Ecology of juvenile turbot and flounder in the Central Baltic Sea Implications for recruitment

Abstract: Our understanding of turbot and flounder ecology in the Baltic Sea is insufficient for sound management decisions. This thesis aims to fill some gaps in current knowledge by providing information of the ecology of turbot and flounder within their juvenile habitat, and to relate these findings to issues assumed relevant for recruitment variation. Main focus is on turbot due to its relatively low abundance and high variability in recruitment. The distribution of both species was studied on different scales, as was environmental effects on food consumption in 0-group turbot. The 0-group turbot display a relatively restricted spatial distribution compared to flounder. This is possibly due to a more specialized diet, which may make them more vulnerable to habitat degradation, especially eutrophication as a strong negative correlation was found with the organic content in the sediment. The species show high temporal and spatial overlap when settling in July-September, with peak abundances in August, and at depths <1 m. Both species display sedentary behavior within the nursery ground. Compared to flounder, turbot was more mobile potentially due to its restricted diet calling for extended searches. For turbot, feeding conditions appear to vary between size groups, which potentially could cause variations in survival between years through size-selective mortality. But, the predation may be low in central Baltic Sea as the abundance of the main predator, brown shrimp are comparatively low during flatfish occupancy. A significant positive relationship was found between the recruitment of turbot and flounder, which suggests that no inter-specific interactions during the juvenile stage affect recruitment. This co-variation also suggests that the recruitment of the species is determined by the same phenomena, potentially by large scale abiotic factors during the egg- and larval stage. For turbot, additional variability is potentially generated during the juvenile stage due to its relatively restricted food and habitat requirements. The specific habitat demands of turbot revealed in this thesis may be used to protect and restore essentially nursery grounds.