Larval transport, vertical distribution, and localized recruitment in anadromous rainbow smelt (Osmerus mordax)

We examined larval dispersal and behaviour in rainbow smelt (Osmerus mordax) from the Salmonier River estuary, Newfoundland, during 2003-2005. Riverine ichthyoplankton surveys indicate that larval transport into the estuary is restricted to a two-week period in late June. A passive drifter experiment and the spatial distribution of larvae and juveniles suggest limited dispersal and low export from the estuary. The estuary exhibited low inflow and stratification in summer and high inflow with a stable freshwater layer in fall. Summer and fall winds are primarily onshore. We hypothesised that larval vertical swimming behaviour limits dispersal and export. Larval vertical position and swimming ability were examined through lab-based experiments conducted 0-10 and 90 days after hatching. Measurements of swim speed indicate an order of magnitude (p < 0.001) improvement in swimming capacity between larvae and juveniles. One-week after hatching, larvae displayed consistent negative geotaxis, whereas juveniles were generally deeper and displayed negative geotaxis in light and positive geotaxis in the dark. The contrasting light-mediated geotactic behavior between development stages corresponds to ontogenetic shifts in swimming abilities and seasonal changes in estuarine circulation. We suggest that the interaction between behaviour, ontogeny, and hydrography results in limited transport and a tendency for estuarine-scale recruitment, despite an extensive pelagic period.