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 (theta = 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.

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 (theta = 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.

Regulations on the exploitation of populations of commercially important fish species and the ensuing consumer interest in sustainable products have increased the need to accurately identify the population of origin of fish and fish products. Although genomics-based tools have proven highly useful, there are relatively few examples in marine fish displaying accurate origin assignment. We synthesize data for 156 single-nucleotide polymorphisms typed in 1039 herring, Clupea harengus L., spanning the Northeast Atlantic to develop a tool that allows assignment of individual herring to their regional origin. We show the method's suitability to address specific biological questions, as well as management applications. We analyse temporally replicated collections from two areas, the Skagerrak (n = 81, 84, 66) and the western Baltic (n = 52, 52). Both areas harbour heavily fished mixed-origin stocks, complicating management issues. We report novel genetic evidence that herring from the Baltic Sea contribute to catches in the North Sea, and find support that western Baltic feeding aggregations mainly constitute herring from the western Baltic with contributions from the Eastern Baltic. Our study describes a general approach and outlines a database allowing individual assignment and traceability of herring across a large part of its East Atlantic distribution.

Clausen, L. A. W., Bekkevold, D., Hatifield, E. M. C., and Mosegard, H. 2007. Application and validation of otolith microstructure as a stock identification method in mixed Atlantic herring (Clupea harengus) stocks in the North Sea and western Baltic. – ICES Journal of Marine Science, 64: 377–385. Herring (Clupea harengus) populations with different spawning times mix in ICES Division IIIa. For stock assessment, otolith microstructure analysis is used to determine the hatching season of individuals, classifying them into hatch type spring, autumn, or winter. The currently applied method uses visual inspection of season-specific daily increment pattern for the larval period. With this method, variability in individual microstructure and a lack of correspondence between hatch and spawning time may lead to classification error. We validate the visual inspection procedure in relation to these potential sources of error. Otoliths from spawning herring were first classified blindly and the results compared with spawning season. In all, 91% of classifications corresponded, and errors represented misclassifications mainly between autumn and winter spawners. However, the estimates may be biased if hatch and spawning times differ, and an objective method of hatch-time estimation based on linear modelling was employed, enumerating unbroken series of daily increments in 0-group herring hatched in different seasons. Visual inspection and objective estimation agreed in 89% of cases, and most of the errors were explained by overlapping hatch periods. Results show that herring older than the 0-group can be classified using multiple linear regression of hatch time on median increment width.

Determining spatio-temporal distributions of fish populations is of interest to marine ecology, in general, and to fisheries science in particular. Genetic mixed-stock analysis is routinely applied in several anadromous fishes for determining migratory routes and timing but has rarely been used for marine fishes, for which population differentiation is commonly weak and the method presumably less powerful. We used microsatellite information for Northeast Atlantic herring Clupea harengus L. populations and mixed stocks to address 2 questions. We used simulated mixture samples and 3 different statistical approaches to determine whether mixed stock composition could be determined with accuracy. Simulations showed that the applied approaches and mixture samples of 100 individuals enabled detailed composition analyses on a regional level, with resolution for tracing the ecologically dominant Rügen (Greifswalder Bodden) herring population. We then estimated spatio-temporal variation in herring migratory behaviour in the Skagerrak from 17 mixed samples collected over 2 seasons and 2 yr, and identified hitherto undescribed differences in distributions among populations that feed and winter in the area.KEY WORDS: Genetic clustering · Genetic stock identification · GSI · Population structure · Simulation analysis · Skagerrak · Baltic Sea · Migration · Clupea harengus Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 442: [187][188][189][190][191][192][193][194][195][196][197][198][199] 2011 stock analysis (MSA) comprises a number of statistical methods developed to estimate the composition of samples of individuals of mixed origin. MSA has been applied widely in anadromous salmonids (e.g. Ruzzante et al. 2004, Beacham et al. 2005, Koljonen et al. 2005, Smith et al. 2005, Gauthier-Ouellet et al. 2009, Miller et al. 2010), but in spite of the method's large potential for determining spatially and temporally explicit migratory behaviour, relatively few studies have yet been conducted in marine fishes (Waples & Naish 2009). One of the reasons for this paucity is that the generally modest levels of differentiation among marine populations (e.g. Ward et al. 1994) limit the statistical resolution for genetic stock identification (GSI), and, therefore, MSA (Manel et al. 2005, Waples & Gaggiotti 2006. However, when detailed sampling of the main populations contributing to mixed aggregations is attainable, approaches can be designed to overcome problems with low genetic resolution among populations (see e.g. Ruzzante et al. 2000Ruzzante et al. , 2006.Atlantic herring Clupea harengus L. is an abundant and widely distributed marine pelagic fish that spawns on substrate in coastal areas throughout most of the north Atlantic (Iles & Sinclair 1982). Most herring populations are migratory and often congregate on common feeding and wintering grounds where aggregations may consist of mixtures of individuals from several populations. One such area is found in the Skagerrak and ea...