Life on the margin: genetic isolation and diversity loss in a peripheral marine ecosystem, the Baltic Sea

Marginal populations are often isolated and under extreme selection pressures resulting in anomalous genetics. Consequently, ecosystems that are geographically and ecologically marginal might have a large share of genetically atypical populations, in need of particular concern in management of these ecosystems. To test this prediction, we analysed genetic data from 29 species inhabiting the low saline Baltic Sea, a geographically and ecologically marginal ecosystem. On average Baltic populations had lost genetic diversity compared to Atlantic populations: a pattern unrelated to dispersal capacity, generation time of species and taxonomic group of organism, but strongly related to type of genetic marker (mitochondrial DNA loci had lost c. 50% diversity, and nuclear loci 10%). Analyses of genetic isolation by geographic distance revealed clinal patterns of differentiation between Baltic and Atlantic regions. For a majority of species, clines were sigmoid with a sharp slope around the Baltic Sea entrance, indicating impeded gene flows between Baltic and Atlantic populations. Some species showed signs of allele frequencies being perturbed at the edge of their distribution inside the Baltic Sea. Despite the short geological history of the Baltic Sea (8000 years), populations inhabiting the Baltic have evolved substantially different from Atlantic populations, probably as a consequence of isolation and bottlenecks, as well as selection on adaptive traits. In addition, the Baltic Sea also acts a refuge for unique evolutionary lineages. This marginal ecosystem is thus vulnerable but also exceedingly valuable, housing unique genes, genotypes and populations that constitute an important genetic resource for management and conservation.     

Distribution, sex ratio and growth of Carassius gibelio (Bloch) in coastal and inland waters of Estonia (north-eastern Baltic Sea)

Gibel carp Carassius gibelio (Bloch) was first introduced into fish ponds and small lakes of Estonia in 1948–49, and first detected in Estonian brackish waters (Gulf of Riga) in 1985. Since the mid‐1990s, the species has spread along the entire Estonian Baltic coastline. Growth rate in the brackish water population does not differ much from freshwater populations, but the freshwater populations are gynogenetic (or show high dominance of females) in contrast to the Baltic Sea population, which presents a normal sex ratio. The recent explosion of this species in the Baltic Sea could be explained by unusually warm summers during the 1990s and by the low abundance of predatory fish.     

Geographical variation in Fucus vesiculosus morphology in the Baltic and North Seas

Effects of stress and disturbance on morphology, reproductive effort, size and sex ratio were studied for Fucus vesiculosus populations from the Baltic Sea at Askö and the North Sea on the west coast of Sweden at Tjäm[otilde]. High morphological variation was found between Fucus populations, with significant differences in length and weight of individuals, thallus breadth, number of branches and receptacles and receptacle weight, not only between Baltic and North Sea populations but also between populations within the same area, differing in wave exposure. With increasing disturbance, individuals in both studied populations were smaller and less branched. Differences were observed in plant size, with longer, broader and more branched plants being found in Askö compared with Tjärnö. Fucus populations at Tjämö allocated more biomass to reproduction and had longer, heavier receptacles than at Askö. Although the observed morphological changes may be partly explained by differences in wave exposure and salinity between the two sites, it is not possible to rule out genetic differences between the Baltic and North Sea populations. However, it is unlikely that the variations observed within the populations and between populations from the same area are genetically determined.     

Locomotory capacity of Baltic Sea and freshwater populations of the threespine stickleback (Gasterosteus aculeatus)

Two freshwater populations and one marine population (Baltic Sea) of threespine stickeback (Gasterosteus aculeatus) from Northeastern Germany were studied with regard to locomotory capacity: sustained swimming performance, activities of key enzymes in axial muscle, pectoral fin muscle and heart, and morphology. We postulated that life history differences between migratory Baltic Sea and resident freshwater populations could have led to a divergence in their locomotory capacity. The activity of citrate synthase (CS) in pectoral muscle correlated with critical swimming speed. Critical swimming speed, aerobic and anaerobic capacity of the pectoral fin muscle were population-specific. The Baltic Sea sticklebacks had a higher locomotory capacity (activity of CS in pectoral muscle, critical swimming speed) than sticklebacks of one freshwater population. However, another freshwater population expressed a similar locomotory capacity as the Baltic Sea population. In addition, Baltic Sea sticklebacks had a greater mass and lower anaerobic capacity of the pectoral fin muscle than the freshwater sticklebacks. The results are interpreted as an indication of a proceeding divergence between marine and resident freshwater populations and between freshwater populations of G. aculeatus originating from marine ancestors. The migratory Baltic Sea sticklebacks had better morphological prerequisites for sustained swimming than both freshwater populations, but there was no general difference in the locomotory capacity between marine and freshwater sticklebacks. However, their morphology could favour a more effective locomotion in the Baltic Sea sticklebacks.     

Absence of population structure of turbot (Psetta maxima) in the Baltic Sea

We found low, albeit significant, genetic differentiation among turbot (Psetta maxima) in the Baltic Sea but in contrast to earlier findings we found no evidence of isolation by distance. In fact temporal variation among years in one locality exceeded spatial variation among localities. This is an unexpected result since adult turbot are sedentary and eggs are demersal at the salinities occurring in the Baltic. Our findings are most likely explained by the fact that we sampled fish that were born after/during a large influx of water to the Baltic Sea, which may have had the consequence that previously locally and relatively sedentary populations became admixed. These results suggest that populations that colonize relatively variable habitats, like the Baltic, face problems. Any adaptations to local conditions that may build up during stable periods may quickly become eroded when conditions change and/or when populations become admixed. Our results indicate that the ability of turbot to survive and reproduce at the low salinity in the Baltic is more likely due to phenotypic plasticity than a strict genetic adaptation to low salinity.     

Spectral sensitivity differences in two Mysis sibling species (Crustacea, Mysida): Adaptation or phylogenetic constraints?

The variation in eye spectral sensitivities of the closely related mysid species Mysis relicta Lovén, 1862 and Mysis salemaai Audzijonyt? and Väinölä, 2005 was studied in sympatric and allopatric populations from the brackish Baltic Sea and from two lakes representing different light environments. In the Baltic Sea the maximum spectral sensitivity of M. relicta, measured by the electroretinogram (ERG) technique, was shifted by ca 20 nm to longer wavelengths than in M. salemaai (564 and 545 nm, respectively). The spectral sensitivity of M. salemaai was closer to that of marine mysid species, which is consistent with its broader euryhalinity and the presumed longer brackish-water history. The species-specific sensitivities in the Baltic Sea were not affected by regional differences in light environments. In two lake populations of M. relicta, the spectral sensitivity was further shifted by ca 28 nm towards the longer wavelengths compared with the conspecific Baltic Sea populations. The spectral sensitivities in the four M. relicta populations were not correlated to the current light conditions, but rather to the phylogeographic histories and fresh- vs. brackish-water environments. A framework to further explore factors affecting spectral sensitivities in Mysis is suggested.     

Diversity of Aphanizomenon flos-aquae (cyanobacterium) populations along a Baltic Sea salinity gradient

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.     

Diversity of luciferase sequences and bioluminescence production in Baltic Sea Alexandrium ostenfeldii

The toxic dinoflagellate Alexandrium ostenfeldii is the only bioluminescent bloom-forming phytoplankton in coastal waters of the Baltic Sea. We analysed partial luciferase gene (lcf) sequences and bioluminescence production in Baltic A. ostenfeldii bloom populations to assess the distribution and consistency of the trait in the Baltic Sea, and to evaluate applications for early detection of toxic blooms. Lcf was consistently present in 61 Baltic Sea A. ostenfeldii strains isolated from six separate bloom sites. All Baltic Sea strains except one produced bioluminescence. In contrast, the presence of lcf and the ability to produce bioluminescence did vary among strains from other parts of Europe. In phylogenetic analyses, lcf sequences of Baltic Sea strains clustered separately from North Sea strains, but variation between Baltic Sea strains was not sufficient to distinguish between bloom populations. Clustering of the lcf marker was similar to internal transcribed spacer (ITS) sequences with differences being minor and limited to the lowest hierarchical clusters, indicating a similar rate of evolution of the two genes. In relation to monitoring, the consistent presence of lcf and close coupling of lcf with bioluminescence suggests that bioluminescence can be used to reliably monitor toxic bloom-forming A. ostenfeldii in the Baltic Sea.     

Conservation genetics and phylogeography of southern dunlins Calidris alpina schinzii

Breeding populations of southern dunlin Calidris alpina schinzii in South Fennoscandia and the Baltic are severely fragmented and declining dramatically. Information on the genetic structure and diversity is therefore of importance for the conservation and management of these populations. Here we present the results of comparative genetic analyses of these populations with other populations of the schinzii , alpina and arctica subspecies in northern Europe. We sequenced the mitochondrial DNA control region and the Z-chromosome intron VLDLR-9, and analyzed microsatellites and AFLPs, for analyses of within-population genetic diversity. We also extended previous analyses of the phylogeographic structure of dunlins in northern Europe with a larger sample of individuals and populations. Our results revealed no evidence of reduced genetic diversity or increased levels of inbreeding in the small and fragmented populations around the Baltic Sea as compared to the more vital and larger populations elsewhere. Nevertheless, their small population sizes and presumably high degree of isolation may lead to local extinctions, indicating that demographic and ecological factors may pose a greater threat to the survival of these populations than purely genetic factors. Phylogeographically, the schinzii populations in Scandinavia and the Baltic do not form a separate genetic clade, but are part of larger cline of genetic variation encompassing several recognized subspecies of dunlins in the western Palearctic region. Only the Icelandic population showed some distinctiveness in genetic structure and might therefore be considered a separate management unit. Our study highlights the general problem of lack of genetic support for subspecies in avian taxonomy and conservation genetics.     

Population genetic structure of mussels from the Baltic Sea

In a macrogeographic survey, the population genetic structure of mussels from various regions of the Baltic Sea, a large semi-enclosed brackish-water basin, was examined with reference toMytilus edulis andM. galloprovincialis samples from the North Sea, Irish coast and southern Portugal. Electrophoretically detectable variation was analysed at 6 polymorphic enzyme loci (Ap, Est-D, Lap-2, Odh, Pgi andPgm). Evidence was provided of a remarkably large amount of biochemical genetic differentiation among ecologically and morphologically divergent mussel populations in the Baltic. Patterns of allele frequencies in low-salinity populations from the area of the Baltic Proper were demonstrated to be widely homogeneous but contrast strongly with those of the western Baltic, the latter resembling populations from marine habitats of the North Sea. Associated with a pronounced salinity gradient, the spatial heterogeneity in gene-pool structure is indicated by steep clines of allele frequency changes in the area of the eastern Danish isles. The adaptive significance of the observed allozymic variation is suggested. From genetic distance estimates, the subdivision of population structure is discussed in relation to the significant amount of differentiation detected withinMytilus populations to date and to the evolutionary time required for the divergence of Baltic mussel populations. The allozymic data provide evidence for the genetic distinctiveness of mussels from the low-salinity areas of the Baltic. Their position at the specific or subspecific level of classification requires further consideration.     

Diversity of Aphanizomenonflos-aquae (Cyanobacterium) Populations along a Baltic Sea Salinity Gradient

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.     

Adaptive and Neutral Genetic Variation and Colonization History of Atlantic Salmon, Salmo salar

Synopsis I combined neutral microsatellite markers with the major histocompatibility complex (MHC) class IIB to study genetic differentiation and colonization history in Atlantic salmon, Salmo salar, in the Baltic Sea and in the north-eastern Atlantic. Baltic salmon populations have lower levels of microsatellite genetic variation, in terms of heterozygosity and allelic richness than Atlantic populations, confirming earlier findings with other genetic markers, suggesting that the Baltic Sea populations have been exposed to genetic bottlenecks, most likely at a founding event. On the other hand, the level of MHC variation was similar in the Baltic and in the north-eastern Atlantic, indicating that positive balancing selection has increased the level of MHC-variation. Both microsatellite and MHC class IIB genetic variation give strong support to the hypothesis that the Baltic salmon are of a biphyletic origin, the southern population in this study is strongly differentiated from both the northern Baltic salmon populations and from the north-eastern Atlantic populations. Salmon may have colonized the northern Baltic Sea either from the south, via the so called “N?rke strait” or from the north, via a proposed historical connection between the White Sea and the northern Baltic. At microsatellites, no significant isolation-by distance was found at either colonization route. At the MHC, populations were significantly isolated by distance when assuming that colonization occurred via the “N?rke strait”.     

Genetic differentiation among natural European populations of Atlantic salmon, Salmo salar L., from drainages of the Atlantic Ocean

Previously published allelic frequencies at four polymorphic protein coding loci were used as a basis for examining genetic relationships among 19 European populations of Atlantic salmon, Salmo salar L.--exclusive of Baltic drainages--from the Barents Sea to Spain. The data did not support a model of distinct ancestral (e.g. Boreal and Celtic) origins, but were consistent with all populations descending from a single ancestral group within this region with genetically diverged populations drawn together through limited local migrations.     

Thiamin dynamics during the adult life cycle of Atlantic salmon (Salmo salar)

Thiamin is an essential water-soluble B vitamin known for its wide range of metabolic functions and antioxidant properties. Over the past decades, reproductive failures induced by thiamin deficiency have been observed in several salmonid species worldwide, but it is unclear why this micronutrient deficiency arises. Few studies have compared thiamin concentrations in systems of salmonid populations with or without documented thiamin deficiency. Moreover, it is not well known whether and how thiamin concentration changes during the marine feeding phase and the spawning migration. Therefore, samples of Atlantic salmon (Salmo salar) were collected when actively feeding in the open Baltic Sea, after the sea migration to natal rivers, after river migration, and during the spawning period. To compare populations of Baltic salmon with systems without documented thiamin deficiency, a population of landlocked salmon located in Lake Vänern (Sweden) was sampled as well as salmon from Norwegian rivers draining into the North Atlantic Ocean. Results showed the highest mean thiamin concentrations in Lake Vänern salmon, followed by North Atlantic, and the lowest in Baltic populations. Therefore, salmon in the Baltic Sea seem to be consistently more constrained by thiamin than those in other systems. Condition factor and body length had little to no effect on thiamin concentrations in all systems, suggesting that there is no relation between the body condition of salmon and thiamin deficiency. In our large spatiotemporal comparison of salmon populations, thiamin concentrations declined toward spawning in all studied systems, suggesting that the reduction in thiamin concentration arises as a natural consequence of starvation rather than to be related to thiamin deficiency in the system. These results suggest that factors affecting accumulation during the marine feeding phase are key for understanding the thiamin deficiency in salmonids.     

Population structure of flounder (Platichthys flesus) in the Baltic Sea: differences among demersal and pelagic spawners

We found significant population structure and isolation by distance among samples of flounder (Platichthys flesus) in the Baltic, Kattegat and Skagerrak seas using microsatellite genetic markers. This pattern was almost entirely due to a difference between flounder that have demersal spawning in the northern Baltic, as compared to pelagic spawners in the southern Baltic and on the west coast of Sweden. Among demersal spawners we found neither genetic differentiation nor any isolation by distance among sampling sites. We speculate that demersal flounder are descendants of a population that colonized the Baltic previous to pelagic spawners. The demersal flounder may thus have had longer time to adapt to the low salinity in the Baltic, and accordingly display egg characteristics that make it possible to reproduce at the low salinity levels in the northern Baltic. Among pelagic spawners significant isolation by distance was detected. Pelagic spawners have previously been shown to display clinal variation in egg size, which allows them to float also at the moderate salinity levels up to the region north of the island Bornholm. Management units for harvesting should ideally be based on true biological populations, and for the commercially important flounder up to 15 different management stocks in the Baltic have been suggested. We could not find a population genetic foundation for such a high number of management units, and our data suggest three management units: the northern Baltic (demersal populations), southern Baltic with the Oresund straits and the most northwestern sampling sites (Skagerrak, Kattegat and North Sea).     

Unfolding of population structure in Baltic sheep breeds using microsatellite analysis

Studies of domestic animals are performed on breeds, but a breed does not necessarily equate to a genetically defined population. The division of sheep from three native and four modern Baltic sheep breeds was studied using 21 microsatellite loci and applying a Bayesian clustering method. A traditional breed-wise approach was compared to that relying on the pattern of molecular diversity. In this study, a breed was found to be inconsistent with a distinct genetic population for three reasons: (i) a lack of differentiation between modern Baltic breeds, since the majority of the studied sheep formed a single population; (ii) the presence of individuals of foreign ancestry within the breed; and (iii) an undefined local Saaremaa sheep was referred to as a breed, but was shown to consist of separate populations. In the breed-wise approach, only the clearly distinct Ruhnu sheep demonstrated low within-breed variation, although the newly identified Saaremaa populations also have low variability. Providing adequate management recommendations for the Saaremaa sheep is not possible without further studies, but the potential harmful effects of inbreeding in the Ruhnu sheep could be reduced through the use of two genetically related Saaremaa populations. In other breeds, excessive crossing appears to be a larger concern than inbreeding. Assigning individuals into populations based on the pattern of genetic diversity offers potentially unbiased means of elucidating the genetic population structure of species. Combining these genetic populations with phenotypic and aetiological data will enable formulation of the most informed recommendations for gene resource management.     

Population differentiation in the redshank (<Emphasis Type="Italic">Tringa totanus</Emphasis>) as revealed by mitochondrial DNA and amplified fragment length polymorphism markers

The redshank (Tringa totanus) is declining throughout Europe and to implement efficient conservation measures, it is important to obtain information about the population genetic structure. The aim of the present study was two-fold. First, we analysed the genetic variation within and between populations in the Baltic region in southern Scandinavia. Evidence of genetic structure would suggest that different populations might require separate management strategies. Second, in an attempt to study large-scale genetic structure we compared the Baltic populations with redshanks from northern Scandinavia and Iceland. This analysis could reveal insights into phylogeography and long-term population history. DNA samples were collected from six breeding sites in Scandinavia presumed to include two subspecies (totanus and britannica) and a further sample from Iceland (subspecies robusta). Two methods were used to study the population genetic structure. Domain II and III of the mitochondrial control region was analysed by DNA sequencing and nuclear DNA was analysed by screening amplified fragment length polymorphism (AFLP) markers. Mitochondrial DNA showed no variation between individuals in domain II. When analysing an 481 bp fragment of domain III seven haplotypes were found among birds. On the basis of mtDNA sequences, redshanks showed some evidence of a recent expansion from a bottlenecked refugial population. Bayesian analyses of AFLP data revealed a significant genetic differentiation between suggested subspecies but not between populations within the Baltic region. Our results indicate that populations of redshanks in Europe constitute at least three separate management units corresponding to the recognised subspecies.     

Population and size-specific distribution of Atlantic salmon Salmo salar in the Baltic Sea over five decades

Population-specific assessment and management of anadromous fish at sea requires detailed information about the distribution at sea over ontogeny for each population. However, despite a long history of mixed-stock sea fisheries on Atlantic salmon, Salmo salar, migration studies showing that some salmon populations feed in different regions of the Baltic Sea and variation in dynamics occurs among populations feeding in the Baltic Sea, such information is often lacking. Also, current assessment of Baltic salmon assumes equal distribution at sea and therefore equal responses to changes in off-shore sea fisheries. Here, we test for differences in distribution at sea among and within ten Atlantic salmon Salmo salar populations originating from ten river-specific hatcheries along the Swedish Baltic Sea coast, using individual data from >125,000 tagged salmon, recaptured over five decades. We show strong population and size-specific differences in distribution at sea, varying between year classes and between individuals within year classes. This suggests that Atlantic salmon in the Baltic Sea experience great variation in environmental conditions and exploitation rates over ontogeny depending on origin and that current assessment assumptions about equal exploitation rates in the offshore fisheries and a shared environment at sea are not valid. Thus, our results provide additional arguments and necessary information for implementing population-specific management of salmon, also when targeting life stages at sea.     

GENETIC STRUCTURE IN POPULATIONS OF FUCUS VESICULOSUS (PHAEOPHYCEAE) OVER SPATIAL SCALES FROM 10 M TO 800 KM1

Recent studies showing consequences of species’ genetic diversity on ecosystem performance raise the concern of how key ecosystem species are genetically structured. The bladder wrack Fucus vesiculosus L. is a dominant species of macroalga in the northern Atlantic, and it is particularly important as a habitat‐forming species in the Baltic Sea. We examined the genetic structure of populations of F. vesiculosus with a hierarchical approach from a within‐shore scale (10 m) to a between‐seas scale (Baltic Sea–Skagerrak, 800 km). Analysis of five microsatellite loci showed that population differentiation was generally strong (average F_ST = 12%), being significant at all spatial scales investigated (10¹, 10³, 10^4–5, 10⁶ m). Genetic differentiation between seas (Baltic Sea and Skagerrak) was substantial. Nevertheless, the effects of isolation by distance were stronger within seas than between seas. Notably, Baltic summer‐reproducing populations showed a strong within‐sea, between‐area (70 km) genetic structure, while Baltic autumn‐reproducing populations and Skagerrak summer‐reproducing populations revealed most genetic diversity between samples within areas (<1 km). Despite such differences in overall structure, Baltic populations of summer‐ and autumn‐reproducing morphs did not separate in a cluster analysis, indicating minor, if any, barriers to gene flow between them. Our results have important implications for management and conservation of F. vesiculosus, and we raise a number of concerns about how genetic variability should be preserved within this species.     

Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis

The study of hybrid zones is central to our understanding of the genetic basis of reproductive isolation and speciation, yet very little is known about the extent and significance of hybrid zones in marine fishes. We examined the population structure of cod in the transition area between the North Sea and the Baltic Sea employing nine microsatellite loci. Genetic differentiation between the North Sea sample and the rest increased along a transect to the Baltic proper, with a large increase in level of differentiation occurring in the Western Baltic area. Our objective was to determine whether this pattern was caused purely by varying degrees of mechanical mixing of North Sea and Baltic Sea cod or by interbreeding and formation of a hybrid swarm. Simulation studies revealed that traditional Hardy-Weinberg analysis did not have sufficient power for detection of a Wahlund effect. However, using a model-based clustering method for individual admixture analysis, we were able to demonstrate the existence of intermediate genotypes in all samples from the transition area. Accordingly, our data were explained best by a model of a hybrid swarm flanked by pure nonadmixed populations in the North Sea and the Baltic Sea proper. Significant correlation of gene identities across loci (gametic phase disequilibrium) was found only in a sample from the Western Baltic, suggesting this area as the centre of the apparent hybrid zone. A hybrid zone for cod in the ecotone between the high-saline North Sea and the low-saline Baltic Sea is discussed in relation to its possible origin and maintenance, and in relation to a classical study of haemoglobin variation in cod from the Baltic Sea/Danish Belt Sea, suggesting mixing of two divergent populations without interbreeding.