An Analysis of the Gravels used by Spawning Salmonids in Ireland

Samples of gravel from 79 sites selected for spawning by Atlantic salmon, 9 selected by sea trout and 13 within spawning areas but known to be unused by spawning salmonids were analysed. For comparison reported data on 81 samples from sites used by Pacific salmon were similarly analysed. No significant difference was found between the range of gravel distributions used by salmon and that used by sea trout. It could not be proved that the sites unused by spawning fish were avoided because of excess coarseness or fineness. The range used by Pacific salmon was found to be wider than that used by Atlantic salmon. A separate analysis was made of the portion of gravel coarser than 4mm in diameter. From this a suitable mixture for use in rehabilitating disrupted spawning areas is proposed. A proposal is made that for spawning gravel analysis the 0scale where 0 = -1.log2 particle size be replaced by a 0 scale where the multiplication by -1 is omitted. Key words: Salmonids, spawning, gravel, site selection