Abstract: Moe, B., Hanssen, S. A., Ytrehus, B., Balk, L., Chastel, O., Christensen-Dalsgaard, S., Gustavsson, H. & Langset, M. 2020.Thiamine deficiency and seabirds in Norway. A pilot study. NINA Report 1720. Norwegian Institute for Nature Research. Thiamine (vitamin B1) is vital for life-sustaining enzymes in cells. Previous studies have reported episodes of thiamine deficiency in marine ecosystems, and suggested that this have contributed to population declines of seabirds breeding in the Baltic Sea and elsewhere. Many Norwegian seabird populations have shown a strong decline in population size, but thiamine status has never been assessed. The objective of this pilot study was, thus, to document thiamin levels in selected species and their associated food webs, and explore methodological issues relevant for future studies or monitoring. The methodological tests showed that storage freezing temperature did not affect thiamine levels in egg yolk samples, and that thiamine levels in eggs could not be corrected for incubation time. Furthermore, the quantified thiamine levels differed between two laboratories, and we developed a predictive equation to convert thiamine levels in egg yolk samples between the laboratories. This pilot study has, for the first time, investigated thiamine levels in seabird eggs from selected species and populations in Norway mainland and Svalbard, and in their food webs. We revealed variation among species, populations and prey types. The lowest levels were found in eggs from common eiders and in blue mussel which is their prey. Eggs from herring gulls had also relatively low levels. The levels for common eiders and herring gulls were higher than previously reported from the Baltic Sea. Nevertheless, the levels from common eiders, herring gulls and blue mussels should be classified as thiamine deficient according to effect-ranges reported in these previous studies. The highest levels were found in eggs from kittiwakes and Atlantic puffins. The diet samples from kittiwakes and Atlantic puffins had higher thiamine levels compared to blue mussels. This is the first time thiamine levels are reported for kittiwakes and Atlantic puffins. This pilot-study cannot answer whether Norwegian seabird population sizes are affected by thiamine levels, but we cannot rule out that thiamine can be a limiting factor for some Norwegian seabird populations. This report identifies knowledge gaps and provides recommendations for future studies and monitoring. We suggest more sampling of levels to better understand variation among years, areas, species and populations, and also clinical examinations and surveys. Studies investigating potential effects on reproduction and survival is ultimately need to better understand potential effects on population dynamics.