A Field Comparison of the Substrate Composition of California Golden Trout Redds Sampled with Two Devices

Abstract

Stream substrates are frequently sampled with several different devices, and it is often assumed that the resulting samples have comparable compositions. We compared substrate samples taken with an excavated corer (15-cm-diameter plastic pipe) with those taken with a freeze corer from redds of California golden trout Oncorhynchus mykiss aguabonita in a stream dominated by small substrates (geometric mean diameter of redd substrates, approximately 6 mm). The proportions of individual particle size-fractions and the percent of fine sediment in the upper portion of excavated cores were similar to those in freeze cores. Geometric mean particle diameters of excavated-core and freeze-core samples were influenced by differences in the proportional weight of particles larger than 12.5 mm; as a result, geometric mean diameters were most similar when these larger size-fractions were omitted. Samplers did not differ in their probability of encountering egg pockets. A power analysis of the substrate size-fraction comparisons showed that the power of our analyses to detect differences between samplers was usually high (≥0.8). Therefore, the general lack of significant differences in the substrate composition from samples taken by the excavated corer and freeze corer is probably the result of few true differences and not of shortcomings in our sampling design. For small streams dominated by fine substrates, we conclude that the inexpensive excavatedcore sampler we used is a viable alternative to the expensive and cumbersome freeze-core sampler, even when sampling in redds, and that is has several advantages over other excavated-core samplers or a shovel when sampling outside of redds.