Sample Sizes Needed to Describe Length-Frequency of Small-Bodied Fishes: An Example Using Larval Pacific Lamprey

Abstract

Abstract The ability to describe the length distribution of a fish population requires sampling an adequate number of individuals, but collecting more fish than needed is inefficient. While fisheries managers have assessed sample size requirements for many sport fishes, these requirements are not routinely described for small-bodied fishes (i.e., maximum length ≤200 mm), particularly larval lampreys. To improve the efficiency of data collection for these fishes, we used resampling analyses to asses sample size requirements for accurately describing length distributions of larval (freshwater-dwelling) Pacific lamprey Entosphenus tridentatus, an anadromous fish native to western North America (total length 60–156 mm). We found that the highest increases in accuracy occurred with sample sizes <50, and that we needed sample sizes of 40 to 130 to describe length frequency with 95% confidence, depending on length interval used for performing length-frequency analyses. From these results, we recommend collecting 100 individuals if using 5-mm length intervals to examine length frequency of larval lamprey. These findings can also be used to estimate the relative accuracy of sample sizes in existing assessments and develop and refine monitoring programs for larval lampreys and other small-bodied fishes.