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Abstract:

Connectivity can be defined as the extent to which populations are linked by the exchange of larvae, juveniles or adults across a species’ range. It plays a fundamental role in local population dynamics and the genetic structure of a species. A good understanding of connectivity is important for population resiliency against changing environments and exploitation, the design of marine reserves and the sustainable conservation of a species.The sole ( Solea solea ), a common marine flatfish inhabiting the North-East Atlantic, is currently under high fishing pressure. It has a long history of exploitation in the North Sea. Since the early 1960s, mortality rates increased considerably due to the introduction of the beam trawl, which increased the catch efficiency for flatfish. Because sole has always been a target species of the North Sea beamtrawl fishery, long time series of fisheries data and large collectionsof historical otoliths (earstones) are available in fisheries institutes. These otoliths were initially collected for ageing purposes but the adhering dried tissue also represents a unique source of historical DNA.Despite the commercial importance of sole, little is known about its population structure and connectivity in the North-East Atlantic Ocean. Furthermore, it was not clear whether the intensification of the fishery has also led to changes in the genetic diversity and if the evolutionary potential of the species is being threatened.This study investigated the connectivity and demographic genetic stability of sole in theNorth-East Atlantic Ocean. In the first part of the thesis, genetic markers and ecological markers were used to assess the population structure, while the second part examined the genetic stability of sole in the North Sea under high fishing pressure. A thorough population genetic analysis using neutral microsatellite markers and a mitochondrial marker showed genetic differences at a large scale, along a latitudinal gradient from the Skagerrak/Kattegat to the Bay of Biscay. At a smaller spatial scale within the North Sea, the subpopulations seemed genetically homogeneous, probably due to a high level of gene flow and/or the high effective population size preventing strong effects of genetic drift (Chapter 1). Besides genetic markers, otolith microchemistry may be applied as a tool for studying population connectivity because it reflects environmental differences experienced by a fish. The analysis of the elemental composition of juvenile sole otoliths showed that fish living in different nurseries had a specific elemental composition. We concluded that the movement of juvenile sole, once settled in a nursery ground, is rather limited (Chapter 2). Subsequently otolith microchemistry and otolith shape were used to discriminate between adult sole from different spawning grounds. Otolith microchemistry was a successful marker for tracing fish back to their spawning site but especially the combination of otolith microchemistry with otolith shape provided a succesfull traceabilitytool (Chapter 3). The integration of genetic markers and otolith microchemistry further improved the assignment for some populations (Chapter 5). The movement of individuals from juvenile to adult habitats provides another critical link. Chapter 4 explored the assignment of adults to their source nursery based on the microchemistry of the juvenile portion of the otolith. This is important to assess the relative contribution of various nursery grounds. A relatively high percentage of self-recruitment was found, suggesting that young fish recruit to the adjacentadult stocks.In the second part of the thesis the temporal genetic stability of the North Sea sole was analyzed using a collection of historical otoliths (dating back to the 1950s). Because of their high value and uniqueness, it is important to obtain as much information as possible (phenotypic and genetic) from these otoliths. Therefore, a DNA extraction protocol was first optimized in Chapter 6. Subsequently, we examined whether a decrease in genetic diversity has taken place due to the intensification of the fishery since the 1960s. A remarkable genetic stability was found from the 1950s up to present. This was reflected in the high effective population size estimate (Ne > 2000) obtained for the North Sea sole, using different methods. The Ne represents an important parameter in conservation genetics because it determines the amount ofgenetic diversity that can be lost due to genetic drift. In theory, theestimate means that genetic drift is probably not an important evolutionary driver in this population. Nevertheless, the ratio of Ne/Nc (censussize) was very small and comparable to other marine fish. It is typicalfor organisms with a type III survival pattern, indicating that drift may occur in populations with high census counts. We conclude that the maintenance of a large population size of mixed age classes (including older individuals) provides the best guarantee for the conservation of solestocks in the North-East Atlantic Ocean.