Relationship between phytoplankton production and cod production on the Faroe shelf

Phytoplankton production on the Faroe Shelf varied dramatically from 1990 to 2003 and a close empirical relationship between phytoplankton production and cod production is demonstrated in this study. The large fluctuations of the biomass of the Faroe Plateau cod stock in the 1990s seem to be explained by variations in primary production, i.e. by a fluctuating carrying capacity of the Faroe Shelf ecosystem. The causal mechanism seems to be a positive relationship between phytoplankton production, zooplankton production and production of food organisms for cod (e.g. benthic crustaceans and especially sandeels). The year-class strength of Faroe Plateau cod seems to be determined during their second winter, corresponding to the time when they leave the nearshore nursery areas and enter the feeding areas of adult cod. The regulating factor seems to be abundance of suitable food organisms for small cod, which seems to be a result of total food availability (determined by the phytoplankton production) and competition from older cod, i.e. density-dependent recruitment. The individual growth of cod older than 2 years seems to be a result of the total food availability and the stock size of cod, i.e. density-dependent growth. Implications for stock assessment and management are discussed.