Pike, Esox lucius
Photo: Le Carlsson/Azote

Contact

Lovisa Wennerström, Department of Zoology, University of Stockholm

Linda Laikre, Department of Zoology, University of Stockholm

Nils Ryman, Department of Zoology, University of Stockholm

Baltic salmon

The salmon, Salmo salar,  reproduces in rivers across the whole Baltic Sea. Juveniles stay in freshwater streams for one to four years and then spend one to several years at sea on a feeding migration before they return to spawn in their natal river. The population structure of Baltic salmon has been known since the mid 1980s. Already then it was clear that the species should be managed on a population by population basis, and that releases were potentially harmful to native gene pools.

  • Manage each population separately. Protect populations within each river.
  • Restore spawning habitats.
  • Avoid mixed fisheries and large scale stocking.
  • Phase out large scale releases. For conservation releases to support or reestablish weak or extinct populations, use genetically close populations.
  • The occurrence of two major lineages within the Baltic Sea should be taken into account. Transplantations between these phylogeographic lineages should be strictly avoided.

The Baltic salmon is subjected to large scale fishing which is expected to affect both fish abundance and genetic diversity. Mixed fisheries and large scale releases threaten remaining gene pools and naturally spawning stocks.

Hatchery breeding often uses too small populations which risks depletion of genetic diversity over time. Genetic homogenization in wild populations from releases is documented and could result in, for example, loss of genetic adaptations.

Power plant constructions have blocked migratory routes between spawning and feeding grounds in many rivers around the Baltic Sea. Only around 30 percent of previous salmon rivers harbour wild populations today.

Salmon is without comparison the genetically most well studied species in the Baltic Sea. The Baltic salmon is genetically differentiated from other salmon populations in Europe and in the Atlantic Ocean. Strong homing to natal spawning grounds results in a pronounced genetic substructure, where each river harbours at least one genetically unique population. A relatively large proportion of the species genetic variation is due to differences between populations inhabiting different rivers.

The majority of the natural populations are lost, and not all of those remaining are sustainable. Effective size of local populations and for the Baltic populations as a whole is depleted.

Baltic salmon exhibit lower genetic variation than Atlantic populations, probably due to bottleneck events during the colonization of the Baltic Sea after the last ice age. The genetic dichotomy in Baltic salmon, where northern and eastern populations form two different genetic groups, is most likely an effect of two colonizing lineages

Genomewide introgressive hybridization patterns in wild Atlantic salmon influenced by inadvertent gene flow from hatchery releases. Ozerov, MY, Gross R, Bruneaux M, Vähä J‐P, Burimski O, Pukk L, & Vasemägi A. Molecular Ecology, 2016.

Report of the Baltic Fisheries Assessment Working Group (WGBFAS), 14–21 April 2015. ICES CM 2015/ACOM:10, 2015.

Restitution and genetic differentiation of salmon populations in the southern Baltic genotyped with the Atlantic salmon 7K SNP array. Poćwierz‐Kotus A, Bernaś R, Kent MP, Lien S, Leliűna E, Debowski P & Wenne R. Genetics Selection Evolution, 2015.

Compromising Baltic salmon genetic diversity. Conservation genetic risks associated with compensatory releases of salmon in the Baltic Sea. Palmé A, Wennerström L, Guban P, Ryman N & Laikre L. Report 2012:18, Swedish Agency for Marine and Water Management, 2012.

Population genetic structure and postglacial colonization of Atlantic salmon (Salmo salar) in the Baltic Sea area based on microsatellite DNA variation. Säisä M, Koljonen ML, Gross R, Nilsson J, Tahtinen J, Koskiniemi J & Vasemägi A. Canadian Journal of Fisheries and Aquatic Sciences, 2005.

Maintenance of genetic diversity of Atlantic salmon (Salmo salar) by captive breeding programmes and the geographic distribution of microsatellite variation. Koljonen M L, Tähtinen J, Säisä M & Koskiniemi J. Aquaculture, 2002.

Matrilinear phylogeography of Atlantic salmon (Salmo salar L.) in Europe and postglacial colonization of the Baltic Sea area. Nilsson J, Gross R, Asplund T, Dove O, Jansson H, Kelloniemi K, Kohlmann K, Loytynoja A, Nielsen EE, Paaver T, Primmer CR, Titov S, Vasemägi A, Veselov A, Öst T & Lumme J. Molecular Ecology, 2001.

Phylogeographic lineages and differentiation pattern of Atlantic salmon (Salmo salar) in the Baltic Sea with management implications. Koljonen ML, Jansson H, Paaver T, Vastin O & Koskiniemi J. Canadian Journal of Fisheries and Aquatic Sciences, 1999.

Genetic population structure of Atlantic salmon. Ståhl G. Chapter in Population Genetics and Fishery Management, eds Ryman N and Utter F. University of Washington Press, Seattle, 1987.

Jumping salmon in a Baltic Sea river
Baltic salmon, Salmo salar
Photo: Ingemar Pettersson/Azote

Contact

Linda Laikre, Department of Zoology, University of Stockholm

Sara KurlandDepartment of Zoology, University of Stockholm

Three-spined stickleback

Three-spined stickleback, Gasterosteus aculeatus, is one of the most abundant fish species in the Baltic Sea, after herring and sprat. The stickleback has an influential position in the ecosystem as food source for piscivorous fishes and is an important predator on grazers, e.g. the Baltic isopod.

During spring spawning, high densities of sticklebacks migrate from offshore into shallow bays and harbors along the coast. Here, they provide a rich food source for adult stages of piscivorous fishes. The sticklebacks themselves also predate on a diverse range of prey items, including e.g. eggs and larvae of coastal piscivorous as well as mesograzers. Thus they affect recruitment on commercial fish and potentially also algal growth.

  • There is no particular genetic management advice communicated or warranted. Management can be of low concern as the high effective population sizes and the observed connectivity should maintain the genetic diversity.
  • However, the sticklebacks are at an influential position as a predator, ecosystem engineer and food source. This ecosystem complexity should be considered in management plans.
  • To maintain the potential to adapt to future conditions, high population sizes and continuous connectivity between those to transfer relevant genetic diversity, are important factors.

There are no immediate threats to populations or genetic variation

There is low spatial genetic differentiation between different populations of Baltic Sea three-spined sticklebacks. This pattern is likely generated by a large effective population size and suggests one global population in the Baltic.

Tolerance to climate change & potential to adapt
Results from the BONUS BAMBI project suggest that three-spined sticklebacks can buffer their offspring from environmental stress caused by salinity change. The mechanism for this acclimation could be transgenerational plasticity, where parents trigger a faster development and growth of the offspring to compensate for high mortality rates in the early stages.

The large genetic effective population and high connectivity between metapopulations should likely maintain the species´evolutionary potential, and thus the long-time survival. Further analyses are needed to fully describe and evaluate the adaptive potential of the three-spined sticklebacks.

Transgenerational plasticity and selection shape the adaptive potential of sticklebacks to salinity change. Heckwolf MJ, Meyer BS, Döring T, Eizaguirre C and Reusch TBH. Evolutionary Applications, 2018.

Inferring spatial structure from population genetics and spatial synchrony in demography of Baltic Sea fishes: implications for management. Östman Ö, Olsson J, Dannewitz J, Palm S and Florin A-B. Fish and fisheries, 2016.

Declining coastal piscivore populations in the Baltic Sea: Where and when do sticklebacks matter? Byström P, Bergström U, Hjälten A, Jonsson D & Olsson J. AMBIO, 2015.

Temporal Stability of Genetic Variability and Differentiation in the Three-Spined Stickleback (Gasterosteus aculeatus). DeFaveri J & Merilä J. PLoS ONE, 2015.

Heterogeneous genomic differentiation in marine threespine sticklebacks: adaptation along an environmental gradient. DeFaveri J, Jonsson PR & Merilä J. Evolution, 2013.

Contrasting population structures in two sympatric fishes in the Baltic Sea basin. DeFaveri J, Shikano T, Ab Ghani NI & Merilä J. Marine Biology, 2012.

Effects of altered offshore food webs on coastal ecosystems emphasize the need for cross-ecosystem managementEriksson BK, Sieben K, Eklöf J, Ljunggren L, Olsson J, Casini M & Bergström U. AMBIO, 2011.

A meso-predator release of stickleback promotes recruitment of macroalgae in the Baltic Sea. Sieben K, Ljunggren L, Bergström U & Eriksson BK. J Exp Mar Biol and Ecol, 2011.

Genetic evidence for male-biased dispersal in the three-spined stickleback (Gasterosteus aculeatus). Cano JM, Mäkinen HS & Merilä J. Mol Ecol, 2008.

Mitochondrial DNA phylogeography of the three-spined stickleback (Gasterosteus aculeatus) in Europe—Evidence for multiple glacial refugia.Mäkinen HS & Merilä J. Mol Phylogenet Evol, 2008.

Genetic relationships among marine and freshwater populations of the European three-spined stickleback (Gasterosteus aculeatus) revealed by microsatellites. Mäkinen HS, Cano JM & Merilä J. Mol Ecol, 2006.

Three spined stickleback
Three spined stickleback, Gasterosteus aculeatus
Photo: Ron Offermans

Contact

Britta MeyerMax Planck Insitute for Evoultionary Biology, Germany

Melanie Heckwolf, GEOMAR Helmholtz centre for ocean research, Germany