Ontogeny and importance of tidal vertical migrations in the retention of larval smelt Osmerus mordax in a well-mixed estuary

Abstract

The retention of larval rainbow smelt Osmerus mordax in the well-mixed, turbid upper estuary of the St. Lawrence River is achieved by active tidal vertical migrations. Time-series analysis of their vertical distribution obtained during two 98 h sampling series conducted in June and July 1986 revealed that larvae were near the surface during flood tides and nearer the bottom during ebbs. The nature of vertical migration changes with larval size. In June, young larvae concentrated at the surface during flood. Their subsequent descent appears to be a passive sinking and dispersal throughout the water column beginning after the flooding tide attains maximum speed. In July, older larvae concentrated during both flood and ebb and the amplitude of their vertical migration increased. Thus, older larvae use currents more efficiently as reflected by their average position in the estuary, which was farther upstream In July than in June, and by the dfferential larval size distribution along the estuary, larger larvae being located farther upstream than smaller larvae in July. Passive vertical transport was not responsible for the observed vertical movements as larvae concentrated in surface or bottom layers when mixing of the water column was maximal. Series of hourly average densities calculated from discrete depth samples revealed cyclic variations associated with tidal advection. As smelt larvae are advected by water masses, their longitudinal distribution was influenced by both active vertical migrations and hdal circulation in the estuary

Influence of turbidity, food density and parasites on the ingestion and growth of larval rainbow smelt Osmerus mordax in an estuarine turbidity maximum

“…The same procedure was done for higher turbidities (55 and 80 NTU). Higher levels of turbidity are typically observed within the St. Lawrence ETM, whereas lower turbidities are more val smelt at each station during their active tidal vertical migration (Laprise & Dodson 1989a). A General Oceanic flowmeter was fitted at the mouth of the gear to measure filtration rate.…”

Plasticity in vertical migration by native and exotic estuarine fishes in a dynamic low‐salinity zone

“…To be retained, organisms must overcome advection seaward due to river flow as well as tidal dispersion. Organisms may accumulate in high abundance by (1) maintaining rapid population growth or accumulation rates within the LSZ relative to predation and advective or dispersive losses (McGurk 1989;Pace et al 1992); (2) migrating laterally to the shoals on ebb tides and to channel habitats on flood tides using diel or tidal cues (Wooldridge and Erasmus 1980); (3) sinking passively in vertically stratified waters (Sclafani et al 1993), or swimming downward into net landward residual flow resulting from gravitational circulation (Weinstein et al 1980;Laprise and Dodson 1989a); and (4) migrating upward on the flood and downward into lower velocity flow, or to the bottom, on the ebb (Laprise and Dodson 1989b;Rowe and Epifanio 1994). Of these four mechanisms, vertical migra- tion behavior is frequently studied.…”

“…Position maintenance strategies often switch during ontogeny in response to changes in physiology and habitat requirements (Laprise andDodson 1989a,b, 1990). This phenomenon is well described for fishes that are spawned offshore and subsequently enter estuaries (Boehlert and Mundy 1988;Shanks 1995).…”

Critical periods and growth-dependent survival of larvae of an estuarine fish, the rainbow smelt Osmerus mordax

“…Larval smelt achieve retention in the ETM by active tidal vertical migration (Laprise & Dodson 1989a). No predator of smelt larvae is known within the ETM.…”

“…High growth rates indicate an important surplus in the energy budget. In order to achieve this surplus, smelt larvae need to actively migrate to the surface layer of the water column to feed (Laprise & Dodson 1989a, Sirois & Dodson 2000. Active vertical migrations also allow smelt larvae to achieve retention within the ETM.…”