Transport and retention of flounder larvae (Platichthys flesus L.) in the Dollard nursery (Ems estuary)

Variability in tidal transport of larval flounder (Platicthys flesus L.) in the Dollard was investigated to estimate the quantity of larvae that are a nnually retained in the nursery based on simultaneous measurements of water transport and larval concentrations. Sampling was done three times per hou r in the main channel in the Dollard, during 35 tidal cycles divided over 1 993, 1993 and 1995. In 1993, additional sampling was carried out simultaneo usly during eight tidal cycles in a subsidiary channel. Larval concentratio ns showed marked tidal variations at all time scales and indicated a patchy spatial distribution. The estimated net tidal transport of larvae was not correlated to the net tidal transport of water. However, a higher net water transport coincided with higher retention, defined as the percentage of ne t larval transport divided by the flood larval transport. Also, relatively longer duration of the flood than the ebb period (tidal asymmetry) was posi tively correlated with both net tidal water transport and larval retention. The wind direction significantly affected the net water transport at the c ross-section in the main channel, whereas wind speed was not significant. T he flood transport, but not the ebb or net transport, was significantly hig her during spring tide following full moon than during the other lunar phas es. The net tidal transport of larvae was much more variable in 1993 than in 19 93 and 1995, frequently resulting in ebb surpluses in larval transport. Des pite higher mean concentrations, the estimated net larval input was lower i n 1994 than in 1993. This corresponded neither with the observed abundance estimates of juvenile flounder in the Dollard in these two years, nor with the abundance index obtained from routine surveys. The total larval input b etter reflects the juvenile Bounder abundance. Variability in water circula tion, influenced by varying wind conditions, and its effect on larval trans port and retention is an important source of variability in larval immigrat ion in the nursery and is held responsible for a poor correlation between n et larval input and juvenile abundance. (C) 2001 Elsevier Science B.V. All rights reserved.