The relationship between vertebral number and water temperature in cod

An inverse relationship between water temperature during early development and mean vertebral number ($VS¯$, includes urostyle) is established for cod populations throughout the North Atlantic, but the extent to which differences in $VS¯$ are inherited is unknown. The $VS¯$ of different year classes of cod from the North Sea shows some dependence on water temperature and the year class should therefore be taken into account, where possible, when seeking evidence of separation between stocks. Values of $VS¯$ for populations around the British Isles are plotted on the chart of mean March surface temperature and in general agree with the established temperature–$VS¯$ relationship. Two areas for which the vertebral numbers seem anomalous are the English Channel and the northeast Irish Sea and these anomalies are discussed.