Het functioneren van de Eems-Dollard als kinderkamer voor platvis: 3. Aantalsverloop en lengtetoename van juveniele schol, bot en tong in Dollard en Eems
Jager, Z.; Kleef, H.L. (1999). Het functioneren van de Eems-Dollard als kinderkamer voor platvis: 3. Aantalsverloop en lengtetoename van juveniele schol, bot en tong in Dollard en Eems. Rapport RIKZ = Report RIKZ, 99(41). Rijksinstituut voor Kust en Zee: The Netherlands. 34 pp.
Part of: Rapport RIKZ = Report RIKZ. Rijksinstituut voor Kust en Zee (RIKZ): s-Gravenhage. ISSN 0927-3980
During the period of April-July young flatfish were found in high densities (flounder: max. 100 per 1000 m2, European flounder: max. 50 per 1000 m2, sole: max. 25 per 1000 m2) in the Dollard. The densities in the Eems were twice as low for the flounder and three times as high for the European flounder opposed to in the Dollard. In the Dollard the average density varied between the different tidal plates: the highest densities for the flounder and the European flounder during both years were found in the LP and the MP strata, plates with mixed sediment and centrally located in the Dollard east of the main berth, the Great Hole. The lowest densities, in the case of flounder, were found in the HZ strata (North of the Dollard, bordering the Eems) and on OP ( muddy highly located plate South East of the Dollard) or ( in the case of the European flounder) in the HZ stratum. The salinity seems to be an important factor for the existence of the flounder and the European flounder, which can be concluded from the special dispersion of the species in the Eems. Within the Dollard itself, special variations in salinity (some promilles in difference) are too small to be of any influence on the spread of young flatfish over the tidal plates. The low salinity from April until July 1994 could explain the fact that the young flounder could only been found in the Dollard for a short period of time that year: after the initial establishment of flounder larvae in the beginning of April there was a fast reduction in the density, possibly caused by emigration in turn caused by the unfavourable circumstances for the young flounders. Within the Eems there was a pronounced special division between the existence of three species of flatfish: the flounder existed only in the polyhalian part ( salinity value above 15 promille), Sole ( in the period around the establishment) especially in the mesohalian zone (salinity between 5 and 15 promille) and the European flounder in the meso and the oligohalian (salinity 0-15 promille) traverse of the river. Compared to 1993, a higher number of young European flounder was found in the Eems than in the Dollard, in 1994. Apparently the distribution of young European flounder varies between the two sub areas from one year to another. The domineering image in both years was that the smallest European flounders were found in the most upstream part of the Eems and the biggest ones in the polyhalian zone. Also within the Dollard the average length varied from one tidal plate to another: for the flounder this was the biggest in the centrally located and sandy stratum HP, and the smallest in the OP stratum that was also populated by low densities of young flounder. The OP stratum seems a less suitable habitat for young flounder. The average length of young European flounder was, like that of the common flounder the biggest in the HP stratum, and the smallest in the sandy and relatively low situated LP and LZ strata. In the course of time a special shifting of the highest densities of young European flounder takes place in the Eems. Combined with the previous observations this leads to the conclusion that the establishment of the larvae take place I the oligo and mesohalian zone, after which gradual seawards (polyhalian zone) migration of the biggest individuals happens. Apart from migration, also a difference in food supply or density depending growth in the Eems can possibly explain the difference in average length. These three factors are difficult to consider separately without further research.
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