Figure 1.

Sampling locations of 20 freshwater populations of the three-spined stickleback (Gasterosteus aculeatus) in the eastern sub-basins (Dijle and Demer) of the Scheldt River in Belgium (see inset). The most downstream population (S4a; not shown) and one more barrier are located 37 km east of population S5a. Red, blue, yellow and green dots represent water mills (n = 46), weirs (n = 57), tunnels (n = 14) and sluices (n = 4), respectively. Small arrows mark flow direction. Codes as in Table 1.