| Summary: | Marine fishes are subdivided into genetically separated units at various geographical scales. The mechanisms contributing to distinct populations may include year-round site fidelity or natal homing of juvenile and adult stages. Restricted dispersal of pelagic eggs and larvae, associated with ocean environmental conditions, can also enhance distinction between populations. In Norwegian waters, Norwegian coastal cod (NCC) and Northeast Arctic cod (NEAC) are managed as separate units, but increasing evidence demonstrates that the NCC may be structured into local populations on a surprisingly small scale, and that each single fjord population should be preserved for genetic diversity and population persistence. Until now, few studies have been done on ecological and physiological differences among local populations. This thesis was motivated by gaining increased understanding of how retention mechanisms of early life stages along the coast and fjord areas of Norway might contribute to the diversity in coastal cod population structure. A key factor here is believed to be egg buoyancy, i.e. the ability of the eggs to float naturally in seawater. Therefore, this thesis addresses how basic physical, chemical and biological properties influence egg buoyancy to ultimately understand population structuring of Atlantic cod. Specifically it is studied whether or not the phenotypes of egg specific gravity (ρegg), which is a key biological factor determining egg vertical distribution, are different among local populations. Regarding basic knowledge of egg buoyancy, it is examined what are physiological and physical determinants of ρegg, i.e., in light of the important roles of egg osmoregulatory capacity influencing ooplasm specific gravity and the chorion (eggshell) thickness and specific gravity, respectively. The data of ρegg are applied to model egg dispersal during the spawning season in Northern Norway. The modelling results are discussed in the context of metapopulation structure (defined here as a set of ...
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