Larval Fish Drift in a Large River with a Comparison of Sampling Methods

Abstract

Larval fish drift in the rocky-bottomed Susquehanna River (northeastern Pennsylvania) was investigated during 1974-1975. Near SSES (Susquehanna Steam Electric Station) at least 18 species of drifting larvae were collected by nets mounted on a stationary boat or by pumping. Maximum densities of 15.4 and 27.1 larvae/10 m3 were found in June 1974 and 1975, respectively. Quillback, Carpiodes cyprinus (56%), minnows (25%), and carp, Cyprinus carpio (14% of the total) were the most abundant larvae caught in 1974 by pumping. The few larvae that drifted during the day were mostly near the bottom. Large numbers of quillback, white sucker (Catostomus commersoni), shorthead redhorse (Moxostoma macrolepidotum), and tessellated darter (Etheostoma olmstedi) larvae drifted near the river surface at night. Drift was maximum at about 2400 h. Overall, the day/night drift ratio was 1/3.8. In 1974 at Falls, the control station upstream of SSES and several intervening coal mine effluents, maximum density of drifting larvae was 1.4 fish/10 m3, less than 10% of that at SSES. Density of spawning-sized fish was about threefold higher at Falls than at SSES. Boat-mounted nets and the pump sampler had equal sampling efficiencies. Condition of larvae in pump samples was related to net material, mesh size, net shape, and pumping duration. Larvae in best condition were in 5-min samples pumped into slender nets (mouth/length ratio 1/10) made of fine-meshed monofilament nylon.