Ecology of the Suldalslågen river in western Norway before its regulation

Abstract

The River Suldalslågen, 22 km in length, situated in Rogaland province in western Norway, has been studied over a period of four years. The macro-invertebrate benthos was low in abundance especially in August and September. The main biomass was contributed by large case-bearing caddis-fly larvae, such as Potamophylax latipenis, which are shredders. The fauna composition of the upper and lower reaches of the river differed widely. Filter feeders, such as the caddis fly Polycentropus flavomaculatus, occurred in large numbers below the lake outlet, while shredders were predominant in the lower reaches. Large quantities of planktonic Crustacea, washed out ot the Suldalsvatn lake, were present in the river during the summer months. The Suldalslågen river is primarily a recruitment area for salmon and sea trout. Interestingly, the diet of the young salmonids did not show a close relationship to the available invertebrate biomass. For example, young salmon almost completely neglected the large caddis-fly larvae, while planktonic Crustacea washed out from Suldalsvatn and adult insects were taken in large numbers. A large proportion of the food supplies of both the salmon fry and parr were thus produced outside the river system. In 1962, for example, 34% of the food of the parr and 43% of that of the fry was produced in other, ecosystems than the river one.