Abstract. The movement pattern and the abundance of perch, Perca fluviatilis L., were studied in the Kleine Nete, a eutrophic, canalized lowland river in north Belgium. Since the behaviour of perch is strongly affected by habitat type, two kinds of river section were chosen, based on physical characteristics: homogeneous sections with regard to depth, bottom type and current speed; and heterogeneous sections with a riffle and pool pattern. The fish were caught by electrofishing and all perch larger than 10 cm were marked individually. The population densities were much higher in the heterogeneous parts. In these sections about 30% of marked perch stayed within a home range, while in the homogeneous sections all were mobile. The older individuals showed a stronger tendency to accept a home range than the 1‐ or 2‐year‐old perch. This difference may be explained by different behaviour: young perch live in small schools, while older perch are often solitary. The low densities and the high mobility of perch in the homogeneous river sections indicate that these parts of the river are not a suitable habitat for perch. Habitat improvement of the homogeneous sections wilt probably result in higher overall population densities.
Modifications to a standard biological water quality assessment method, used in Belgium since 1979, were studied.As a result, we recommend the following: -samples should be processed live -samples should not be washed through a series of sieves -systematic units represented by only one individual should be included in the calculation of the biotic index.Using this modification, the biotic index can be calculated more rapidly and shows a higher correlation with a chemical water quality assessment index.
METHODS AND MATERIALSThe data for the present study were collected in April and September of 1977 and in April of 1978. Altogether 58 samples were taken in 23 brooks belonging to the river basin of the Kleine Nete, and classified as the lowland-type of flowing waterways having an EEC~code of Wl, G4, H2-3 and a temperature T which is low in winter and rather high in summer (PERSOONE, 1977). The discharge of the rivulets changes within a great range, because most of their water is supplied by rain. During dry periods the brooks exhibit low water levels; they might even dry up.The invertebrates, mainly benthic organisms, were collected from plants and the beds of the brooks. A standard hand-net with a mesh width of 0.5 mm and an opening diameter of 30 cm was moved upstream along the bottom and the banks of the brooks, mostly covering a distance of 10 m. Each habitat was sampled in relation to its relative appearance within the sample; e.g. when Elodea covered 40 % of the bottom surface, 40 % of the sample was taken in the Elodea beds. The animals were preserved in 80 % ethyl-alcohol.Before the samples were taken, the temperature, the flow rate of the water and a cross section of the brooks were measured and the different substrates were noted.
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