Variability of northeast Arctic cod (Gadus morhua) distribution on the main spawning grounds in relation to biophysical factors

Early life stages may on average gain a survival benefit from the regular return of parent fish to the same spawning grounds. Such areas may have a considerable spatial extent, occupation of which varies between spawning seasons. Northeast Arctic cod (Gadus morhua) is an ideal candidate to pinpoint the most influential biophysical factors in these respects due to its data richness and apparently strong "homing" behaviour. Their main spawning grounds, off northern Norway, are split in two by the Lofoten archipelago, and in the previous decade strong spawning activity was observed on the seaward continental shelf (Yttersida), while in recent years spawning activity in the wide fjord (Vestfjorden), that separates the islands from the Norwegian mainland, has again increased. Time-series of egg distribution were available from the 1980s, 1990s, and 2004-2012. We used several fixed and random factors to construct a statistical model for the distribution of recently spawned eggs, and hence, spawning adults in either area. We show that the most relevant covariates for the spatial distribution of eggs in Vestfjorden are local temperature and bathymetry, while at Yttersida influence affecting the whole stock, like regional ocean climate and age structure appear more important. Therefore, spawning inside the fjord may be constrained by suboptimal environmental conditions. Yttersida may be occupied by fish which have been displaced out of Vestfjorden when preferred conditions there are of limited extent.