THE SIZES OF SALMONID SPAWNING GRAVELS

The availability of suitably sized spawning gravels limits salmonid (s almon and trout) populations in many streams. We compiled published an d original size distribution data to determine distinguishing characte ristics of spawning gravels and how gravel size varies with size of th e spawning fish. Median diameters of 135 size distributions ranged fro m 5.4 to 78 mm, with 50% falling between 14.5 and 35 mm. All but three spawning gravel size distributions were negatively skewed (on a log-t ransformed basis), with 50% of the skewness coefficients falling betwe en -0.24 and -0.39. Fewer than 20% of the distributions were bimodal. Although tending to be coarser, spawning gravels had sorting and skewn ess values similar to other fluvial gravels reported in the literature . The range of gravel sizes used by fish of a given species or length is great, but the relation between fish size and size of gravel can be described by an envelope curve. In general, fish can spawn in gravels with a median diameter up to about 10% of their body length.