Spatial variation in abundance, size composition and viable egg production of spawning cod (Gadus morhua L.) in Icelandic waters

Changes in geographical location of spawning and/or in spatial abundance of spawners may influence offspring survival. In Iceland, the spatial dynamics of spawning cod (Gadus morhua L.) are complicated due to unequal distribution of size classes on the main spawning grounds. Information on abundance, age, and size distributions during peak spawning, potential fecundity, egg size, and estimated larval viabilities were used to contrast the reproductive potential of three adjacent spawning areas within the main spawning ground of cod in the Icelandic waters. The three areas differed with respect to age and size distribution, abundance, mean production per female and an index of total egg production. Larger and faster growing cod spawned closer to the coast (area 1). Smaller cod spawned in deeper water out on Selvogsbanki (area 2), and the smallest and youngest cod tended to spawn along the continental edge (area 3). Relative abundance was higher in area 2 than in areas 1 and 3 (640, 153, and 334 kg/net, respectively). Mean production of eggs per female was highest in area 1, while the index of total egg production was highest in area 2. The estimates of viable egg production did not change the general results. Estimates of the proportion of viable eggs produced were slightly higher in area 1 than in areas 2 and 3. However, the influence of the relative abundance of spawners in the different areas overrode any trends due to higher mean production of viable eggs. The results demonstrate that egg production, based on abundance and size and age composition of spawning cod, may vary extensively among different spawning areas. Such information, in conjunction with documentation of spatial and temporal variation in oceanic and environmental conditions, may provide an important contribution towards the understanding of recruitment processes necessary for management of commercially important fish species.