Fishing the gene pool : Genetic structure, admixture and behavioural complexity in fisheries management

Prudent management of marine fish resources relies on separation and management of biologically meaningful groups of conspecifics, termed fishery stocks, often depicted as self-sustaining, spatiotemporally separated and demographically independent entities. Such stock discrimination has however proven challenging. Although most marine fish species form local spawning populations (in relation to hydrological and topographical features that promote local juvenile retention), the level of genetic and demographic connectivity among them is difficult to quantify, given that marine fish often display low genetic differentiation, implying gene flow among local populations mediated by dispersal and migrations in a “barrier free” environment. In this thesis I assess regional genetic structure and phenotypic divergence in a “high gene flow” fish species, the Atlantic cod (Gadus morhua), and re-evaluate the current stock division into an Eastern Baltic (EB) and Western Baltic (WB) cod stock. I also assess genetic consequences of population size reductions in a regional population assemblage of brown trout (Salmo trutta), used as a model system to draw conclusions relevant to overexploited fishery stocks.I examine genetic structure in a regional assemblage of local cod populations, making up a transitional admixture zone that separates the non-admixed North Sea stocks from non-admixed Baltic Sea stocks. In spite of a gradient in genetic admixture, which coincides with a gradient in salinity, local cod populations form two differentiated genetic clusters (stock complexes); a North Sea stock complex (Kattegat & Öresund), and a Baltic Sea proper stock complex (Arkona & Bornholm basins). This division is supported by recent genetic work, but is not congruent with the current EB and WB stock division. I also assess stock mixing in the Öresund management subdivision, an interface between inferred stock complexes, seeing increasing stock mixing (proportion Baltic genotypes) with decreasing distance from the Baltic Sea proper, and non-negligible but stable stock mixing throughout a single season. The combination of non-negligible migrant influx and maintained genetic differentiation might imply regionally prevailing spatiotemporal spawning separation and homing.I assess phenotypic and behavioural divergence in a local cod population, manifested through biased catches with regard to body condition, growth and body morphology. Evidence suggest that the observed divergence constitute within-stock behavioural components (not genetically divergent populations) most likely induced by increased intraspecific competition within the Baltic Sea proper.The inconsistent spatial genetic structures historically inferred for Baltic cod raises questions regarding the spatiotemporal stability of an admixture zone. To examine how population size reductions (and ensuing density dependent migrations and interactions) influence the genetic integrity of populations and the regional genetic admixture, I assess genetic variability within an assemblage of trout populations that suffered recent region-wide population decimations and recoveries. Recovered populations were characterized by lower genetic differentiation and higher genetic admixture, implying that population size reductions could endanger the genetic integrity of local populations. Such density-dependent genetic integrity is of relevance also to cod, given the drastic declines in Baltic cod stocks from overexploitation in the 1980s and 1990s.In summary, my work suggests that stock discrimination and stock division must be more dynamic, especially for population assemblages experiencing genetic admixture, exploitation, and stock size reductions. Given that stock size reductions can increase genetic admixture from density-dependent gene flow, and that gene flow can counteract local adaptation, overexploitation can have unforeseen detrimental effects on local populations