No abstract

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.

Sources of pollution at the north end of Lake Tanganyika are concentrated around Bujumbura, Burundi . The impact of River Ntahangwa on the water quality of this part of Lake Tanganyika was compared with that of an unpolluted river, the Mugere . A shallow bay, not influenced by a river, was used as a reference station . Chemical analyses were carried out at four week intervals during nearly a year . Samples were taken at different distances from the shore and at different depths . River impact was detected up to 100 m from both river mouths . Even at 50 m from the mouth of the polluted River Ntahangwa, no important decrease of dissolved oxygen was detected . Chlorophyll-a maxima were found 100 m from the Ntahangwa mouth, but this was also the case 100 m from shore in the unpolluted bay . Lake water at 60 m depth was saturated with oxygen during the dry season, as it used to be in 1956 . The rivers studied descent initially within the lake due to greater sediment load and lower water temperature . Nevertheless, it appears that these rivers mix completely in the lake within 100-300 m from the river mouths . The direct effects from organic pollution of the Ntahangwa on the lake seem to be rather negligible . Appropriate anti-erosion measures should reduce massive sediment discharge into the lake .

No abstract

The abundance/biomass comparison (ABC) method, proposed by Warwick (1986) for detecting pollution effects on marine macrobenthic communities, was tested on distribution patterns of numbers and biomass among fish species in regulated and non-regulated lowland rivers in Belgium. The biomass and abundance of the fish populations were determined by electrofishing. The sample sites represent a range of physically disturbed watercourses, from natural meandering to heavily channelized reaches. Sites upstream and downstream from a polluting discharge were also evaluated. The physical habitat and water quality were measured on each site. Our data show a significant correlation between the qualitatively measured physical habitat and an ABC index, calculated from the ABC graph data. A significant correlation was also found between the pollution level and the ABC index. It is concluded that the ABC method is applicable to the assessment of disturbance in fish communities in rivers, and that the method gives information about both pollution and physical disturbance. The ABC method is a useful instrument to assess the status of a fish community before and after river channel works or natural and human induced river restorations.