Tests of Drift Samplers for Estimating Abundance of Recently Hatched Walleye Larvae in Small Rivers

Abstract The efficiency of drift nets for assessing abundance of larval walleyes in small rivers was tested during the springs of 1985–1987 by capture of hatchery-reared larvae released upstream of various arrays of drift nets. Catches of larvae in 18 trials with small drift samplers (17.8-cm mouth diameter) averaged only 5.3% (95% confidence interval, 3.3–7.2%) of expected values when larvae were released 1 or 5 km upstream of the nets. Short-term, short-distance release–capture experiments (10 min) with pairs of these samplers and a barrier net indicated that near 100% efficiency could be obtained with these nets in optimal conditions. Patterns of capture by the nets suggested that larval behavior, predation, and mortality were largely responsible for the poor capture rates. Also, fluctuations in stream discharge, heterogeneous distribution of drifting larvae, and distance from the source of larvae affect the effectiveness of sampling gear.