Visbestandopnames op de Demer (2006)

We surveyed 13 locations of the River Demer on 3, 4, 5 and 20 April 2006 (Table 1 and Map). Fish assemblage data were obtained by electric fishing from a boat using a 5 kW generator (DEKA 7000) with an adjustable output voltage ranging from 300 to 500 V. The pulse frequency is 480 Hz. Electric fishing was carried out along both banks over a distance of 250 m. The locations and methodology used are given in table 2.

Abiotic parameters were recorded, they are pH, oxygen concentration (DO), conductivity, temperature and stream velocity (Table3). In Lummen a low DO was recorded (0.8 mgl^-1). Conductivity was high in Bilzen and downstream from Aarschot onwards.

Fish data include species, individual total length and weight (Tables 4 till 7). The Index of Biotic Integrity (IBI) was calculated for this and previous campaigns (1995, 1999, 2003) (Table 8). Figure 1 gives the evolution of the number of species collected at a site level for the period 1995-2006. Figure 2 gives the evolution for the number of individuals (per 100 m) over the same period.

In total we collected 21 different fish species: ten-spined stickleback, three-spined stickleback, perch, stone loach, Bitterling, roach, topmouth gudgeon now called stone moroko, bream, gibel carp, carp, silver bream, chub, eel, ruffe, rainbow trout, rudd, gudgeon, pike, ide, tench and pumpkinseed. The most common species were: gudgeon (fished in all locations), three-spined stickleback (12) and stone moroko (12). Roach was present in 11 locations. Upstream catches were more important than downstream. In 2003 most common species were roach (fished in all 9 locations), perch (8 locations) and stone moroko (8 locations). For the 9 locations surveyed in 2003 and 2006 the number of individuals caught in 2006 is double that of 2003. However species diversity was highest in 1999. Species diversity in 2006 is comparable with that of 1995 or it declined compared to 2003 and 1999.

The IBI is poor in 8 locations and moderate in the other sites. This is comparable with the results obtained in 2001-2003. This status quo in water quality vis à vis 2001-2003 or the decrease in biotic integrity compared to 1999 indicates that we must remain alert and should at all cost put effort to improve the water quality of the River Demer.