WEIR BUILDING IN A REGULATED WEST NORWEGIAN RIVER: LONG-TERM DYNAMICS OF INVERTEBRATES AND FISH
✍ Scribed by FJELLHEIM, ARNE ;RADDUM, GUNNAR G.
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 568 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0886-9375
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✦ Synopsis
In the period [1975][1976][1977][1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989][1990] long-term studies of succession and dynamics of invertebrates and fish were conducted in a weir basin area in the strongly regulated River Ekso. During the years of the study, the invertebrate community in the basin was subjected to great changes. In the first years after weir building, biomass was greatest in the riffles due to a higher abundance of lotic species like the mayfly, Baetis, blackflies and many stonefly larvae. The biomass of oligochaetes and chironomids was similar both in the riffles and in the deeper and more lentic weir basin. In the following years the biomass of lentic chironomid species increased dramatically in the basin. In 1984 the fauna was dominated by Stictochir- onomus pictulus. In 1988 another species, Chironomus melanotus, also became very abundant. At this time net benthic animal production in the basin had increased 10-fold compared with 1975-1976. A high flow situation during the summer of 1989 altered the weir basin community dramatically. The mean autumnal biomass decreased 4.5 times compared with 1988, dominant lentic species disappeared and lotic/semi-lotic species like the stoneflies Amphinemura sulcicollis, Leuctra fusca and Capnia pygmaea increased in density.
Prior to regulation the density of brown trout in the riffle, which later constituted the weir basin area, was 2.5 individuals 100 m-2. During the first years after regulation and weir building, fish density increased to 11.1. In 1983 a density of 23.0 trout 100 m-* was achieved. The trout were stunted and showed marked tendencies towards food depletion. During 1984-1985 most of the brown trout population of the basin were removed and used as stock material in the reservoirs of the hydropower station. This resulted in a higher growth rate in the remaining weir basin population. The strong reduction in trout density was followed by major immigration of small (2+ and 3+) trout from the surrounding riffles to the basin. The trout population was now harvested, while a small population of adult spawners was retained.
Weir basins increase the area of pool habitats in strongly regulated rivers, and are of major benefit for trout populations, especially by segregating size classes and increasing winter survival. The presence of intermittent riffle sections is also very important, both as spawning and nursery areas and for fish food production.