In 1985, there was fishing taking place on a more or less monthly basis, in the shallow sub littoral (3-4 m at low water) of the sandy beach, in front of the nature reserve of “het Zwin”, in the neighbourhood of the Wester Scheldt estuary. With the help of a 1.90 m fine-meshed (10 mm straightened mesh) tree-dredge-net and pulled by a vessel with little profoundness, every morning at low water a known surface was being fished. The fish were immediately benumbed and then fixed, considering the stomach analysis. For the used fishing tools, the net efficiency is known for certain species so that absolute densities of the demersale fish fauna could be calculated. Seeing the limited ability for capture of larger (<120mm) fish, apart from the non-commercial species, juveniles of commercial species were taken into account when calculating. 30 different fish species were caught. Two difficulties arose with the identification. A comparative study of postlarvae and herring-like (Clupeidae) turned out to be necessary because the available literature data were not sufficient for determination. It was found that only counting of the myotomes offers sufficient support to the discrimination herring (Clupea harengus) and sprat (Sprattus sprattus), at least for length classes up to 35 mm. In agreement with Hamerlynck & Janssen (1985) we noted, apart from Pomatoschistus minutus also P. lozanoi in that area. The latter is not being considered separately in certain, even recent, publications. For P. minutus, P. lozanoi, Pleuronectes platessa and Clupea harengus the observations were regularly spread over the course of the year, so to be able to follow the life history of these species. For the two latter species, we only have data of the 0+ and the 1+ year classes. Amongst other, we can deduce that Pleuronectes platessa, with the literature. For some commercially important species, the area has an important function as nursey: Pleuronectes platessa, Solea solea,Limanda limanda, Clupea harengus and Sprattus sprattus. In winter time immigration can be noted from brackish water species, originating from the Zwin and/or the Wester Scheldt: P. microps, Gasterosteus aculeatus and Pungitius pungitius. Two species of sand groundels together (P. minutus and P. lozanoi) reach an average density each year of 1 individual per 2 m2 in this area. The density of P. lozanoi lies 4x higher than the density of P. minutus. On the second pace we have the plaice (Pleuronectes platessa) with 1 individual per 10 m². Densities of Solea solea and Limanda limanda reach in certain months also high values of 1 ind./ m². The biomass of the most important species was calculated according to the length-axe free dry weight (AFDW) regressions originating from calculated absolute densities. P. platessa, represented in biomasses, the highest value namely 0.8 g AFDW per m² per year. P. minutus and P. lozanoi together reach a value that is half as low. To calculate the production, the classic method of Allen amongst others, turned out not to be suitable for the data that are gathered this way. Only the Crisp (1984) method turned out strong enough to be used on these data. Because the fitting of a grow comparison to the insufficient dataset could not be done in a satisfactory way, it was opted for exponential interpolation of the grow curb, separating the cohorts “the visu”, for the determination of G (the instantaneous growth rate). The production of 0+ P.platessa grind out in the winter of 1984-1985 is being estimated at 0.06 9 AFDW / m² per year. The total yearly consumption of the sandground is and the plaice together is being estimated at 10 g AFDW / m². The latter guarantees 65% of that value. Expressed in energy-units, the two ground species consume together more or less 20 |
