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No abstract available. In press.

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No abstract available. In print.

We employed DNA sequence variation at two mitochondrial (control region, COI) regions from 212 individuals of Galaxias platei (Pisces, Galaxiidae) collected throughout Patagonia (25 lakes/rivers) to examine how Andean orogeny and the climatic cycles throughout the Quaternary affected the genetic diversity and phylogeography of this species. Phylogenetic analyses revealed four deep genealogical lineages which likely represent the initial division of G. platei into eastern and western lineages by Andean uplift, followed by further subdivision of each lineage into separate glacial refugia by repeated Pleistocene glacial cycles. West of the Andes, refugia were likely restricted to the northern region of Patagonia with small relicts in the south, whereas eastern refugia appear to have been much larger and widespread, consisting of separate northern and southern regions that collectively spanned most of Argentinean Patagonia. The retreat of glacial ice following the last glacial maximum allowed re-colonization of central Chile from nonlocal refugia from the north and east, representing a region of secondary contact between all four glacial lineages. Northwestern glacial relicts likely followed pro-glacial lakes into central Chilean Patagonia, whereas catastrophic changes in drainage direction (Atlantic . Pacific) for several eastern palaeolakes were the likely avenues for invasions from the east. These mechanisms, combined with evidence for recent, rapid and widespread population growth could explain the extensive contemporary distribution of G. platei throughout Patagonia.

A suite of 10 microsatellite loci was used to examine genetic divergence between two sympatric morphs of Arctic charr Salvelinus alpinus (‘dark’ and ‘pale’) inhabiting Gander Lake, Newfoundland. Results can be summarized as follows: (1) the morphs are strongly reproductively isolated – gene flow–migration estimates were consistently low in long and short-term evolutionary timescales of analysis; (2) intermorph divergence based on allele size (RST) was significantly larger than those based on allele state (y) implying a cumulative effect of stepwiselike mutations; (3) historical (coalescent) and current (linkage disequilibrium) point estimates of effective population size (Ne) were consistently higher for dark than for pale S. alpinus. The first and second findings lend support to the hypothesis that divergence between forms may have preceded the last glacial period (ending c. 12 000 years BP). The third finding argues for significant differences in habitat quantity and quality between morphs, which were emphasized in a previous study. Overall, these analyses underscore the importance of genetic assessment and monitoring in the conservation of fish diversity, with emphasis on ‘rare’ or under-represented forms among temperate species pairs.

Genetic stochasticity due to small population size contributes to population extinction, especially when population fragmentation disrupts gene flow. Estimates of effective population size (Ne) can therefore be informative about population persistence, but there is a need for an assessment of their consistency and informative relevance. Here we review the body of empirical estimates of Ne for wild populations obtained with the temporal genetic method and published since Frankham’s (1995) review. Theoretical considerations have identified important sources of bias for this analytical approach, and we use empirical data to investigate the extent of these biases. We find that particularly model selection and sampling require more attention in future studies. We report a median unbiased Ne estimate of 260 (among 83 studies) and find that this median estimate tends to be smaller for populations of conservation concern, which may therefore be more sensitive to genetic stochasticity. Furthermore, we report a median Ne/N ratio of 0.14, and find that this ratio may actually be higher for small populations, suggesting changes in biological interactions at low population abundances. We confirm the role of gene flow in countering genetic stochasticity by finding that Ne correlates strongest with neutral genetic metrics when populations can be considered isolated. This underlines the importance of gene flow for the estimation of Ne, and of population connectivity for conservation in general. Reductions in contemporary gene flow due to ongoing habitat fragmentation will likely increase the prevalence of genetic stochasticity, which should therefore remain a focal point in the conservation of biodiversity.

An Atlantic herring major histocompatibility class II A (Clha-DAA) cDNA sequence has been characterized and was shown to encode a leader peptide, alpha-1 domain, alpha-2 domain, connecting peptide, transmembrane and cytoplasmic region. The Clha-DAA protein sequence has all the characteristics of a teleost class II A protein with conserved cysteines in both the alpha-1 and the alpha-2 domains and two potential N-linked glycosylation sites. Exon 2 sequences encoding the polymorphic alpha-1 domain from different individuals were analysed and revealed the presence of at least two loci. The Clha-DAA gene consists of four exons and three short introns. Four unique intron 3 sequences from multiple individuals were obtained and were shown to contain a (TG)n microsatellite sequence. Primers were optimized such that only a single microsatellite locus designated Clha-DAA-INTR3 was ampli.ed. Four herring populations from the North Sea and the Baltic Sea were genotyped for Clha-DAA-INTR3. In total, 16 Clha-DAA-INTR3 alleles were detected; the distribution of the alleles showed no deviation from Hardy–Weinberg expectation. Levels of genetic differentiation among samples were of similar magnitude as have been reported earlier for neutral microsatellite loci between northern North Sea and Baltic Sea herring populations.

Changes in lake and stream habitats during the growth and retreat of Pleistocene glaciers repeatedly altered the spatial distributions and population sizes of the aquatic fauna of the southern Andes. Here, we use variation in mtDNA control region sequences to infer the temporal dynamics of two species of southern Andean fish during the past few million years. At least five important climate events were associated with major demographic changes: (i) the widespread glaciations of the mid-Pliocene (c. 3.5 Ma); (ii) the largest Patagonian glaciation (1.1 Ma); (iii) the coldest Pleistocene glaciation as indicated by stacked marine d18O (c. 0.7 Ma); (iv) the last southern Patagonian glaciation to reach the Atlantic coast (180 ka); and (v) the last glacial maximum (LGM, 23–25 000 years ago). The colder-water inhabitant, Galaxias platei, underwent a strong bottleneck during the LGM and its haplotype diversity coalesces c. 0.7 Ma. In contrast, the more warm-adapted and widely distributed Percichthys trucha showed continuous growth through the last two glacial cycles but went through an important bottleneck c. 180 000 years ago, at which time populations east of the Andes may have been eliminated. Haplotype diversity of the most divergent P. trucha populations, found west of the Andes, coalesces c. 3.2Ma. The demographic timelines obtained for the two species thus illustrate the continent-wide response of aquatic life in Patagonia to climate change during the Pleistocene, but also show how differing ecological traits and distributions led to distinctive responses.

Thirteen polymorphic microsatellite loci are described for the South American freshwater fish Percichthys trucha. Number of alleles per locus ranged from two to 21 and observed heterozygosities ranged from 0.304 to 0.915 in a sample of 47 individuals from four different sampling locations.

Abstract

Baltic Sea herring (Clupea harengus) have been shown to exhibit morphological differences across the marked salinity and temperature gradients in the region. Here we analyse genetic (nine microsatellite loci), morpho metric (skull shape), and meristic (pectoral fin rays and number of vertebrae) variations across seven samples of spawning herring collected from four spawning locations in the Baltic Sea to examine whether morphological variation correlates with genetic and (or) environmental factors. Results suggest that herring is adapting to its environment through a combination of selection and plastic responses. Skull shape, including and excluding size variation, differed significantly among samples, both temporally and spatially. Genetic and morphometric distances were correlated, especially when size variation was excluded from the analysis. When size variation was included, skull shape variation was more closely correlated with environmental distances among spawning locations. Vertebrate number differed among samples and was correlated with environmental distances, whereas the number of fin rays was not. Genetic and geographic distances among samples were not correlated.

Abstract

Metapopulation dynamics are increasingly invoked in management and conservation of endangered species. In this context, asymmetrical gene flow patterns can be density dependent, with migration occurring mainly from larger into smaller populations, which may depend on it for their persistence. Using genetic markers, such patterns have recently been documented for various organisms including salmonids, suggesting this may be a more general pattern. However, metapopulation theory does not restrict gene flow asymmetry to ‘source-sink’ structures, nor need these patterns be constant over longer evolutionary timescales. In anadromous salmonids, gene flow can be expected to be shaped by various selective pressures underlying homing and dispersal (‘straying’) behaviours. The relative importance of these selective forces will vary spatially and for populations of different census size. Furthermore, the consequences of life-history variation among populations for dispersal and hence gene flow remain poorly quantified. We examine population structure and connectivity in Atlantic salmon (Salmo salar L.) from Newfoundland and Labrador, a region where populations of this species are relatively pristine. Using genetic variation at 13 microsatellite loci from samples (N =1346) collected from a total of 20 rivers, we examine connectivity at several regional and temporal scales and test the hypothesis that the pre dominant direction of gene flow is from large into small populations. We reject this hypothesis and find that the directionality of migration is affected by the temporal scale over which gene flow is assessed. Whereas large populations tend to function as sources of dispersal over contemporary timescales, such patterns are often changed and even reversed over evolutionary, coalescent-derived timescales. These patterns of population structure furthermore vary between different regions and are compatible with demographic and life-history attributes. We find no evidence for sex-biased dispersal underlying gene flow asymmetry. Our findings caution against generalizations concerning the directionality of  gene flow in Atlantic salmon and emphasize the need for detailed regional study, if such information is to be meaningfully applied in conservation and management of salmonids.

Movement of the South American perch Percichthys trucha (Perciformes: Percichthyidae) was studied in a set of interconnected mountain lakes in northern Patagonia. Fish were tracked using attached acoustic tags and a fixed hydrophone array over two seasons, corresponding to the spawning (1999) and prespawning periods (2000). Percichthys trucha was found to move throughout the lake system, and to use a small, shallow, connected lake as a principal spawning area. In addition, detailed behavioural information was obtained for the spawning period. Most fish spent the initial 2 weeks descending from the littoral zone to depths of 5–8 m, presumably while feeding. Fish changed depth slowly, spending several days at each depth, and moving throughout the lake. A few days prior to the full moon, there was a change in behaviour thought to be associated with spawning, where the fish moved toward the vegetated littoral areas. Marked daily behaviour also began at this time, with movement to particular near-shore locations at dusk, and return to slightly greater depths offshore at or before dawn. Study of the behaviour of P. trucha during the breeding season has highlighted the conservation significance of the main locations within the lake system.

Abstract

Allelic variation at nine microsatellite loci and the Pan I locus provides evidence that Atlantic cod (Gadus morhua) around Iceland is genetically structured (F_ST = 0.003 and F_ST = 0.261, respectively). A total of 2534 cod were sampled at 22 spawning locations. For both types of markers, most of the significant pairwise F_ST resulted from northeastern–southwestern comparisons. A multidimensional scaling analysis based on F_ST, a spatial hierarchical analysis of molecular variance (SAMOVA) and a hierarchical analysis of molecular variance (AMOVA), conducted on both types of markers confirmed a genetic differentiation between cod from the northeastern and southwestern regions. Genetic discontinuities were revealed across two main current fronts in the southeast and northwest, where the cold and warm water masses meet. The AMOVA also detected genetic differences with depth. Expected and observed heterozygosity of microsatellite loci significantly decreased with depth, whereas the B allele frequency at the Pan I locus increased. A tagging experiment of spawning fish conducted during the same years as the genetic work revealed that tagged individuals released in the southwestern region seldom migrated to the northeastern region and vice versa, suggesting that the southwestern and northeastern populations of Atlantic cod around Iceland represent two distinct spawning components

We used molecular evidence to examine the roles that vicariance mechanisms (mountainbuilding and drainage changes during the Pleistocene) have played in producing phylogeographical structure within and among South American fish species of the temperate perch family Percichthyidae. The percichthyids include two South American genera, Percichthys and Percilia, each containing several species, all of which are endemic to southern Argentina and Chile (Patagonia). Maximum-likelihood phylogenies constructed using mitochondrial DNA (mtDNA) control region haplotypes and nuclear GnRH3-2 intron allele sequences support the current taxonomy at the genus level (both Percichthys and Percilia form strongly supported, monophyletic clades) but indicate that species-level designations need revision. Phylogeographical patterns at the mtDNA support the hypothesis that the Andes have been a major barrier to gene flow. Most species diversity occurs in watersheds to the west of the Andes, together with some ancient divergences among conspecific populations. In contrast, only one species ( Percichthys trucha ) is found east of the Andes, and little to no phylogeographical structure occurs among populations in this region. Mismatch analyses of mtDNA sequences suggest that eastern populations last went through a major bottleneck c. 188 000BP, a date consistent with the onset of the penultimate and largest Pleistocene glaciation in Patagonia. We suggest that eastern populations have undergone repeated founder-flush events as a consequence of glacial cycles, and that the shallow phylogeny is due to mixing during recolonization periods. The area of greater diversity west of the Andes lies outside the northern limit of the glaciers. mtDNA mismatch analysis of the genus Percilia which is restricted to this area suggests a long-established population at equilibrium. We conclude that patterns of genetic diversity in these South American genera have been primarily influenced by barriers to gene flow (Andean orogeny, and to a lesser extent, isolation in river drainages), and by glacial cycles, which have resulted in population contraction, re-arrangement of some watersheds, and the temporary breakdown of dispersal barriers among eastern river systems.

Small populations at the edge of a species’ distribution can represent evolutionary relics left behind after range contractions due to climate change or human exploitation. The distinctiveness and genetic diversity of a small population of bottlenose whales in the Gully, a submarine canyon off Nova Scotia, was quantified by comparison to other North Atlantic populations using 10 microsatellites and mitrochondrial DNA (mtDNA) control region sequences (434 bp). Both markers confirmed the distinctiveness of the Gully (n= 34) from the next nearest population, off Labrador ( n= 127; microsatellites –FST= 0.0243, P< 0.0001; mtDNA – ÖST= 0.0456, P < 0.05). Maximum likelihood microsatellite estimates suggest that less than two individuals per generation move between these areas, refuting the hypothesis of population links through seasonal migration. Both males and females appear to be philopatric, based on significant differentiation at both genomes and similar levels of structuring among the sexes for microsatellites. mtDNA diversity was very low in all populations ( h= 0.51, ð= 0.14%), a pattern which may be due to selective sweeps associated with this species’ extreme deep-diving ecology. Whaling had a substantial impact on bottlenose whale abundance, with over 65 000 animals killed before the hunt ceased in the early 1970s. Genetic diversity was similar among all populations, however, and no signal for bottlenecks was detected, suggesting that the Gully is not a relic of a historically wider distribution. Instead, this unique ecosystem appears to have long provided a stable year-round habitat for a distinct population of bottlenose whales.

Eight tetranucleotide microsatellite loci were characterized for white hake, Urophycis tenuis, a commercially exploited demersal gadoid fish distributed in the Northwest Atlantic. The eight loci are polymorphic with number of alleles and observed heterozygosities ranging from seven to 40 and from 0.544 to 0.978, respectively, in samples of between 39 and 47 individuals collected from throughout the species range. These loci were developed for population genetic studies.

Information on statistical power is critical when planning investigations and evaluating empirical data, but actual power estimates are rarely presented in population genetic studies. We used computer simulations to assess and evaluate power when testing for genetic differentiation at multiple loci through combining test statistics or P values obtained by four different statistical approaches, viz. Pearson’s chi-square, the log-likelihood ratio G -

test, Fisher’s exact test, and an FST -based permutation test. Factors considered in the comparisons include the number of samples, their size, and the number and type of genetic marker loci. It is shown that power for detecting divergence may be substantial for frequently used sample sizes and sets of markers, also at quite low levels of differentiation. The choice of statistical method may be critical, though. For multi-allelic loci such as microsatellites, combining exact P values using Fisher’s method is robust and generally provides a high resolving power. In contrast, for few-allele loci (e.g. allozymes and single nucleotide polymorphisms) and when making pairwise sample comparisons, this approach may yield a remarkably low power. In such situations chi-square typically represents a better alternative. The G -test without Williams’s correction frequently tends to provide an unduly high proportion of false significances, and results from this test should be interpreted with great care. Our results are not confined to population genetic analyses but applicable to contingency testing in general.

The existence of biologically differentiated populations has been credited with a major role in conferring sustainability and in buffering overall productivity of anadromous fish population complexes where evidence for spatial structure is uncontroversial. Here, we describe evidence of correlated genetic and life history (spawning season linked to spawning location) differentiation in an abundant and highly migratory pelagic fish, Atlantic herring, Clupea harengus, in the North Sea (NS) and adjacent areas. The existence of genetically and phenotypically diverse stocks in this region despite intense seasonal mixing strongly implicates natal homing in this species. Based on information from genetic markers and otolith morphology, we estimate the proportional contribution by NS, Skagerrak (SKG) and Kattegat and western Baltic (WBS) fish to mixed aggregations targeted by the NS fishery. We use these estimates to identify spatial and temporal differences in life history (migratory behaviour) and habitat use among genetically differentiated migratory populations that mix seasonally. Our study suggests the existence of more complex patterns of intraspecific diversity than was previously recognized. Sustainability may be compromised if such complex patterns are reduced through generalized management (e.g. area closures) that overlooks population differences in spatial use throughout the life cycle.

The marine environment is characterized by few physical barriers, and pelagic fishes commonly show high migratory potential and low, albeit in some cases statistically significant, levels of genetic divergence in neutral genetic marker analyses. However, it is not clear whether low levels of differentiation reflect spatially separated populations experiencing gene flow or shallow population histories coupled with limited random genetic drift in large, demographically isolated populations undergoing independent evolutionary processes. Using information for nine microsatellite loci in a total of 1951 fish, we analyzed genetic differentiation among Atlantic herring from eleven spawning locations distributed along a longitudinal gradient from the North Sea to the Western Baltic. Overall genetic differentiation was low (u 5 0.008) but statistically significant. The area is characterized by a dramatic shift in hydrography from the highly saline and temperature stable North Sea to the brackish Baltic Sea, where temperatures show high annual variation. We used two different methods, a novel computational geometric approach and partial Mantel correlation analysis coupled with detailed environmental information from spawning locations to show that patterns of reproductive isolation covaried with salinity differences among spawning locations, independent of their geographical distance. We show that reproductive isolation can be maintained in marine fish populations exhibiting substantial mixing during larval and adult life stages. Analyses incorporating genetic, spatial, and environmental parameters indicated that isolating mechanisms are associated with the specific salinity conditions on spawning locations.

Abstract

The Atlantic herring Clupea harengus has played a pivotal role in the formulation of ideas relating to population structuring in marine fishes, yet considerable uncertainty remains as to the extent to which phenotypic and genetic differentiation coincide in such a highly mobile species. In this study, we examined genetic population structure across the major herring spawning aggregations In the North Sea and adjacent waters over 2 years, 2002 and 2003. We analysed 1660 spawning individuals across 9 microsatellite loci. Data were analysed using several approaches, taking into account the effect of location, year-class and sex, as well as pooling all individuals together, making no assumption as to the number of populations present in the data set. The results suggest the presence of a genetically homogeneous unit off Northern Scotland, and a temporally stable pattern of isolation by distance determined predominantly by the divergence of the English Channel samples and,

in 2003, by the Norwegian spring spawners. Our data suggest that the current view of North Sea herring as a unit-stock might be adequate, but confirm the considerable degree of demographic independence of the herring populations in the English Channel. Despite major recent population collapses, genetic data indicated no evidence of bottlenecks affecting the genetic diversity of extant North Sea herring populations. Finally, despite evidence of weak population structuring, we discuss the risks of underestimating population differentiation in marine fish of large population sizes, and with reference to herring population history and dynamics, we attempt to reconcile the existing theories on herring population structure.

Abstract

Numerically small but statistically significant genetic differentiation has been found in many marine fish species despite very large census population sizes and absence of obvious barriers to migrating individuals. Analyses of morphological traits have previously identified local spawning groups of herring (Clupea harengus L.) in the environmentally heterogeneous Baltic Sea, whereas allozyme markers have not revealed differentiation. We analysed variation at nine microsatellite loci in 24 samples of spring-spawning herring collected at 11 spawning locations throughout the Baltic Sea. Significant temporal differentiation was observed at two locations, which we ascribe to sympatrically spawning butgenetically divergent ‘spawning waves’. Significant differentiation was also present on a geographical scale, though pairwise FST values were generally low, not exceeding 0.027. Partial Mantel tests showed no isolation by geographical distance, but significant associations were observed between genetic differentiation and environmental parameters (salinity and surface temperature) (0.001 <P=0.099), though these outcomes were driven mainly by populationsin the southwestern Baltic Sea, which also exhibits the steepest environmental gradients. Application of a novel method for detecting barriers to gene flow by combining geographical coordinates and genetic differentiation allowed us to identify two zones of lowered gene flow. These zones were concordant with the separation of the Baltic Sea into major basins, with environmental gradients and with differences in migration behaviour. We suggest that similar use of landscape genetics approaches may increase the understanding of the biological significance of genetic differentiation in other marine fishes.

Abstract

We examined the morphological variability (23 morphometric traits) among individual Galaxias platei (N = 380) collected from 20 postglacial lakes in the southern Andes. The lakes were chosen to cover the latitudinal range of the species in Patagonia. Diet examined for a subset of these fish ( N = 261) collected during the summer months (January through March), differed among river basins but there were no clear latitudinal trends in the consumption of any prey type. Diet may, however, have partially shaped morphology, as pelvic measures were negatively correlated with consumption of amphipods. Substantial differentiation among populations was observed, primarily in the shape and dimensions of the head, in caudal morphology and in fin length. Our results indicate that the morphology of G. platei varies with latitude, and may be related to risk of predation and diet.

Abstract

We examined polymorphism at seven microsatellite loci among sea trout (Salmo trutta) (n = 846) collected from three areas in the Limfjord (Denmark). We then assessed their potential population source by comparing, using a mixed stock analysis (MSA) Bayesian framework, their genetic composition to that of brown trout collected from 32 tributaries pooled into nine geographical regions (n = 3801) and two hatcheries (n = 222) used for stocking. For each of the three regional sea trout groups (western, central and eastern Limfjord, n = 91, n = 426, n = 329, respectively), MSA was conducted with (i) all individuals in the group, (ii) with the subset of spawning sea trout only and (iii) with the subset of foraging, nonspawning individuals only, a subset that consisted primarily of sea trout caught during their first year at sea. For all three regional sea trout groups, a higher proportion of individuals (regardless of whether they were foraging or spawning) appear to have originated from the rivers that drain locally, than from the rivers that drain in other parts of the Limfjord. This suggests (1) that sea trout, at least during their first year at sea, undertake limited migrations within the Limfjord system and (2) that sea trout on their spawning run were caught close to their natal rivers. The proportion of sea trout of hatchery origin varied widely among all three Limfjord areas and broadly reflected regional stocking histories, with high proportions of sea trout of domestic origin in the east (39.3%), where stocking with domestic trout was practised intensely at the time of sampling, and in the west (57.2%), where a programme of coastal stocking of post smolts took place over several years in the early 1990s. In contrast, in the central Limfjord, where stocking with domestic trout was largely abandoned in the early 1990s, the proportion of sea trout of domestic origin was only 8.5%. Interestingly, for all three regional sea trout groups, virtually no sea trout of hatchery origin were found among the spawning individuals, which were on average larger than the nonspawning sea trout. These results suggest that stocked domestic brown trout that become anadromous experience high mortality at sea and are therefore largely absent among the larger, spawning individuals. We conclude that sea trout of domestic origin exhibit much reduced ability to reproduce and are unlikely to contribute significantly to the local gene pool largely because of a relatively high mortality at sea before the onset of maturity.

Abstract
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.

Abstract

Variation in fatty acid (FA) composition of blubber collected in 1998 from 170 common minke whales (Balaenoptera acutorostrata) was used to study population structure in the North Atlantic. Samples from seven IWC management units were analysed: West Greenland (‘WG’, n = 69); East Greenland (‘CG’, n = 3); Jan Mayen (‘CM’, n = 24); Svalbard (‘ES’, n = 16); the Barents Sea (‘EB’, n = 30); Vestfjorden/Lofoten (‘EC’, n = 7); and the North Sea (‘EN’, n = 21). FA analyses were conducted on both deep and superficial blubber with a one-step extraction and esterification method followed by gas-chromatography. The 43 FAs identified comprised 93-99% of total FAs. CART and MANOVA analyses on FA signatures in both blubber sections suggested a ‘3-geographic Regions model’ where the regions were Greenland (WG, CG), the Northeast Atlantic (CM, ES, EB, EC) and the North Sea (EN). This is in general agreement with a genetic study on the same samples and suggests that differences in FA signatures can be used for studying population structure in minke whales. Potential variation in FA signatures caused by internal and environmental factors needs to be better understood. It is recommended that future studies of blubber FA signatures in minke whales include samples from their entire North Atlantic range (including Canadian and Icelandic waters). Samples should be collected from a pre-specified body site to rule out possible internal variation and during a narrow time-window in the same year to rule out seasonal exchange between areas.

Abstract
Within-species differentiation in phenotypic characters related to resource use (resource polymorphism) is frequently thought to result from divergent natural selection in a heterogeneous environment with 'open niches'. In this study we found consistent resource-based polymorphism within three different populations of Percichthys trucha , a lake-dwelling fish native to the southern Andes. In each of three lakes we found two morphotypes that could be clearly distinguished by differences in gill raker length. However, the magnitude of the polymorphism, and the suite of phenotypic characteristics associated with the polymorphism, differed between lakes. Patterns of divergence were more similar between the two northern lakes which ultimately drain into a common river, than between these two lakes and a more southern, unconnected lake. The southern population, which had the largest divergence in gill raker length (32% vs. 16% and 19%), also showed substantial differences in diet. Evidence from the southern population suggests that polymorphism in P. trucha is present early during ontogeny. We conclude that while there are some strong parallels among lakes in the development of a trophic polymorphism, differences in environmental ­conditions and/or colonization history have led to substantial differences in the evolutionary history, resulting in ­different ecological roles of common morphotypes within different lakes.

Abstract
We examined the long-term temporal (1910s to 1990s) genetic variation at eight microsatellite DNA loci in brown trout (Salmo trutta L) collected from five anadromous populations in Denmark to assess the long-term stability of genetic composition and to estimate effective population sizes (N_e). Contemporary and historical samples consisted of tissue and archived scales, respectively. Pairwise θ_ST estimates, a hierarchical analysis of molecular variance () and multidimensional scaling analysis of pairwise genetic distances between samples revealed much closer genetic relationships among temporal samples from the same populations than among samples from different populations. Estimates of N_e, using a likelihood-based implementation of the temporal method, revealed N_e ≥ 500 in two of three populations for which we have historical data. A third population in a small (3 km) river showed N_e ≥ 300. Assuming a stepping-stone model of gene flow we considered the relative roles of gene flow, random genetic drift and selection to assess the possibilities for local adaptation. The requirements for local adaptation were fulfilled, but only adaptations resulting from strong selection were expected to occur at the level of individual populations. Adaptations resulting from weak selection were more likely to occur on a regional basis, i.e. encompassing several populations. N_e appears to have declined recently in at least one of the studied populations, and the documented recent declines of many other anadromous brown trout populations may affect the persistence of local adaptation.

Abstract
Among 158 Galaxias platei individuals, collected from nine deep Andean lakes and from one shallow lake on the Patagonian steppe, morphological characters associated with caudal fin length were negatively correlated with predation intensity as measured by the incidence of piscivory.  Fish from lakes with high rates of piscivory had relatively short caudal fins and those from lakes with low piscivory had long caudal fins.  It is hypothesized that in this species caudal fin length affects swimming ability and thus, ability to escape from predators.  The population of G. platei from Lake Espejo with an intermediate level of piscivory contained individuals with both long and short caudal fins.  Morphological variation within this lake was related to the exploitation of food resources and habitat; individuals with shorter tails ate more amphipods and inhabited shallower habitats.  The results indicated that while the among-lakes morphological variation in G. platei was influenced largely by differences in predation intensity, within-lake morphological variation, where predation was intermediate, appeared to be affected by both predation and differences in the exploitation of food resources.

Abstract
We examined polymorphism at 12 microsatellite loci in 807 harbour porpoises , Phocoena phocoena, collected from throughout the central and eastern North Atlantic to the Baltic Sea. Multilocus tests for allele frequency differences, assignment tests, population structure estimates (F_ST) and genetic distance measures (D_LR and D_C) all indicate six genetically differentiated populations/sub-populations after pooling sub-samples within regions. Harbour porpoises from West Greenland, the Norwegian Westcoast, Ireland, the British North Sea, the Danish North Sea and the inland waters of Denmark (IDW) are all genetically distinguishable from each other. A sample of harbour porpoises collected off the Dutch coast (mainly during winter) was genetically heterogeneous and likely comprised a mixture of individuals of diverse origin. A mixed stock analysis indicated that most of the individuals in this sample (~77%) were likely migrants from the British and Danish North Sea.

Abstract
We report on evidence of long term stability in the geographic pattern of genetic differentiation among cod (Gadus morhua) collected from 5 spawning banks off Newfoundland and Labrador over a period spanning three decades (1964-1994) and 2 orders of magnitude of population size variation. Six microsatellite DNA loci amplified from archived otoliths (1964 and 1978) and contemporary (1990s) tissue samples revealed fidelity to natal spawning banks over this period. A two level (spawning bank and decade) hierarchical and multilocus AMOVA indicated that 1.55% of the total variation in allele frequencies could be attributed (P = 0.036) to spatial structure while no variance component could be attributed to temporal changes. A finer scale analysis among cod from just 3 of these spawning banks reveals, however, evidence consistent with some post-collapse mixing between cod from two banks. In the context of fisheries management and conservation, the survival of the spatial pattern of genetic differentiation during the population collapse suggests that if recovery eventually occurs it will likely be through population re-growth in situ rather than by migratory influx.

Abstract
We examined polymorphism at seven microsatellite loci in 4023 brown trout (Salmo trutta) collected from 32 tributaries to the Limfjord, Denmark (~200 km) and from two hatcheries used for stocking. Populations differ in their estimated sizes and stocking histories. Mean individual inbreeding coefficients do not differ among locations within rivers. Relatedness varies between sites within rivers indicating varied local dynamics at a very small geographical scale. Relatedness is sometimes lower than expected among an equal number of simulated individuals with randomized genotypes, suggesting structure within locations. Five per cent of the genetic variance is distributed among rivers (F_ST = 0.049), but in the western, less heavily stocked, area of the Limfjord a higher proportion of the genetic variance is distributed among rivers than among locations within rivers. The reverse is true of the eastern, more heavily stocked, area of the Limfjord. Here, a higher proportion of the genetic variance is distributed among locations within rivers than among rivers. Assignment tests reveal that the majority of trout (mean 77% of all fish) are more probably of local origin than hatchery origin but this proportion varies regionally, with rivers in the western area of the Limfjord showing a relatively high (mean 88%) and those in the eastern area showing a relatively low (mean 72%) proportion of locally assigned trout. These results can be interpreted as reflecting stocking impact. Also, the proportion of locally assigned trout correlates with the populations' stocking histories, with rivers presently subjected to stocking (hatchery trout) showing low (mean ~0.73), and rivers where stocking was discontinued showing high (mean ~0.84) proportions of local fish, probably reflecting lower survival of hatchery than of wild trout. There is evidence for isolation by distance at a large geographical scale when individual river populations are pooled into nine geographical regions but not at a small geographical scale when populations are considered individually. We reject the null hypothesis that stocking has had no impact on population structure but the relatively high proportion of locally assigned trout in populations where stocking with domestic fish no longer takes place suggests limited long-term success of stocking.

Abstract
The genetic integrity of many salmonid fish populations is threatened by stocking of domesticated conspecifics. The purpose of this study was to assess the utility of microsatellite DNA markers for detecting loss of genetic diversity in hatchery strains, for estimating their genetic relationships, and for monitoring the genetic impact of stocking activity on wild populations of salmonid fishes. Brown trout from ten hatchery strains, one supportive breeding “strain,” and five wild populations were screened for variation at eight loci. In most hatchery strains, genetic variation was comparable to that of wild populations, but three strains showed loss of allelic variation. In six of the hatchery strains, significant differentiation was observed between age classes. Genetic differentiation among all populations was moderate (F_ST = 0.065, ρ_ST = 0.076), and only a minor part of genetic diversity was distributed between the wild and hatchery populations. We assessed whether wild populations were introgressed by stocked hatchery trout by performing assignment tests to determine population of origin and estimating maximum potential introgression rates. The results suggested that genetic introgression by hatchery trout had occurred for only two of the five populations potentially influenced by stocking. In one of these two rivers, microsatellite data obtained from a limited number of old scale samples indicated that individuals from the original population were genetically divergent from those of the present population, suggesting that extinction of the original population had taken place. The study demonstrates that microsatellite analysis provides a useful tool for distinguishing heavily introgressed populations from those unaffected by stocking. The information can be used to assist in (1) prioritizing populations for conservation and (2) the choice of wild populations to be used as sources for the reintroduction of salmonids in areas where local, indigenous gene pools have been extirpated.

Abstract
Stocking with offspring of local wild fish, so-called supportive breeding, is often advocated as an alternative to stocking domesticated fish. However, it is important to ensure that supportive breeding does not result in inbreeding and loss of genetic variability. We analysed eight microsatellite loci in samples of wild and hatchery-reared brown trout (Salmo trutta) from three populations subject to supportive breeding. For calibrating statistical procedures, we included two test samples of reared offspring for which the precise number of parent fish was known and a sample from a further wild reference population. Three different statistical procedures were used to detect population bottlenecks and loss of variability: (i) a randomization test for comparing allelic diversity between samples; (ii) estimates of effective number of breeders from gametic-phase disequilibrium; and (iii) a test for assessing population bottlenecks based on detecting deviations from mutation-drift equilibrium. All three procedures were useful but they also exhibited different strengths and limitations, with the test for population bottlenecks probably being the single most useful procedure for routine monitoring. In two populations subject to supportive breeding, there were strong indications of reduced effective population sizes, and significant genetic differentiation was observed between different samples from the same population.

Abstract
The collapse of various stock complexes of cod (Gadus morhua) in the northwest Atlantic has prompted a clarification of relationships among stock components. Here we examine the genetic composition of >2300 cod collected during 1994–1997 in the Gulf of St. Lawrence and its approaches to determine whether: (1) stock components can be genetically identified; (2) population structure is temporally stable; (3) components are always separated and, if not, where and when are they mixed; and (4) component contributions to mixtures can be estimated. We use polymorphism at six microsatellite DNA loci from cod collected on or near their spring and summer spawning grounds to examine structure and then employ maximum likelihood analyses to estimate contributions of each component to mixtures overwintering near the entrance to the Gulf. Estimates of genetic structure (F_ST and R_ST) reveal significant differences among cod populations during stock-separated periods, and the structure appears to be temporally stable. Multidimensional scaling analysis of estimates of genetic distance (D_A) suggest that the structure results from differences among cod collected within the Gulf of St. Lawrence and those collected near the entrance to the Gulf on either side of the Laurentian Channel in the Cabot Strait, as well as among cod collected south of Newfoundland along the north side of the Channel. Weak genetic heterogeneity among seven regional mixed-stock collections during the overwintering period suggests that cod aggregations characteristically found in the overwintering region represent population mixtures that differ in the proportion of cod contributed to them by the various stock components. Maximum likelihood estimates indicate no significant temporal changes in component contributions to the mixed-stock samples between 1996 and 1997 when all of the winter mixed-stock samples were pooled. The combined contribution of cod from the southern and northern Gulf of St. Lawrence to the mixed-stock samples ranged between 46% and 71% (expected 64%). More precise estimates of contributions from these two regions are precluded by the weak genetic differentiation detected in our samples. The contribution by cod from the Cape Breton Island region was small and estimated at 3%. Contributions by cod from the eastern Scotian Shelf, southwest Newfoundland and south-central Newfoundland were in the range of 13–14%, 4%, and 8%, respectively. Contributions by inshore cod from Placentia and Fortune Bays in south Newfoundland were small to negligible (~3% each). The results indicate that future management could be designed around the spatial and temporal scale of the stock structure identified during the stock-separated period and around the spatially varying contributions to the overwintering mixed-stock fishery.

Abstract
The effects of stocking hatchery trout into wild populations were studied in a Danish river, using microsatellite and mitochondrial DNA (mtDNA) markers. Baseline samples were taken from hatchery trout and wild trout assumed to be unaffected by previous stocking. Also, samples were taken from resident and sea trout from a stocked section of the river. Genetic differentiation between the hatchery strain and the local wild population was modest (microsatellite F_ST = 0.06). Using assignment tests, more than 90% of individuals from the baseline samples were classified correctly. Assignment tests involving samples from the stocked river section suggested that the contribution by hatchery trout was low among sea trout (< 7%), but high (46%) among resident trout. Hybrid index analysis and a high percentage of mtDNA haplotypes specific to indigenous trout observed among resident trout that were assigned to the hatchery strain suggested that interbreeding took place between hatchery and wild trout. The latter result also indicated that male hatchery trout contributed more to interbreeding than females. We suggest that stronger selection acts against stocked hatchery trout that become anadromous compared to hatchery trout that become resident. As most resident trout are males this could also explain why gene flow from hatchery to wild trout appeared to be male biased. The results show that even despite modest differentiation at neutral loci domesticated trout may still perform worse than local populations and it is important to be aware of differential survival and reproductive success both between life-history types and between sexes.

Abstract
Divergent natural selection is often believed to be the driving force behind phenotypic differentiation in characters related to resource acquisition, leading to trophic polymorphism in fishes. Here we use variation in the fatty acid composition of adipose and muscle tissues to look at differences in resource use by two recently described sympatric morphs of Percichthys trucha, a common freshwater fish of the Andean and Patagonian regions of South America. Because dietary fatty acids are often stored in carnivorous animals with little modification after consumption, they can be used to infer information about dietary habits of individuals. We found that the two morphs differed in the overall composition of fatty acids in both adipose and muscle tissue, but that there were some differences in how the morphs differed in lakes from the northern vs southern part of the range. Furthermore, we found that certain fatty acids were correlated with diet as determined by gut content analysis. Consumption of anisopteran larvae was highly correlated with 14:0 in adipose and muscle tissue; and higher levels of longer chain unsaturated fatty acids (i.e. 20 and 22 carbons) were correlated with the presence of fish and also amphipods in the diets. Taken together, the results suggest that there are marked differences in the foraging ecology of the two morphs of P. trucha inhabiting southern Andean lakes.

Abstract
Polymorphisms at five microsatellite DNA loci provide evidence that Atlantic cod Gadus morhua inhabiting Gilbert Bay, Labrador are genetically distinguishable from offshore cod on the north-east Newfoundland shelf and from inshore cod in Trinity Bay, Newfoundland. Antifreeze activity in the blood suggests that Gilbert Bay cod overwinter within the Bay. Gilbert Bay cod are also smaller (weight and length) for their age and consequently less fecund for their age, than cod elsewhere within the northern cod complex. The productivity and recruitment potential of coastal cod off Labrador may thus be much lower than that of offshore northern cod or of inshore cod farther south, implying that a more conservative management strategy may be required for cod from coastal Labrador than traditionally practised for northern cod inhabiting less harsh environments. Relatively high F_STand R_STmeasures of population structure suggest that important barriers to gene flow exist among five components that include two inshore (Gilbert and Trinity Bay) and three offshore cod aggregations on the north-east Newfoundland Shelf and the Grand Bank. D_AandD_SW estimates of genetic distance that involve Gilbert Bay cod are approximately three- and 10-fold larger, respectively, than estimates not involving Gilbert Bay cod. The differences between inshore cod from Gilbert Bay and Trinity Bay raise the possibility that other genetically distinguishable coastal populations may exist, or may have existed prior to the northern cod fishery collapse. Harvesting strategies for northern cod should recognize the existence of genetic diversity between inshore and offshore components as well as among coastal components.

Abstract
We review our recent studies on the genetic structure of Atlantic cod (Gadus morhua) populations in the NW Atlantic. Our conclusions are based on knowledge of polymorphism at microsatellite DNA loci combined with known aspects of cod biology and ecology and with known oceanographic features in the NW Atlantic. Three case studies illustrate genetic heterogeneity between cod populations at the meso- and large-scales of coastal embayments and offshore banks and at the small-scale of oceanographic features. Our results generally highlight the importance of combining genetic with physiological, ecological, and oceanographic information, when assessing the genetic structure of highly abundant, widely distributed, and high gene-flow marine fish species. We highlight the role that oceanographic features (e.g., gyre-like systems) and known spatio-temporal differences in spawning time may play as barriers to gene-flow between and among neighboring and often contiguous cod populations in the NW Atlantic. We suggest that bathymetric and hydrodynamic/oceanographic structure represents a rational starting point for developing hypotheses aimed at assessing the genetic structure of high gene-flow marine fish species.

Abstract
Divergent natural selection affecting specific trait combinations that lead to greater efficiency in resource exploitation is believed to be a major mechanism leading to trophic polymorphism and adaptive radiation. We present evidence of trophic polymorphism involving two benthic morphs within Percichthys trucha, a fish endemic to temperate South America. In a series of lakes located in the southern Andes, we found two morphs of P. trucha that could be distinguished on the basis of gill raker length and five other morphological measures, most of which are likely associated with the use of food resources. The differences were consistent across all lakes examined, and were correlated with habitat use and diet. Individuals with longer gill rakers were more abundant in the littoral zone (littoral morph) while the short gill-raker morph was more abundant at 10 m depth and deeper (deep benthic morph). Both morphs fed primarily on benthic invertebrates, but the littoral morph fed more on larval Anisoptera than did the deep benthic morph. Phenotypic correlations among traits were high for the littoral morph, but low and non-significant for the deep-benthic morph. We suggest that gill raker length may influence the relative efficiency of suction feeding for the two morphs. This is the first evidence of trophic polymorphism in fishes from temperate South America.

Abstract
Variation at five microsatellite DNA loci scored in ~1300 individuals provided evidence of genetic structure among 14 cod (Gadus morhua) populations spanning the range of the species in the northwest (NW) Atlantic. Using D_A and D_SW measures of genetic distance, as well as F_ST and R_ST measures of population structure, differences are revealed among populations at continental shelf scales (NE Newfoundland Shelf, Grand Banks, Flemish Cap, Scotian Shelf, Georges Bank) where regions are separated by submarine saddles, channels and trenches. However, we also provide evidence of genetic structure at spawning-bank scales consistent with variation in oceanographic features and in the spatiotemporal distribution of spawning, each of which may represent barriers to gene flow among geographically contiguous populations inhabiting a highly advective environment. The differences described are consistent with postdispersal spawning fidelity to natal areas, a behaviour that may be facilitated by topographically induced gyre-like circulations that can act as retention mechanisms. Significant degrees of substructure among neighbouring and contiguous cod populations may be most easily explained by the associated oceanographic features and processes that conceivably form the template for the evolution of the structure. We suggest that bathymetric and hydrodynamic structure represents a rational starting point for developing hypotheses to examine the processes that lead to the genetic structuring of marine fish species.

Abstract
Because of their rapid mutation rate and resulting large number of alleles, microsatellite DNA are well suited to examine the genetic or demographic structure of fish populations. However, the large number of alleles imply that large sample sizes are required for accurate reflection of genotypic frequencies. Estimates of genetic distance are often biased at small sample sizes, and biases and sampling variances can be affected by the number of, and distances between, alleles. Using data from a large collection of larval cod (Gadus morhua) from a single area, I examined the effect of sample size on seven genetic distance and two structure metrics. Pairs of samples (equal or unequal) of various sizes were drawn at random from a pool of 856 individuals scored for six microsatellite loci. (δμ)², D_SW, R_ST, and F_ST were the best performers in terms of bias and variance. Sample sizes of 50 <= N <= 100 individuals were generally necessary for precise estimation of genetic distances and this value depended on number of loci, number of alleles, and range in allele size. (δμ)² and D_SW were biased at small sample sizes.

Abstract
The genetic difference between inshore overwintering Atlantic cod (Gadus morhua) from Trinity Bay, Newfoundland, and offshore overwintering cod from the Grand Bank region (Ruzzante et al. 1996; Can. J. Fish. Aquat. Sci. 53: 634-645) has remained stable during 1992-1995. Cod collected inshore during 1995 in Trinity Bay (N = 150) were again genetically distinguishable (using (δμ)² and D_SW) from offshore cod (N = 140) collected between 1992 and 1994 and were genetically indistinguishable from inshore cod (N = 123) also collected between 1992 and 1994. Farm-held cod (N = 30), captured inshore in 1992 and pen reared until 1995 were most likely to have been drawn from an inshore overwintering population; they were genetically different from offshore cod and were indistinguishable from wild inshore cod showing high antifreeze activity and from cod collected in relatively cold (~0.2°C) water in December 1995. The farm-held cod were genetically different from cod collected in relatively warm (~3.3°C) water but were indistinguishable from cod with low antifreeze activity. Despite evidence of weak genetic heterogeneity within the pool of wild cod collected inshore in 1995, which was not associated with antifreeze activity nor with water temperature, the magnitude of the genetic differences between inshore and offshore cod has remained unchanged during the period 1992-1995.

Abstract
Allele frequency data from eight microsatellite loci provide evidence of highly significant genetic differentiation among stocks of Atlantic salmon Salmo salar L. from the Bay of Fundy, eastern and north-western Nova Scotia and Newfoundland. Estimates of genetic structure (R_ST and Θ) were significant both among all samples taken from the different geographical locations and among samples from geographical regions for which more than one stock was sampled. Samples from the Bay of Fundy taken from stocks which are phenotypically and behaviourally diverse showed particularly high levels of genetic structure. Rogers', allele sharing and (δμ)² distances also revealed significant differences among stock samples and were significantly correlated [Rogers' and (δμ)²] with sea distance between rivers. Results suggest that stocks of Atlantic salmon in eastern Canada are highly diverse genetically and that this should be an important consideration in any management programme for stocks in the area.

Abstract
Polymorphism at six microsatellite DNA loci among cod larvae sampled repeatedly over a 3-week period from an aggregation on Western Bank of the Scotian Shelf provided evidence of several heterogeneous groups within the aggregation. There was strong evidence of heterozygote deficiency and departure from Hardy-Weinberg expectations for the larval aggregation as a whole (N = 1337) and for all larvae sampled within a single water mass (CW larvae), but not for a subset of these larvae considered to be part of a single cohort on the basis of age at length. These results suggest that both the entire aggregation and the CW subset originated from several distinct spawning events involving spawners with heterogeneous allelic compositions, but that the larvae forming the cohort originated from a single spawning event. Our results establish a link between the ecological match-mismatch hypothesis and the genetic "sweepstakes" selection hypothesis. There was no evidence that the larvae originated from different populations as measured by (δμ)² distance, R_ST and F_ST estimates among subsets. Additional analyses showed the larval cohort to have greater genetic similarity to adult cod sampled on Western Bank 2 years later than to adult cod sampled on Banquereau Bank (~150 km away) also 2 years later. These results suggest that the genetic composition of cod on Western Bank remains stable over time.

Abstract
Allelic variation in six highly polymorphic microsatellite loci (mean heterozygosity, 86%) provided evidence that cod (Gadus morhua) in the northwest Atlantic belong to multiple genetically distinguishable populations, and further that genetic differences may also exist between northwestern and southeastern cod aggregations within the northern cod stock complex off Newfoundland. Cod were sampled from winter aggregations ranging from Hamilton Bank to the northern Grand Bank in the northern cod complex, and from the Flemish Cap, the Scotian Shelf, and the Barents Sea. Tests of allele frequency homogeneity (χ²), F_ST , and (δμ)², allele sharing, and Rogers' distance measures revealed significant differences among northern cod, Flemish Cap, Scotian Shelf, and Barents Sea samples. Within the northern cod complex, two pooled samples, NORTH (Hamilton, Funk, and Belle Isle banks) and SOUTH (northern Grand Bank area), were distinguishable using χ², (δμ)², and allele sharing measures. Both (δμ)² and Rogers' distances clustered western Atlantic samples in two groups distinct from the divergent Barents Sea sample; one comprised NORTH, SOUTH, and Scotian Shelf, and the other, Flemish Cap.

Abstract
Microsatellite DNA provided evidence that Atlantic cod overwintering in inshore Newfoundland are genetically distinguishable from cod overwintering offshore. We compared variation in five loci in samples from inshore locations around Trinity Bay, Newfoundland, and from an offshore region on the northern Grand Bank (North Cape). Cod collected inshore were divided into two groups on the basis of antifreeze level in blood: those with high antifreeze levels presumed to have overwintered in cold (<0°C) inshore waters and those with low antifreeze levels presumed to have overwintered offshore in warmer (>2°C) waters. Fish overwintering inshore differed from offshore fish in allele sharing and in (δμ)² distances, while fish with low antifreeze levels did not. Subpopulation structure (R_ST) was detected when offshore cod were compared with inshore cod with high levels of antifreeze but not when compared with those with low levels of antifreeze. These results suggest that cod overwintering inshore are genetically distinct from offshore cod. Inshore and offshore cod from the areas studied remain genetically distinct despite the fact that individuals from the two populations intermingle inshore during the summer and fall feeding migration. Thus, we found evidence of population structure at a finer geographical scale than has been shown to date for this species.

Abstract
Behavioral and evolutionary ecologists often attempt to quantify monopolization of mates or food items using indices such as the variance, the coefficient of variation (CV), the coefficient of dispersion (CD), or the opportunity for selection (I). Because of the tendency for the variance to increase with the mean and because of the relationship between variance and the number of competitors, such indices are of limited value for comparisons between systems that differ in the mean number of resources per competitor or in the number of competitors. Here we examine an alternative index of relative monopolization, Q, in which the observed variance in resource distribution in expressed as a fraction of the maximum possible variance (assuming discrete resouce items) for a given resource abundance, both corrected by an estimate of the variance expected under a random distribution of resources. Q = 1 when the variance in resource distribution is maximum, and Q = 0 when resources are randomly distributed. We demonstrate analytically that (1) on average, Q is independent of mean resource abundance for overdispersed systems and (2) Q can be used to compare systems with different numbers of competitors as long as the total number of resource units is not larger than the number that can be monopolized by a single individual. We illustrate the advantages of this method using data from studies on feeding competition in fish and on mating competition in crickets.