Estimates of Larval-Fish Abundance: Diurnal Variation and Influences of Sampling Gear and Towing Speed

Abstract

The influence of net-towing speed on estimates of the abundance of larval Atlantic menhaden Brevoortia tyrannus, spot Leiostomus xanthurus, pinfish Lagodon rhomboides, and Atlantic croaker Micropogonias undulatus was studied in nearshore and estuarine areas of North Carolina. A modified Miller high-speed sampler was towed at speeds ranging from 2 to 12 m/second. In one series, catch increased monotonically with increasing tow speed up to 7 m/second; in a second series, catch increased with speed up to 8 m/second but decreased at higher speeds, possibly because larvae were extruded through the net or deflected by the pressure wave. In additional studies, a slowly towed (2 m/second) 20-cm bongo net was compared to the high-speed sampler during daylight; day and night samples from the high-speed sampler also were compared. These data imply that visual avoidance by larval fish biases estimates of both their abundance and their vertical distribution. The data from all three studies suggest the need to calibrate nets for towing-velocity effects, particularly for larger ichthyoplankton (10–16-mm spots and 19–26-mm Atlantic menhaden) that can avoid slowly towed sampling gear more easily than can earlier life stages.