Spawning origins of pelagic juvenile cod Gadus morhua inferred from spatially explicit age distributions:: potential influences on year-class strength and recruitment

Incongruity between the reconstructed spawning period of pelagic juvenile (0-group) cod Gadus morhua and the observed spawning period for adult cod on the main spawning grounds south of Iceland have indicated that large numbers of the surviving juvenile population may, in some years, originate from other smaller inshore spawning grounds located within fjords of the west, north and east coasts. We examine this hypothesis by using age/length and temperature/growth relationships, based on aging of 0-group cod from 1995 and 1997, to analyze historical demographic data from 0-group fish surveys (1970 to 1998) and determine the spawning distribution and origin of pelagic juvenile cod. Data on early life-history characteristics in conjunction with temporal and spatial distribution of pelagic juvenile cod are used to create new abundance indices representing the inshore and offshore juvenile cod-population components. Partitioning of the historical 0-group index into these population components at a finer geographic scale than has been used previously provides new information for estimating the relative contribution of the main spawning component versus the smaller in-fjord spawning components towards variable recruitment levels in the Icelandic cod stock. Spawning diversity, or multiple spawning components, are important for recruitment success as they disperse the mortality risk of the early Life-history stages, which are thought to be the principal determinants of year-class strength.