Range expansions can lead to increased contact of divergent populations, thus increasing the potential of hybridization events. Whether viable hybrids are produced will most likely depend on the level of genomic divergence and associated genomic incompatibilities between the different entities as well as environmental conditions. By taking advantage of historical Baltic cod (Gadus morhua) otolith samples combined with genotyping and whole genome sequencing, we here investigate the genetic impact of the increased spawning stock biomass of the eastern Baltic cod stock in the mid 1980s. The eastern Baltic cod is genetically highly differentiated from the adjacent western Baltic cod and locally adapted to the brackish environmental conditions in the deeper Eastern basins of the Baltic Sea unsuitable for its marine counterparts. Our genotyping results show an increased proportion of eastern Baltic cod in western Baltic areas (Mecklenburg Bay and Arkona Basin)-indicative of a range expansion westwards-during the peak population abundance in the 1980s. Additionally, we detect high frequencies of potential hybrids (including F1, F2 and backcrosses), verified by whole genome sequencing data for a subset of individuals. Analysis of mitochondrial genomes further indicates directional gene flow from eastern Baltic cod males to western Baltic cod females. Our findings unravel that increased overlap in distribution can promote hybridization between highly divergent populations and that the hybrids can be viable and survive under specific and favourable environmental conditions. However, the observed hybridization had seemingly no long-lasting impact on the continuous separation and genetic differentiation between the unique Baltic cod stocks.

Coexistence of fish populations (= stocks) of the same species is a common phenomenon. In the Baltic Sea, two genetically divergent stocks of Atlantic cod (Gadus morhua), Western Baltic cod (WBC) and Eastern Baltic cod (EBC), coexist in the Arkona Sea. Although the relative proportions of WBC and EBC in this area are considered in the current stock assessments, the mixing dynamics and ecological mechanisms underlying coexistence are not well understood. In this study, a genetically validated otolith shape analysis was used to develop the most comprehensive time series of annual stock mixing data (1977-2019) for WBC and EBC. Spatio-temporal mixing analysis confirmed that the two stocks coexist in the Arkona Sea, albeit with fluctuating mixing proportions over the 43-year observation period. Depth-stratified analysis revealed a strong correlation between capture depth and stock mixing patterns, with high proportions of WBC in shallower waters (48-61% in <20m) and increasing proportions of EBC in deeper waters (50-86% in 40-70m). Consistent depth-specific mixing patterns indicate stable differences in depth distribution and habitat use of WBC and EBC that may thus underlie the long-term coexistence of the two stocks in the Arkona Sea. These differences were also reflected in significantly different proportions of WBC and EBC in fisheries applying passive gears in shallower waters (more WBC) and active gears in deeper waters (more EBC). This highlights the potential for fishing gear-specific exploitation of different stocks, and calls for stronger consideration of capture depth and gear type in stock assessments. This novel evidence provides the basis for improved approaches to research, monitoring and management of Baltic cod stocks.

Recent studies have uncovered patterns of genomic divergence in marine teleosts where panmixia due to high gene flow has been the general paradigm. These signatures of divergent selection are often impacted by structural variants, acting as "supergenes" facilitating local adaptation. The highly dispersing European plaice (Pleuronectes platessa)-in which putative structural variants (i.e., inversions) have been identified-has successfully colonized the brackish water ecosystem of the Baltic Sea. Thus, the species represents an ideal opportunity to investigate how the interplay of gene flow, structural variants, natural selection, past demographic history, and gene flow impacts on population (sub)structuring in marine systems. Here, we report on the generation of an annotated draft plaice genome assembly in combination with population sequencing data-following the salinity gradient from the Baltic Sea into the North Sea together with samples from Icelandic waters-to illuminate genome-wide patterns of divergence. Neutral markers pointed at large-scale panmixia across the European continental shelf associated with high gene flow and a common postglacial colonization history of shelf populations. However, based on genome-wide outlier loci, we uncovered signatures of population substructuring among the European continental shelf populations, i.e., suggesting signs of ongoing selection. Genome-wide selection analyses (xp-EHH) and the identification of genes within genomic regions of recent selective sweeps-overlapping with the outlier loci-suggest that these represent the signs of divergent selection. Our findings provide support for genomic divergence driven by local adaptation in the face of high gene flow and elucidate the relative importance of demographic history versus adaptive divergence in shaping the contemporary population genetic structure of a marine teleost. The role of the putative inversion(s) in the substructuring-and potentially ongoing adaptation-was seemingly not substantial.

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.

The use of growth estimation methods that depend on unreliable age data has previously hindered the quantification of perceived differences in growth rates between the two cod stocks inhabiting the Baltic Sea. Data from cod tagged in different regions of the Baltic Sea during 2007-2019 were combined, and general linear models were fit to investigate inter-regional (defined as area of release) and inter-stock (assigned to a subset of recaptures using genetic and otolith shape analyses) differences in individual growth. An average-sized cod (364 mm) caught in the western Baltic Sea and assigned to the western Baltic cod stock grew at more than double the rate (145 mm year(-1)) on average than a cod of the same size caught in the eastern Baltic Sea and assigned to the eastern Baltic cod stock (58 mm year(-1)), highlighting the current poor conditions for the growth of cod in the eastern Baltic Sea. The regional differences in growth rate were more than twice as large (63 mm year(-1)) as the stock differences (24 mm year(-1)). Although the relative importance of environmental and genetic factors cannot be fully resolved through this study, these results suggest that environmental experience may contribute to growth differences between Baltic cod stocks.

The assignment of individual fish to its stock of origin is important for reliable stock assessment and fisheries management. Otolith shape is commonly used as the marker of distinct stocks in discrimination studies. Our literature review showed that the application and comparison of alternative statistical classifiers to discriminate fish stocks based on otolith shape is limited. Therefore, we compared the performance of two traditional and four machine learning classifiers based on Fourier analysis of otolith shape using selected stocks of Atlantic cod (Gad us morhua) in the southern Baltic Sea and Atlantic herring (Clupea harengus) in the western Norwegian Sea, Skagerrak, and the southern Baltic Sea. Our results showed that the stocks can be successfully discriminated based on their otolith shapes. We observed significant differences in the accuracy obtained by the tested classifiers. For both species, support vector machines (SVM) resulted in the highest classification accuracy. These findings suggest that modern machine learning algorithms, like SVM, can help to improve the accuracy of fish stock discrimination systems based on the otolith shape.

: For sustainable fisheries management, fish individuals in a mixing area need to be separated according to their stock affiliation. The assignment of individuals to one of the stocks requires reliable stock discrimination methods with high assignment accuracy. In the Baltic Sea, 2 genetically differentiated Atlantic cod Gadus morhua stocks, the western (WBC) and eastern Baltic cod (EBC), coexist in the Arkona Basin, inducing uncertainties in the stock assessments. Here, we evaluated a suite of non-molecular stock discrimination techniques (otolith shape analysis, stable isotope analysis on otolith nuclei, otolith readability and diameter of translucent zones [TZs]) on the same set of genetically validated Baltic cod baseline samples from the mixing area (Arkona Basin) and adjacent areas (Belt Sea, Øresund and Bornholm Basin). Otolith shape and stable oxygen isotope analyses showed the highest classification accuracies; between 80 and 84% of cod individuals were correctly assigned to their respective stock of origin. Stable carbon isotope analysis, otolith readability and the diameter of the first 2 TZs yielded classification accuracies of only 52 to 61%. Given the high assignment accuracy and the availability of archived otoliths, otolith shape and stable oxygen isotope analyses on otolith nuclei are powerful separation methods that allow for high-throughput quantification of present and past mixing proportions of Baltic cod stocks. This study provides the most comprehensive approach of genetically validated stock discrimination techniques currently available for Baltic cod, and evaluates the applicability and reliability of otolith-based methods for future research studies and for fisheries management purposes.

Tag–recapture data contain valuable information about individual fish growth, which can enhance the estimation of growth parameters. Tag–recapture data are especially useful when age-determination uncertainties throw age-based growth estimates and stock assessments into question, as is the present situation with several important gadoid stocks. Length-based (GROTAG method) and age-based (LEP method) growth functions were fit to a large data set of tagged and recaptured cod (Gadus morhua) from an artificial reef in the western Baltic Sea to calculate improved growth parameter estimates. The LEP method allowed integration of different data formats, including tag–recapture, length frequency, and length-at-age, to estimate a more robust and comprehensive growth function (von Bertalanffy growth parameters: L∞ = 154.56 cm, k = 0.11, t0 = –0.13). Two heavily exploited cod stocks inhabit the Baltic Sea, subsisting at the upper thermal and lower salinity limits of the species. Otolith shape analyses indicated that, unexpectedly, individuals from both populations were resident at the reef. Compared with cod populations elsewhere, cod in the western Baltic Sea grow relatively slowly and with weak seasonal fluctuations in growth rates, potentially due to adverse conditions for growth.

Atlantic cod (Gadus morhua) is a species of great ecological and economical importance in the Baltic Sea. Here, two genetically differentiated stocks, the western and the eastern Baltic cod, display substantial mechanical mixing, hampering our understanding of cod ecology and impeding stock assessments and management. Based on whole-genome re-sequencing data from reference samples obtained from the study area, we designed two different panels of Single Nucleotide Polymorphisms markers (SNPs), which take into account the exceptional genome architecture of cod. A minimum panel of 20 diagnostic SNPs and an extended panel (20 diagnostic and 18 biologically informative SNPs, 38 in total) were developed and validated to distinguish unambiguously between the western and the eastern Baltic cod stocks and to enable studies of local adaptation to the specific environment in the Baltic Sea, respectively. We tested both panels on cod sampled from the southern Baltic Sea (n = 603) caught in 2015 and 2016. Genotyping results showed that catches from the mixing zone in the Arkona Sea, were composed of similar proportions of individuals of the western and the eastern stock. Catches from adjacent areas to the east, the Bornholm Basin and Gdańsk Deep, were exclusively composed of eastern Baltic cod, whereas catches from adjacent western areas (Belt Sea and Öresund) were composed of western Baltic cod. Interestingly, the two Baltic cod stocks showed strong genetic differences at loci associated with life-history trait candidate genes, highlighting the species’ potential for ecological adaptation even at small geographical scales. The minimum and the extended panel of SNP markers presented in this study provide powerful tools for future applications in research and fisheries management to further illuminate the mixing dynamics of cod in the Baltic Sea and to better understand Baltic cod ecology.

The ICES Baltic Fisheries Assessment Working Group (WGBFAS) met 19-26 April 2017(Chair: Tomas Grohsler, Germany and Co-chair: Michele Casini, Sweden), with 41 participants and 9 countries represent ...

Journal of Fish DiseasesVolume 39, Issue 8 p. 1001-1005 Short Communication Ocean acidification leads to deformations of caudal vein angio-architecture in juvenile threespine stickleback, Gasterosteus aculeatus Linnaeus H Ahnelt, Corresponding Author H Ahnelt Department of Theoretical Biology, University of Vienna, Vienna, AustriaCorrespondence H Ahnelt, Department of Theoretical Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria (e-mail:harald.ahnelt@univie.ac.at)Search for more papers by this authorF M Schade, F M Schade Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, List, GermanySearch for more papers by this authorM Wegner, M Wegner Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, List, GermanySearch for more papers by this author H Ahnelt, Corresponding Author H Ahnelt Department of Theoretical Biology, University of Vienna, Vienna, AustriaCorrespondence H Ahnelt, Department of Theoretical Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria (e-mail:harald.ahnelt@univie.ac.at)Search for more papers by this authorF M Schade, F M Schade Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, List, GermanySearch for more papers by this authorM Wegner, M Wegner Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, List, GermanySearch for more papers by this author First published: 16 October 2015 https://doi.org/10.1111/jfd.12417Citations: 3Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume39, Issue8August 2016Pages 1001-1005 RelatedInformation

Marine environmental conditions are naturally changing throughout the year, affecting life cycles of hosts as well as parasites. In particular, water temperature is positively correlated with the development of many parasites and pathogenic bacteria, increasing the risk of infection and diseases during summer. Interannual temperature fluctuations are likely to alter host parasite interactions, which may result in profound impacts on sensitive ecosystems.In this context we investigated the parasite and bacterial Vibrionaceae communities of four common small fish species (three-spined stickleback Gasterosteus aculeatus, Atlantic herring Clupea harengus, European sprat Sprattus sprattus and lesser sand eel Ammodytes tobianus) in the Northern Wadden Sea over a period of two years. Overall, we found significantly increased relative diversities of infectious species at higher temperature differentials. On the taxon-specific level some macroparasite species (trematodes, nematodes) showed a shift in infection peaks that followed the water temperatures of preceding months, whereas other parasite groups showed no effects of temperature differentials on infection parameters.Our results show that even subtle changes in seasonal temperatures may shift and modify the phenology of parasites as well as opportunistic pathogens that can have far reaching consequences for sensitive ecosystems. (C) 2015 Elsevier B.V. All rights reserved.

Background Pathogens are a major regulatory force for host populations, especially under stressful conditions. Elevated temperatures may enhance the development of pathogens, increase the number of transmission stages, and can negatively influence host susceptibility depending on host thermal tolerance. As a net result, this can lead to a higher prevalence of epidemics during summer months. These conditions also apply to marine ecosystems, where possible ecological impacts and the population-specific potential for evolutionary responses to changing environments and increasing disease prevalence are, however, less known. Therefore, we investigated the influence of thermal stress on the evolutionary trajectories of disease resistance in three marine populations of three-spined sticklebacks Gasterosteus aculeatus by combining the effects of elevated temperature and infection with a bacterial strain of Vibrio sp . using a common garden experiment. Results We found that thermal stress had an impact on fish weight and especially on survival after infection after only short periods of thermal acclimation. Environmental stress reduced genetic differentiation (Q ST ) between populations by releasing cryptic within-population variation. While life history traits displayed positive genetic correlations across environments with relatively weak genotype by environment interactions (GxE), environmental stress led to negative genetic correlations across environments in pathogen resistance. This reversal of genetic effects governing resistance is probably attributable to changing environment-dependent virulence mechanisms of the pathogen interacting differently with host genotypes, i.e. G Pathogen xG Host xE or (G Pathogen xE)x(G Host xE) interactions, rather than to pure host genetic effects, i.e. G Host xE interactions. Conclusion To cope with climatic changes and the associated increase in pathogen virulence, host species require wide thermal tolerances and pathogen-resistant genotypes. The higher resistance we found for some families at elevated temperatures showed that there is evolutionary potential for resistance to Vibrio sp. in both thermal environments. The negative genetic correlation of pathogen resistance between thermal environments, on the other hand, indicates that adaptation to current conditions can be a weak predictor for performance in changing environments. The observed feedback on selective gradients exerted on life history traits may exacerbate this effect, as it can also modify the response to selection for other vital components of fitness.

Some studies have demonstrated that elevated CO2 concentrations in ocean waters negatively impact metabolism and development of marine fish. Particularly, early developmental stages are probably more susceptible to ocean acidification due to insufficient regulations of their acid-base balance. Transgenerational acclimation can be an important mechanism to mediate impacts of increased CO2 on marine species, yet very little is known about the potential of parental effects in teleosts. Therefore, transgenerational effects were investigated on life history in juvenile three-spined sticklebacks Gasterosteus aculeatus by acclimating parents (collected in April 2012, 55°03′N, 8°44′E) and offspring to ambient (~400 µatm) and elevated (~1,000 µatm) CO2 levels and measured parental fecundity as well as offspring survival, growth and otolith characteristics. Exposure to elevated CO2 concentrations led to an increase in clutch size in adults as well as increased juvenile survival and growth rates between 60 and 90 days post-hatch and enlarged otolith areas compared with fish from ambient CO2 concentrations. Moreover, transgenerational effects were observed in reduced survival and body size 30 days post-hatch as well as in enlarged otoliths at the end of the experiment, when fathers or both parents were acclimated to the high-CO2 environment. These results may suggest that elevated CO2 concentrations had rather positive effects on life-history traits of three-spined sticklebacks, but that parental acclimation can modify these effects without improving offspring fitness. Although the mechanistic basis of such transgenerational acclimation remains unclear, selective gradients within generations seem to determine the direction of transgenerational effects.

The ICES Benchmark Workshop on Baltic Cod Stocks (WKBALTCOD2) met at ICES Headquarters in Copenhagen, Denmark on 4–8 February 2019, following a data evaluation workshop (Chair Johan Lovgren) and several preparatory web conference meetings. The meeting, cochaired by Meaghan Bryan, USA (External Chair) and Michele Casini, Sweden (ICES Chair), was attended by two invited external experts, Vladlena Gertseva, USA, and Verena Trenkel, France, and 47 participants from 10 countries. Participants represented a diversity of groups including industry, NGOs, managers, and scientists. The objectives of WKBALTCOD2 were to evaluate the appropriateness of the data and the assessment methods to determine stock status for the Western Baltic cod (SD 22-24) and the Eastern Baltic cod (SD 24-32) stocks, evaluate the short-term forecasting methods, re-examine and update the reference points, and update the stock annex as appropriate to these stocks. The workshop started with a group discussion of data issues and decisions that were integral to both assessments. Subsequent, stock-specific discussions were held in smaller subgroups and the main results and conclusions were presented in plenary. Around 15–20 persons, together with at least one external reviewer/co-chair, participated in each of the subgroups. A single modelling approach, the state-space stock assessment model (SAM), was presented for Western Baltic cod. This model has been used previously to determine stock status of Western Baltic cod and the panel agreed that this model should be used for the current assessment. The main issues that emerged and were addressed during the workshop for this stock were stock mixing in SD 24, the inclusion of new recreational data from Sweden and Denmark, the inclusion of a German pound-net survey index of age-0 fish in the assessment model, the extension of the time-series of the assessment back in time, and the update of reference points. Future efforts should be made to update the mixing proportion of Eastern and Western Baltic cod in SD 24 by length and season, and continue improving the stock splitting methods and the geographical coverage of the samples. An analytical quantitative assessment had been lacking since 2014 for the Eastern Baltic cod stock. The key issues addressed during the benchmark included how to best account for changes in productivity (e.g. growth, mortality, maturity) in the assessment model, stock splitting in SD 24, the use of age–length keys in the assessment, and ageing error and bias. Stock assessment models using Stock Synthesis and the stochastic surplus production model (SPiCT) were put forward for the benchmark as two possible model candidates. The panel agreed that the Stock Synthesis model assuming time-varying growth, natural mortality, and maturity to account for changes in productivity was acceptable to provide scientific advice, while the SPiCT model should be maintained as an alternative approach. The accepted model exhibited some residual patterns that were likely due to assuming that ages were precisely known. Stock Synthesis can accommodate an ageing error matrix to account for precision and bias. Ageing error and bias statistics should be developed and thoroughly reviewed for the next benchmark. Additionally, future work should focus on improving the growth estimates, which will allow a more precise separation between growth and natural mortality. The Stock Synthesis model would also benefit from information on sample size associated with length distributions of commercial catches. Estimates of fishing mortality compatible with a precautionary FMSY was not attainable for this stock, therefore probabilistic forecasts with Markov chain Monte Carlo methods were proposed to be used instead.