Relationship between feeding incidence and vertical and longitudinal distribution of rainbow smelt larvae (Osmerus mordax) in a turbid well-mixed estuary

We tested the hypothesis that the active vertical migration of the larvae of anadromous rainbow smelt (Osmerus n~ordax) observed in the turbid, well-mixed portion of the St. Lawrence Estuary is an adaptation to improve feedlng incidence by retaining larvae in productive, up-estuary waters. We documented the vertical distribution, diet and feeding incidence of larvae of 3 length classes over 4 tidal cycles at each of 3 stations located along the longitudinal axis of the upstream portion of the estuary. The percentage of smelt larvae with gut contents increased with larval length and with distance upstream for all 3 length classes. Vertical migrations of larvae were not related to feeding incidence in the vertical plane, except in the case of the smallest length class, indicating that active vertical migrations were not behavioral responses to vertical changes in prey availability. The predominance of calanoid copepods and freshwater cladocerans in the diets of larvae with the highest feeding frequencies and the predominance of mysids in the diets of larvae with the lowest feeding frequencies suggested that larvae were more successful feeding on copepods and cladocerans than on the larger mysids. Apparent tidal rhythmicity in diet and feeding success was largely due to the tidal displacement of water masses at fixed sampling stations. Feeding incidence at the 2 sampling stations situated within the maximum turbiditv zone was greatest in larvae advected from upstream at the end of ebbing tide. We conclude that active vertical migration of smelt larvae is an adaptation to maximize the longitudinal retention of high densities of larvae in a zone of high prey biomass.