Fickian Transport and the Dispersal of Fish Larvae in Estuaries

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Abstract
We studied the regulation of ichthyoplankton dispersion in the two-layer circulation of the St. Lawrence upper estuary by determining larval abundance and vertical distribution during high frequency sampling at three stations in May, June, and July, 1979. Monthly variations in capelin (Mallotus villosus) and Atlantic herring (Clupea harengus harengus) abundance were in agreement with the seasonal trends previously reported. Capelin larvae were concentrated in the surface layer, a situation which resulted in seaward drift. No significant growth was observed over the 60-d sampling period indicating continuous recruitment to, and removal from, the sampling area. Herring larvae were concentrated in the deep layer and were carried landward. The average size of herring larvae increased from the downstream to the upstream stations. Short-term fluctuations in the abundance and vertical distribution of the two species were interpreted in terms of the Fickian representation of transport for partially mixed estuaries. The major source of variation in abundance, at a given station, was the tidal advection of horizontal gradients. Capelin larvae and herring larvae smaller than 10 mm did not actively cross the pycnocline and were not submitted to the diffusive effect of the vertical current shear. The dispersal of these larvae was apparently passive. Herring larvae larger than 10 mm performed diel vertical migrations across the pycnocline and were dispersed in the horizontal plane at a faster rate than a passive contaminant of the environment. We conclude that the Fickian approach can be profitably applied to studies of dispersal and mortality of early larval stages of fish in estuaries.Key words: ichthyoplankton, St. Lawrence estuary, dispersion, transport, vertical distribution, diel migrations, variability, abundance, tidal mixing, Fickian