Exploring the multidimensional nature of stock structure: a case study on herring dynamics in a transition area

Abstract

Fish are not just fish. Differences within marine fish species in terms of morphology, behaviour, life history and certainly also genetic differentiation have been shown for an impressive number of species, including herring (Clupea harengus). These differences persist despite marine fish usually occupy areas without much environmental structuring and extensive mixing between populations occur. Many species of marine fishes have the capacity of dispersing over vast geographical areas, either passively by drifting eggs and larvae following ocean currents, or actively by migration of juveniles and adults, however, even for highly migratory species, significant population structure have been documented. Thus population structures are maintained despite extensive mixing of populations across vast distances; however the structuring factors are not easily disentangled. The factors behind this structure of populations have in some cases been referred to as spatial distance between populations, when the distribution of the species is larger than the dispersal range of individuals. Also oceanographic processes and the topography of the ocean floor have been linked to population structure in a number of species, yet few studies have tested specifically for relationships between environmental parameters of adaptive significance and population structuring in marine migratory fish, and even fewer have examined evidence of local adaptation. The relative roles of migratory behaviour and local differences in environmentally induced selective pressures in effecting such structure remain elusive. Maintaining population structures is of vital importance for the resilience of fish populations to changes in the environment and their exploitation. The preservation of intraspecific population integrity is a prerequisite for maintaining population and life history diversity which in turn affect the performance of individual species in providing important ecosystem services. In this PhD thesis, I explore the population complexity of the herring stock called the Western Baltic Spring Spawning herring; localized in the transition area between the North Sea and the Baltic. I analyse which herring populations that are available to a mixed herring fishery in the area and their spatial and temporal occurrence. I explore the potential structuring factors causing the population diversity in the area and discuss the mechanisms behind these structuring factors. The results in this present thesis contribute to the understanding of the dynamics of the herring populations in the mixed pool of herring in the transition area between the North Sea and the Baltic. I identify several genetically different herring populations which are available for a fishery; their occurrence is structured by divergent migration strategies driven primarily by growth potential and the persistence of a genetic population differentiation is linked to the environmental heterogeneity in terms of salinity facilitating homing to spawning site. Such insight will aid a sustainable aggregated management of a fishery on a mixed herring stock. It will facilitate protecting the weaker populations from over harvesting in a mixed fishery and thus maintain the diversity and in turn the resilience of the stock to a fishery