Spatial partitioning of relative fishing mortality and spawning stock biomass of Icelandic cod

Fishing mortality affects demographic characteristics of a population, and is a major determinant in loss of spawners from a fish stock; directly influencing stock productivity and recruitment variability. We investigated the effects of fishing on the composition of the spawning stock of cod (Gadus morhua) in Icelandic waters by partitioning relative fishing mortality and spawning stock biomass (SSB) into finer spatial components than those used previously. Geographic regions of reproductive importance were identified around the country, in contrast to traditional paradigms of cod stock structure in Icelandic waters. SSB and relative fishing mortality were unevenly distributed in waters around Iceland, where gill nets selectively removed larger spawners from inshore waters of the main spawning grounds, whose demographic characteristics were more conducive to progeny survival. Changes in stock structure and demographic characteristics caused by changes in spatially explicit exploitation patterns can significantly affect stock productivity through differential loss of reproductive potential. Our approach of partitioning SSB into finer spatial units than those used previously, enabled a more detailed analysis of the distribution of spawners and exploitation of the Icelandic cod stock, and will enable the development of spatially disaggregated models of total egg production, that in turn may assist in differentiating the inherent variability within stock-recruitment relationships.