Across-channel distribution of flounder larvae (Platichthys flesus L.) in the Eems-Dollard estuary and its effects on larval transport estimates

Abstract

Until recently estimates of larval transport of the flounder (Platichthys flesus L.) in the Eems-Dollard estuary were based on the assumption of homogeneous concentrations across the width of the tidal channel. This needed further investigation because there were indications from other studies that flatfish larvae are heterogeneously distributed across a tidal channel. The opportunity of large-scale current measurements involving many ships in the central part of the estuary was seized so that a study of the larval distribution of flounder across the tidal channel could be made and the assumption tested. In addition, larval concentrations were measured at two positions on another cross-section of the Dollard further inland and which was the sampling location for extensive studies of larval flounder. The hypothesis that larval concentrations are homogeneous across the width of the channel was rejected. Larval concentrations showed heterogeneous tidal variations at the different positions on the cross-sections in the middle part of the estuary, and moreover, mean concentrations differed. Larvae tend to concentrate in the main stream. Given the large variability in larval concentrations the data were not sufficient to determine tidal patterns in the across-channel distribution. The accuracy of the estimates of overall transport through the channel section improved by increasing the number of sampling positions per cross-section. However, some of the individual sampling positions can provide transport estimates that approach the overall transport based on all four sampling positions. These are generally the positions in the centre of the channel with the largest water transport. In the more landward cross-section of the Dollard tidal variations in larval concentrations were similar and mean concentrations differed between the positions only during part of the tidal cycle. Concentrations tended to be higher in the flood-branch of the channel. The estimates of the net transport were affected in an unpredictable way, although measurements at the ebb-dominated position on the channel cross-section underestimated both the flood and ebb transport of flounder larvae. The data do not allow a reliable correction of larval-transport estimates that are based on only one position per cross-section. © 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.