Historical DNA reveals the demographic history of Atlantic cod (Gadus morhua) in medieval and early modern Iceland
Atlantic cod (Gadus morhua) vertebrae from archaeological sites were used to study the history of the Icelandic Atlantic cod population in the time period of 1500-1990. Specifically, we examine the genetic structure at the Cytochrome b (cytb) and Pantophysin I (PanI) loci, using coalescence modelling to estimate population size and fluctuations. The models are consistent with an expanding population during the warm medieval period with medieval effective population size (NE) magnitudes larger than observed in current times, a marked bottleneck event at 1400-1500 with continuing decrease in NE in early modern times. The model results are corroborated by reduction of haplotype and nucleotide variation over time and pair-wise population distance, which partitions a significant portion of nucleotide variation across the 1500 time mark. The mean age of the historical fished stock is high in medieval times with a truncation in age in early modern times. The population size crash coincides with a period of known cooling in the North Atlantic and we conclude that the collapse may be related to climate or climate induced ecosystem change with subsequent effects of fishing.