Environmental and stock effects on spawning origins and recruitment of cod Gadus morhua

Recent studies modelling back-calculated birth date distributions of pelagic juvenile (0-group) cod Gadus morhua have indicated differential regional spawning components in waters off the west, north and east coasts of Iceland, in contrast to traditional paradigms of stock structure. The relative proportions of pelagic juvenile cod that were estimated to originate from these regional spawning components were hypothesized to influence recruitment and vary in response to the inflow of Atlantic water from the main spawning grounds off the southwest coast to the main nursery grounds off the north coast. We examine this hypothesis by analyzing annual (1971 to 1998) coastal current data in relation to the total relative abundance, mean total length, and proportions of pelagic juvenile cod that were estimated to originate from the main spawning component off the southwest coast of Iceland. Additional variation in environmental (temperature, salinity, and zooplankton biomass) and spawning stock biomass (SSB) effects were also examined in relation to the 0-group variables and recruitment at Age 3 to enable a detailed analysis of the stock-recruitment relationship. The coastal current index was the most dominant covariate affecting the spawning origins and recruitment of cod in Icelandic waters. The freshwater-induced coastal current provided a predictable transport mechanism for pelagic eggs and larvae derived from the main spawning component off the southwest coast to the main nursery grounds off the north coast. In years when the coastal current were strong, more abundant and larger pelagic juveniles derived from the main spawning component were found on the main nursery grounds, typically resulting in corresponding years of good recruitment. Differentiating the interplay of the physical environment with that of a spawning stock is fundamental to understanding recruitment variability in marine fish stocks.