Samenvatting

Population size increase at the Balgzand intertidal in the western Dutch Wadden Sea suggested a growth decrease in both plaice and flounder populations over summer. Research into the seasonal growth of 0-group plaice, Pleuronectes platessa, confirmed this suspicion. The years 1995 and 1996 had contrasting winter temperature regimes. However, as both years showed summer growth reduction it is suggested that benthos activity in summer might be the underlying mechanism. To further investigate the extent of this suspicion the seasonal growth patterns of 0-group flounder, Platichthys flesus, was researched.

Samples were taken on a grid of 36 stations using a 1.9m beam trawl (1 ticker chain, net mesh size of 5x5mm) at frequent intervals (2-4 weeks) from February to August in 1995 and March to August in 1996. In the laboratory samples were sorted and all individuals measured to the nearest mm total length and fixated in 96% ethanol. Per sampling campaign +/-100 individuals of different size classes (mm) were selected in relation to their abundance. Daily growth increments of the otoliths were analysed using a light microscope. The observed growth of the 0-group flounder population were compared to DEB simulations showing maximum potential growth.

Results show that water temperatures were much lower in February and March of 1996 compared to 1995 (ANCOVA, p<0.05). Larvae settled about 4 weeks earlier in 1995, densities of the entire 0-group population was higher in 1995 compared to 1996, however, peak densities of newly settled flounder was about the same in both years. The growth curves of 1995 and 1996 seem to be different. Comparing observed growth to predicted growth from the DEB models it is seen that individuals in 1995 were able to reach maximum potential growth (realized growth ratio (RGR) ±1), while in 1996 observed growth is even below the male DEB prediction (RGR average max 0.7).

Data analysis indicates that in 1995 the DEB model is a good fit with the observed growth while in 1996 it is not. However, it can be said that the upward trend in RGR of 1996 suggest that individuals were not able to reach maximum potential growth in the beginning but over time were moving more towards the predicted growth.
The differences seen between the plaice and flounder study might be explained by species specifics such as settlement habitat selection of which it is known that juvenile flounder inhabit the more silty tidal flat systems. Movement variation among size classes in flounder along abiotic factor gradients such as salinity, temperature variations and depth, might also explain the differences. Lastly, it could be caused due to differences in prey composition.

All in all, it can be said that for European flounder at the Balgzand summer growth reduction does not take place. Taking all findings of both researches into account the underlying mechanism is suggested to be the difference in composition and activity of the benthos over time. Further research into this topic should shed light on the underlying mechanism of population size decrease in the summer months.