Seasonal and spatial variation in brown trout Salmo trutta L. abundance, density and biomass were studied in 29 pools of varying size in an afforested catchment together with the physical characteristics of those pools. A movement of 0+ trout towards the pools as the year progresses was detected. Water volume of the pool accounted for a significant amount of the variation in metrics across all seasons. Cover provided by overhanging vegetation also explained a significant amount of variation, especially during the summer. Water velocity, percentage of undercut bank and substrate composition had little explanatory power in the distribution of trout in the pools. In all seasons significant relationships between both fish biomass (g m(-2)) and fish number and water volume in the pool were found. However. in summer and autumn there was also a significant correlation between both fish density (fish m(-2)) and biomass and water volume in the pool described by a power function with a coefficient >1. These relationships were consistent across the subset of pools studied over a 2-year period. Thus there was a proportionally greater number of fish in deeper pools than in the shallower ones in summer and autumn, suggesting that trout use the available habitat (i.e. the pool) as a three dimensional space in which an increase in the third dimension (depth) leads to a proportionally greater number of fish per unit area. (C) 2001 The Fisheries Society of the British Isles.