Fish communities and migrations in the vicinity of fishways in a regulated river (Enz, Baden-Württemberg, Germany)

We evaluated the effects of fishway design and stream habitat on fish distribution and migrations within a 50 km section of the Enz River (108 km long, 16.7 m³ · s⁻¹ mean annual discharge), a second order tributary of the Rhine River. On 12 different occasions between August 1994 and September 1995, we collected fish electrically upstream, downstream, and inside three different types of fish-passes: a concrete channel (site I), an artificial stream (site II), and a step and pool fishway (site III). During electrofishing, species identity and fish lengths were recorded separately for specific habitats or sections within the fishways, and traps were installed for 3–5 week long periods at the upstream outlets of the fishways. More than 3550 fish >5 cm and approximately 5800 young-of-year individuals of 28 species were captured. 772 fish marked by subcutaneous injection of alcian blue dye were released downstream of two fishways. Almost 1150 fish, including 5 marked individuals, were captured in the traps, providing direct evidence for migration through the fishway. The total number of species recorded decreased downstream from 21 at site I to 18 at site II, and 17 at site III. This shift in numbers and also in species composition between sites could be partially related to differences in the available habitat. There were pronounced changes in the abundance of most of the dominant species downstream and upstream of the fishway at site III, and, to a lesser degree at sites I and II, indicating species (and size) specific differences in the effectiveness of the three designs for fish passage. Furthermore, marked differences in relative fish abundance between downstream locations and inside the fishway itself, and in the relative numerical composition of the trap catch at sites I and II, clearly demonstrate that some species (Cottus gobio, Barbatula barbatula, Salmo trutta, Leuciscus cephalus) use these fishways as a habitat, whereas others (Phoxinus phoxinus, Gobio gobio) simply migrate through. Only inside the artificial stream did we find early life stages of seven species, suggesting that passage of the other fishways is restricted to older fish. Our results demonstrate that fishway design is crucial in providing continuous fish passage for all species and life-stages, and that a nature-like fishway is the most functional solution.