Comparison of Net Mesh Sizes for Estimating Abundance of the Opossum Shrimp Mysis relicta from Vertical Hauls

Abstract

We compared abundance estimates of the opossum shrimp Mysis relicta collected in vertical tow nets having mesh sizes of 333 and 1,000 μm for evidence of size-related displacement or avoidance. A timed haul rate (mean = 0.44 ms·s^–1), reportedly near optimum for the 1,000-μm-mesh net, was used with both nets to collect M. relicta from Lake Pend Oreille, Idaho. Estimates of adult and juvenile densities (numbers·m^–3) and lengthfrequency distributions were similar for both mesh sizes. We found no evidence that bow waves, created by water resistance through the finer mesh net (333 μm), displaced more mysids than waves created by the larger mesh size (1,000 μm). Net efficiencies were 66% for the 333-um-mesh net and 71% for the 1,000-μm-mesh net. Consequently, at haul rates up to 0.44 ms·s^–1, choice of net mesh sizes should rely on research goals. Larger mesh sizes will facilitate sampling; finer mesh nets will also collect other zooplankton.