The risk of thiamine deficiency in common eider ducklings increases when females consume large prey.

Episodic mass mortality due to thiamine (vitamin B1) deficiency is a major threat to marine top-consumers worldwide. Recent reports have shown that thiamine deficiency can lead to high mortality rates of common eider (Somateria mollissima) ducklings in the Baltic population. It is unclear whether the drastic reduction in offspring recruitment is linked to the changes in egg thiamine levels. This would have an additional negative impact on the Baltic population of the common eider, which is mainly driven by increased predation pressure. As thiamine concentration decreases with the size of organisms at the interspecific level, we investigated whether the consumption of larger individuals of the dominant prey species may have a detrimental effect on adults and egg thiamine levels. We modelled female thiamine status in the life history perspective of a capital breeding sea duck, which accumulates body reserves prior to the nesting season. Thiamine uptake by model females was based on the empirical analysis of the thiamine concentrations in blue mussels Mytilus edulis trossulus, a predominant species in the diet of eiders. The model predicted that consumption of larger prey reduced thiamine levels in females and eggs, as thiamine concentration decreased with mussel size. Mechanisms of thiamine excretion were more important than seasonal variation and other life history determinants in shaping the egg thiamine levels in response to consumed prey size. The model also predicted higher levels of thiamine in the eggs of females that reduced their clutch size in response to changes in breeding phenology. These results suggest that the variation in thiamine concentration within prey species in relation to body size, is an important determinant of thiamine balance in top consumers. A reduction in clutch size is expected to increase offspring quality in females with poor thiamine status. Importantly, the consumption of large prey may predispose consumers to limitation of essential metabolic micronutrients.