|On the ecology of juvenile plaice on a tidal flat in the Wadden Sea|In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
The ecology of juvenile plaice, Pleuronectes platessa L., of the O-, I- and II-group in the western Wadden Sea has been studied on the Balgzand intertidal flats during the years 1972 through 1975. The present paper deals especially with density, growth and food-intake on 4 representative sampling squares, intensively sampled from March to October 1973.The O-group arrived as new settlers in the intertidal zone in April and reached maximum densities in June; the I- and II-group repopulated the area as early as March, reaching peak densities in May. In June and July densities of all age groups decreased rapidly, on the one hand as the result of mortality (the O-group) and on the other through emigration (I- and II-group). Since mortality in O-group decreased abruptly in July, from this age group relatively large numbers remained in the intertidal zone until October (end of observation period).Growth of O-group plaice on the Balgzand was relatively high as compared to that of the German Wadden Sea and British coastal waters. In I-group and particularly in II-group plaice, growth may be under-estimated due to gradual emigration of larger individuals.Study of the length-weight relation showed isometric growth in I- and II-group, and an allometric growth in the O-group; the smallest plaice were “too heavy” for their length.From density and growth data the productivity of the plaice populations was followed in the course of the seasons. Top values were found in April–May (II-group), May–June (I-group) and in June–July (O-group). Total production of plaice tissue was estimated at about 5 grammes wet-weight per m2 in the growing period (March through October).Data on stomach contents and gastric digestion led to a description of the food composition, and to estimates on individual feeding rates, food conversion and population food-intake. When individual feeding rate is expressed relatively to weight of the fish, feeding appeared to be most intensive in spring. The same applies to the conversion of food into body tissues. Superimposed on this seasonal effect, both feeding rate per gramme fish and food conversion are negatively related to the age (or size) of the plaice.From data on population density and the individual feeding rate, population food-intake—predation pressure—was followed in the course of the season. Total food-intake in grammes ashfree dry-weight per m2 from March to October amounted to 0.25 g (O-group), 0.43 g (I-group) and 4.35 g (II-group); in total to about 5 grammes. Predation on the benthic invertebrate populations appeared to be most intensive from April to June. The biomass of the macro-benthos, however, showed in the same period an increase from 80% to 120% of the yearly average. Apparently maximum predation coincides with maximum productivity of the benthic invertebrates.Since survival and growth conditions seem to be very favourable on the Balgzand, the importance of the Wadden Sea as a nursery for the North Sea plaice was discussed.