We surveyed 12 locations in tributaries of the River Maas on 13, 14, 15 March and 18 April 2006 (Table 1 and Map). Fish assemblage data were obtained by electric fishing from a boat or wading (Table2). Abiotic parameters were recorded, they are pH, oxygen concentration (DO), conductivity, temperature, turbidity and stream velocity (Table3). Fish data include species, individual total length and weight (Table 4). We also included results from previous campaigns (1996 and 2002). We give the morphometric specifications in table 5 and the catch per unit effort (CPUE) in table 6. The Index of Biotic Integrity (IBI) was calculated for this and previous campaigns. In table 7 we give the IBI for the River Jeker and tributaries and in table 8 for the River Ziepbeek and tributaries.
In the River Jeker we caught three-spined stickleback, stone moroko, gibel carp, carp, rudd and ide. In the Oude Jeker only three-spined stickleback was collected and no fish in the Beek. In 1996 we still collected stone loach and gudgeon in the River Jeker. At present fish diversity and densities are low and reflect the poor ecological quality the River Jeker and its tributaries have. The River Ziepbeek has more potential than the River Jeker which is reflected by the presence of the brook lamprey (upstream). However, the presence of exotic species such as brown bullhead, striped mudminnow and pumpkinseed decrease the calculated IBI value even though other species such as roach, perch and tench were collected. In the tributaries no fish or only two specimens of brook lamprey were caught.