Fish species richness is an important indicator of river ecological condition but it is particularly difficult to estimate in large unwadeable rapidly flowing rivers. Intensive multi-gear sampling is time consuming, logistically complex and expensive. However, insufficient sampling effort underestimates species richness and yields inaccurate data about the ecological condition of river sites. We raft-electrofished 10 large river sites in 10 different ecoregions and six western USA states for distances equal to 300 times their mean wetted channel widths (MCWs) to estimate the effort needed to approach asymptotes in fish species richness. To collect 90% of the observed fish species at the sites, we found that an average of 150 MCWs (ranging 80–210 MCWs) were needed, with the number of MCWs increasing in rivers with a higher proportion of spatially rare species. Frequently, the second or third additional 100 MCWs produced only one or two additional singletons or doubletons (species occurring only once or twice at a site). Before initiating sampling programs for estimating species richness, we recommend assessing sampling effort, particularly if rare or uncommon species are expected or desired.

鱼类物种丰富度是河流生态状况的重要指标，但在不可涉水的快速流动的大型河流中估计尤其困难。密集的多齿轮采样非常耗时、逻辑复杂且昂贵。然而，采样工作不足低估了物种丰富度，并产生了有关河流生态状况的不准确数据。我们对 10 个不同生态区和美国西部 6 个州的 10 个大型河流进行了筏式电捕鱼，捕鱼距离等于其平均湿河道宽度 (MCW) 的 300 倍，以估算接近鱼类丰富度渐近线所需的努力。为了收集现场观察到的 90% 的鱼类物种，我们发现平均需要 150 个 MCW（范围为 80-210 个 MCW），并且在空间稀有物种比例较高的河流中，MCW 的数量会增加。通常，第二个或第三个额外的 100 个 MCW 仅产生一个或两个额外的单态或双态（物种在一个地点仅出现一次或两次）。在启动估计物种丰富度的采样计划之前，我们建议评估采样工作，特别是在预计或需要稀有或不常见物种的情况下。