Allocation of Sampling Effort to Optimize Efficiency of Watershed-Level Ichthyofaunal Inventories

Abstract

Sampling protocols for ichthyofaunal inventories have received considerable study at the stream reach level; however, no studies have examined the optimal allocation of sampling effort for watershed-level inventories. To address this question, we used data from surveys of four Great Lakes watersheds to compare rates of species accumulation among several watershed-level scenarios of sampling effort allocation (sampling strategies). To allow for quantitative comparisons among strategies, we (1) divided each sampling site into smaller sampling units and separately recorded fish capture data at the smallest unit; (2) stratified the allocation of sampling effort throughout each watershed by stream order; and (3) recorded all sampling and travel costs (i.e., time spent on different tasks). We used a Monte Carlo simulation program to resample the data from each of the four watershed surveys and calculated rates of species accumulation. This technique allowed us to quantitatively compare sampling strategies. Using this program, we determined the optimal values for (1) the length of stream to sample at each reach; (2) the allocation of sampling effort (time) among stream order strata; and (3) the allocation of effort between seasons (spring and summer) for each of the four watersheds. On average, sampling 15, 9, and 9 stream widths per reach in first-, second-, and third-order reaches, respectively, maximized the rates of species accumulation across watersheds. Allocating 50-70% of the sampling effort to third-order strata of each watershed yielded the highest rates of species accumulation without increasing the number of species systematically missed in either the lower or upper portions of the watershed. Focusing sampling effort in only one season, either spring or summer, did not consistently increase the rates of species accumulation. Our results provide useful guidance for the design of watershed-scale ichthyofaunal surveys in Great Lakes basin watersheds. We recommend similar assessments in other regions with contrasting patterns of fish diversity.