Underestimation of Standing Crop by the Surber Sampler1

Before the flow of water from Jackson Lake into the Snake River was stopped by closure of a dam, invertebrates were collected in five Surber samples from a downstream riffle. After the water had receded from the riffle, five 0.093‐m2 areas of the exposed streambed substrate were hand‐collected and the invertebrates removed. Predrawdown samples contained 4,286 invertebrates weighing 6.3 g while postdrawn samples contained 15,490 weighing 13.6 g. Large differences between submerged and exposed samples resulted from passage of smaller insects through the fine mesh of the Surber sampler and from backwash created by the sampler.