Mechanisms promoting upriver transport of larvae of two fish species in the Hudson River estuary

Abstract

Bay anchovy Anchoa mitchilli and naked goby Gobiosoma bosc larvae have been reported to move up-estuary. In the present study, we examined depth preferences and periodic vertical move- ments that might promote such along-estuary transport in these 2 species. We conducted 2 cruises of 3 d each in the Hudson River estuary, USA. The cruises were 1 wk apart, coinciding with spring and neap tides. We sampled every 2 h with an ichthyoplankton trawl to permit tests of time, depth, and lateral position on larval concentration. We also collected data on water-column structure with a CTD, and cur- rent velocity with an acoustic Doppler current profiler (ADCP). We briefly sampled at several sites over a distance of 25 km along the river, and found that larvae of both species were uniformly abundant along this section although salinity decreased sharply with increasing distance upriver. Bay anchovy larvae were more abundant than goby larvae (median concentration 234 vs 6.6 ind. 100 m -3 ). Most sampling was conducted at an oligohaline location (mean salinity = 3 to 5 psu). Larvae were typically more concentrated at greater depths; among anchovy larvae during neap tide conditions, and goby larvae during neap and spring tide conditions, larvae were more concentrated at 6 and 8 m than at the surface by a factor of 2 to 9. Large larvae showed a stronger depth preference than small larvae. During spring tide, the water column was less stratified, and anchovy larvae under these conditions were uniformly distributed vertically. There were slight lateral differences in larval concentration, with fewer larvae in shallow water over the shoals than in similar depths in the main channel. We evaluated periodic cycles in flow and larval dis- tributions via harmonic regression. Tidal constituents of the depth-averaged current flow included the K1 (period = 23.9 h), M2 (period = 12.4 h), and the M4 (period = 6.2 h) tides. Harmonic regression explained >95% of the observed variability in mean flow. Diel periodicity in depth-averaged larvae con- centration was evident, particularly among large anchovy and goby larvae during neap tide conditions. Larvae were more abundant in the sampled depths at night than during the day by a factor of 3 to 10. There was also diel periodicity in the mean depth of goby larvae, such that larvae were about 2 m shal- lower at night than during the day. There was no periodic variability in the mean depth of anchovy larvae. We suggest that diel periodicity in larval concentration and mean depth reflects diel migration to shallower water at night, noting that temporal variability in net avoidance may also contribute to the periodicity. We conclude that anchovy and goby larvae exhibit a depth distribution and vertical migra- tion behavior that promotes upriver transport. Transport should be most rapid during neap tide periods.