DNA from archived otoliths was used to explore the temporal stability of the genetic composition of two cod populations, the Moray Firth (North Sea) sampled in 1965 and 2002, and the Bornholm Basin (Baltic Sea) sampled in 1928 and 1997. We found no significant changes in the allele frequencies for the Moray Firth population, while subtle but significant genetic changes over time were detected for the Bornholm Basin population. Estimates of the effective population size (Ne) generally exceeded 500 for both populations when employing a number of varieties of the temporal genetic method. However, confidence intervals were very wide and Ne's most likely range in the thousands. There was no apparent loss of genetic variability and no evidence of a genetic bottleneck for either of the populations. Calculations of the expected levels of genetic variability under different scenarios of Ne showed that the number of alleles commonly reported at microsatellite loci in Atlantic cod is best explained by Ne's exceeding thousand. Recent fishery-induced bottlenecks can, however, not be ruled out as an explanation for the apparent discrepancy between high levels of variability and recently reported estimates of Ne << 1000. From life history traits and estimates of survival rates in the wild, we evaluate the compatibility of the species' biology and extremely low Ne/N ratios. Our data suggest that very small Ne's are not likely to be of general concern for cod populations and, accordingly, most populations do not face any severe threat of losing evolutionary potential due to genetic drift.

A number of evolutionary mechanisms have been suggested for generating low but significant genetic structuring among marine fish populations. We used nine microsatellite loci and recently developed methods in landscape genetics and coalescence-based estimation of historical gene flow and effective population sizes to assess temporal and spatial dynamics of the population structure in European flounder (Platichthys flesus L.). We collected 1062 flounders from 13 localities in the northeast Atlantic and Baltic Seas and found temporally stable and highly significant genetic differentiation among samples covering a large part of the species' range (global F(ST) = 0.024, P < 0.0001). In addition to historical processes, a number of contemporary acting evolutionary mechanisms were associated with genetic structuring. Physical forces, such as oceanographic and bathymetric barriers, were most likely related with the extreme isolation of the island population at the Faroe Islands. A sharp genetic break was associated with a change in life history from pelagic to benthic spawners in the Baltic Sea. Partial Mantel tests showed that geographical distance per se was not related with genetic structuring among Atlantic and western Baltic Sea samples. Alternative factors, such as dispersal potential and/or environmental gradients, could be important for generating genetic divergence in this region. The results show that the magnitude and scale of structuring generated by a specific mechanism depend critically on its interplay with other evolutionary mechanisms, highlighting the importance of investigating species with wide geographical and ecological distributions to increase our understanding of evolution in the marine environment.

Most of the world's cod (Gadus morhua) fisheries are now tightly regulated or closed altogether. Being able to link individual fish to their population of origin would assist enormously in policing regulations and in identifying poachers. Here we show that microsatellite genetic markers can be used to assign individual cod from three different populations in the northeastern Atlantic Ocean to their population of origin.

Cod eggs in the Baltic Sea are neutrally buoyant at depths exceeding 55 m. When these eggs hatch the larvae must enter the upper photic portion of the water column to locate and capture sufficient prey to feed and grow. In this study we investigated the time during ontogenetic development at which this vertical migration occurs. The vertical distribution of cod larvae, microzooplankton, light intensity and the physical characteristics of the water column in the Bomholm Basin were investigated during 3 cruises in May, June and July 1994. Larvae designated as pre-feeding were usually located at the depths where they had hatched. After larvae had begun to feed, their distributions moved closer to the water's surface. Since larvae are negatively buoyant relative to the density of water in the upper layers of the Baltic, this migration requires active swimming. Hence the hydrographic structure of the water column in the Baltic likely imposes a modest metabolic cost on larvae. We also investigated factors determining the vertical hstribution of feeding larvae. The distribution of these larvae was poorly correlated with prey abundance (i.e. concentration of copepod stages). However, distributions were correlated with prey availability as estimated by combining measures of light-dependent larval feeding incidence with the measured prey concentrations. Our observations suggest that a vertical migration among Baltic cod larvae is necessary for 2 reasons. This migration enables larvae to obtain suitable feeding conditions, and to avoid mortality that could be induced by exposure to the low oxygen conditions typical for the sub-halocline layer.

The occurrence of natal homing in marine fish remains a fundamental question in fish ecology as its unequivocal demonstration requires tracking of individuals from fertilization to reproduction. Here, we provide evidence of long-distance natal homing (>1000 km) over more than 60 years in Atlantic cod (Gadus morhua), through genetic analysis of archived samples from marked and recaptured individuals. Using a high differentiation single-nucleotide polymorphism assay, we demonstrate that the vast majority of cod tagged in West Greenland and recaptured on Icelandic spawning grounds belonged to the Iceland offshore population, strongly supporting a hypothesis of homing. The high degree of natal fidelity observed provides the evolutionary settings for development of locally adapted populations in marine fish and emphasize the need to consider portfolio effects in marine fisheries management strategies.

Parallel evolution and the extent to which it involves gene reuse have attracted much interest. Whereas it has theoretically been predicted under which circumstances gene reuse is expected, empirical studies that directly compare systems showing high and low parallelism are rare. Three-spined stickleback (Gasterosteus aculeatus), where freshwater populations have been independently founded by ancestral marine populations, represent prime examples of phenotypic and genomic parallelism, but cases exist where parallelism is low. Based on RAD (restriction site associated DNA) sequencing, we analysed SNPs and chromosome inversions in populations in Denmark and Greenland showing low and high parallelism, respectively. We identified parallelism across freshwater populations in Greenland at genomic regions previously identified to be associated with marine-freshwater divergence. These same markers also separated Danish marine and freshwater sticklebacks, albeit to a weaker extent. Hence, parallelism was not absent in Denmark but possibly constrained by spatially and temporally varying selection. Divergence time estimates found one Danish freshwater population to be much older than the others. It also deviated strongly with respect to parallelism and may represent earlier postglacial colonization based on a different pool of standing variation and eliciting different adaptive responses to freshwater conditions. These findings provide empirical support to previous suggestions that the time since replicate populations had access to a common pool of standing variation is a major factor determining gene reuse. At last, based on the observed parallelism in the Greenlandic system we discuss the predictability of adaptive responses in newly established populations.

Newly hatched Baltic cod Gadus morhua larvae are typically found at depths >60 m. This is a region of low light and prey availability, hence generating the hypothesis that larvae have to migrate from hatching depth to the surface layer to avoid starvation and improve their nutritional condition. To test this hypothesis, Baltic cod larvae were sampled during the spawning seasons of 1994 and 1995 with depth-resolving multiple openingiclosing nets. Each larva was aged by otolith readings and its RNA/DNA ratio was determined as a measure of nutritional condition. The RNMDNA ratios of these larvae aged 2-25 days (median 10 days) ranged from 0.4 to 6.2, corresponding to levels exhibited by starving and fast-growing larvae in laboratory calibration studies (starvation. protein growth rate, G,,= -12.2% day I; pastgrowing larvae, Gpi= 14.1% day-') respectively. Seventy per cent of the field caught larvae had RNNDNA ratios between the mean values found for starving and fed laboratory larvae. Only larvae aged 8-11 days had higher mean RNAlDNA ratios above 45 m than below (t-test, P

A study was carried out to investigate the diet and feeding strategies of age 0 year juvenile flounder Platichthys flesus in two different micro-tidal habitats in the nutrient enriched Mariager Fjord on the Danish east coast. Juvenile flounder and benthic macrofauna were sampled monthly from June to October 1999 in a bare sand habitat and a habitat covered by filamentous and mat forming macroalgae. The presence of the 'opportunistic' macroalgae created a shift in the dominance of surface dwelling prey such as epifaunal amphipods to more infaunal groups such as oligochaetes and polychaetes. The diet of the flounder varied considerably between the two habitats mainly reflecting prey availability relative to their abundance, prey spatial distribution, habitat structure and ontogenetic prey shifts as a function of total length. Flounder in the vegetated site fed on a diverse diet of copepods, polychaetes and oligochaetes, whereas those caught in the bare sand site fed primarily on the amphipod Corophium volutator which was numerically dominant at this site. During the growth season, two diet shifts were observed: from copepods early in the season to macrofauna organisms and, later in the season, the inclusion of more hyperbenthic prey such as Mysidea spp; Idotea spp. and the common goby Pomatoschistus microps. # 2005 The Fisheries Society of the British Isles