SPRING ABUNDANCE AND DISTRIBUTION OF THE CTENOPHORE PLEUROBRACHIA-PILEUS IN THE SEINE ESTUARY - ADVECTIVE TRANSPORT AND DIEL VERTICAL MIGRATION

Several sets of plankton and hyperbenthos samples were collected from the Seine estuary in late May-early June of 1991, 1992 and 1994, and t he abundance, size-frequency, horizontal and vertical distribution, an d short-term fluctuations of Pleurobrachia pileus (O.F. Muller, 1776) were determined. In the Seine estuary, P. pileus was more abundant tha n reported for any other European shallow waters. Its depth-averaged d ensity was 33 individuals m(-3) in 1991 and 32 individuals m(-3) in 19 92, with a maximum density of 808 individuals m(-3). The ctenophores d isplayed passive tidal advection and active diel vertical migration. I n the outer estuary, high abundances were observed around low tide, an d diel vertical migration was very pronounced. However, many individua ls remained aggregated near the bottom at all times. In spring, the di stribution of P. pileus coincided with the 33 and 15 parts per thousan d, S isohalines, with high abundances on the marine side of the estuar y. It is suggested that aggregation of ctenophores near the bottom and diel vertical migration play an important role in the retention of P. pileus in the estuary. Two schemes of P. pileus life cycle in the Bay of Seine are proposed.