Spatial Configurations of Young Herring (Clupea harengus harengus) larvae in the St. Lawrence Estuary: Importance of Biological and Physical Factors

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Abstract
The phenomenon of larval retention has recently been identified as central to the structuring of herring stocks. It has been shown that retention of larval fish populations in estuaries is dependent on active vertical migration, the capacity for which develops with growth, but the present study shows that mechanisms for spatial structuring of larval herring (Clupea harengus harengus) populations act at the earliest (yolk sac and post-yolk sac) stages. The study was carried out on the Isle-Verte stock of the St. Lawrence estuary in 1981 and 1982. Length–frequencies and tide-related abundance fluctuations suggested that larvae are retained in the study area. As a result, larvae remain aggregated throughout the sampling period of approximately 1 mo. Yolk sac larvae were significantly more abundant near the bottom than in the water column, while post-yolk sac larvae were significantly less abundant in the suprabenthic layer than in the water column. Larvae, being negatively buoyant, tend to sink in slack water but are transported upwards by turbulence resulting from tidal flows. The extent of vertical transport of larvae in the water column is greatest for larvae with lesser specific gravities; specific gravity is at a maximum at hatching and decreases to a minimum at yolk sac absorption. We conclude that larval specific gravity, current velocity, and the two-layer residual circulation are the major factors causing retention and aggregation of larvae.

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