Passage of Gizzard Shad and Threadfin Shad Larvae through a Larval Fish Net with 500-μm Openings

Abstract

Accuracy of larval fish entrainment estimates at power plants depends on the effectiveness of sampling gear. Simulated tows in a laboratory flume were made to evaluate retention of gizzard shad (Dorosoma cepedianum) and threadfin shad (Dorosoma petenense) larvae in a larval fish net with 500-μm square mesh openings that is regularly used by the Tennessee Valley Authority and other investigators. Retention of 1-d-old gizzard and threadfin shad larvae (3.0-5.0 mm total length) averaged less than 15%. Average retention was 32% over all tests with tow velocity, tow duration, species, and age as variables. Retention was significantly greater at a tow speed of 1.0 m/s than at a speed of 0.5 m/s. No effect of tow duration on retention was observed. Tests conducted in static water revealed about 40% of 1- and 3-d-old threadfin shad swam through the 500-μm openings during a 5-min period and about 10% were extruded through the net when it was washed down. These results suggested that (1) the use of larval fish nets with 500-μm or larger openings does not provide representative estimates of the densities of shad larvae present in natural water bodies, (2) comparisons of larval densities based on samples collected at different towing velocities or with stationary nets versus towed nets could lead to invalid conclusions, and (3) a significant proportion of larvae are lost (extruded through the net) during net washdown.