The mechanism of retention of pelagic tomcod, Microgadus tomcod, larvae and juveniles in the well-mixed part of the St. Lawrence Estuary

Abstract

SynopsisWe tested the hypothesis that the mechanism of retention of tomcod, Microgadus tomcod, larvae and juveniles in the well-mixed part of the St. Lawrence Estuary is similar to that of sympatric smelt, Osmerus mordax, larvae who actively migrate to the surface during flood tides and to the bottom during ebb tides so as to minimize net downstream displacement. The vertical distribution of tomcod larvae and juveniles was documented during two 98-h sampling series at 2 anchor stations in June and July, 1986. An hourly index of the center of mass of fish in the water column calculated to take into account daytime net avoidance in surface waters suggested that tomcod remained deep in the water column and that their accumulation at the head of the estuary was the result of passive upstream transport by net residual circulation rather than active tidal migrations. For both series, depth of fish was inversely related to density of the water suggesting that the buoyancy of fish influenced their vertical distribution. Tomcod larvae and juveniles were advected by tidal currents. In June, larger larvae were found at low slack water indicating that they were located upstream of smaller larvae. In July, larger juveniles were located downstream of smaller juveniles, the difference in mean length between low and high slack water attaining 20 mm. Ontogenetic buoyancy changes may be responsible for these differences in the vertical distribution of tomcod. Comparisons of the early life-history stages of tomcod and smelt retained in the same area under the same hydrodynamical conditions indicate that more than one mechanism permits retention in a well-mixed estuary and that the observed species-specific patterns of vertical distribution are not simply interpretable as adaptations to retention.

Ontogenetic Habitat Shifts of the Atlantic Tomcod (Microgadus tomcod) Across an Estuarine Transition Zone

“…This observation complements those of Dodson (1989, 1990) who noted that whereas larger pre-metamorphic YOY tomcod were located upstream of smaller tomcod in June, larger postmetamorphic YOY tomcod were located downstream of smaller tomcod in July. These authors suggested that the downstream movement of larger YOY tomcod was related to the dynamics of the retention mechanism with large YOY tomcod more apt to be advected downstream (Laprise and Dodson 1990). Whatever the cause, the present observations suggest that although this situation persists until September, it does not persist through to the following summer.…”

Nature of environmental variability experienced by benthic and pelagic animals in the St. Lawrence Estuary, Canada

“…We chose these species because they exhibit different patterns of vertical distribution. Tomcod do not migrate vertically but remain in the deeper layers of the water column (Laprise & Dodson 1990). Smelt larvae undertake tidal vertical migrations, moving to surface waters during floods and closer to the bottom during ebbs (Laprise & Dodson 1989a).…”

Trophic coupling across the St. Lawrence River estuarine transition zone

“…The occurrence of several estuarine Neomysis species in ETMs and low-salinity waters (< 5 psu) is associated with the interaction between vertical distribution and estuarine circulation (N. americana: Hulburt 1957, Siegfried et al 1979, Orsi 1986). Finally, the larvae of rainbow smelt are retained in the St. Lawrence ETM by tidal stream transport (Laprise & Dodson 1989), whereas tomcod in the same estuary are retained by remaining in the deep, upstream, residual circulation (Laprise & Dodson 1990). The observed discontinuity in the relative proportions of autotrophs and heterotrophs along the leading edge of the transition zone is due to a variety of mechanisms.…”