The River Suldalslagen, which holds a population of large Atlantic salmon, has been regulated twice for hydropower production. The first regulation occurred in 1968 and the second in 1980. Present problems include the reduced density of benthic fauna, the reduced growth rate of young salmon, the low survival of O+ fish and the increased time required for smoltification. A programme of habitat restoration includes building a rearing channel system where water flow and the substrate can be controlled. The salmon fry are stocked in the rearing channel and in an adjacent tributary stream. The effects on macrobenthos of introduced dead organic material were also studied. Improvement of physical habitat increased the density of benthic animals, and the survival of 1+ salmon was about 30%. Experiments that included adding of 115 g wheat/m2 resulted in a threefold increase in benthic fauna compared with a control area. The largest increase in numbers was in Chironomidae in August-September, when benthic Crustacea also showed a significant increase. An increase in macrobenthos is expected to increase the growth and survival of young salmon fry.

Zonation studies of the benthic invertebrate fauna of stony substrates were made along a longitudinal gradient (600 km) in the Glomma river which drains 13% of Norway's land area (41767 km2). Wide differences in the faunal composition were found between the upper, middle and lower reaches of the river. Plecoptera were most abundant in the upper and Chironomidae and Trichoptera in the lower reaches. Ephemeroptera were abundant throughout the river. The faunal composition showed a dominance of shredders in the upper, grazers in the middle and collectors in the lower reaches. Some local effects of human influence were observed, but no general changes from the natural faunal composition and zonation seem to have taken place. The benthic data are in general agreement with the predictions of the River Continuum Concept (Vannote et al. 1980).

SUMMARY. 1. The duration of egg incubation (Y) in Dinocras cephalotes and Siphonoperla burmeisteri was related to constant temperatures from 4 to 24°C, by the regression equations Y=2382 T −1, 402(r2=0.992, P<0.001) and y= 2683 T−1.667 (r2=0.994, P<0.001), respectively. No diapause was observed in either species.2. Egg incubation in D. cephaloles was slow and took 784.9±92.7 (mean ± SD) degree days between 12 and 20°C. significantly more than in S. burmeisteri(445±76.17 degree days: t= 7.44. d.f.=13,P<0.001).3. For D. cephalotes hatching occurred at temperatures between 12 and 24°C, and for S. burmeisteri between 8 and 20°C. The mean volume of the eggs of D. cephalotes was about 5 times greater than that of S. burmeisteri and the mean body lengths of the newly‐hatched nymphs were 1.13 mm and 0.95 mm respectively.4. This study shows that the freshwater fauna of northern Fennoscan‐ dia also contains species with warm stenotherm eggs. D. cephalotes. which is of a Mediterranean origin (Zwick, 1981a), may exist at the limit of its distribution in northern Fennoscandia.

The duration of the egg incubation period, the hatching success and nymphal growth of three local Norwegian populations of Leuctra hippopus Kempney in relation to temperature were studied in the laboratory. Eggs were reared at temperatures between 2 and 24°C, nymphs between 4 and 20°C. Intraspecific differences existed in egg survival rates at the different temperatures, and in the body size of the newborn nymphs. The populations also had specific optimal hatching temperatures. However, no significant differences were found between the populations in regard to temperature (T, in °C) and the length of the egg incubation period (y, in days). This relationship, on a logarithmic scale, was linear and could be approximated by the regression equation y = 519T -1·167. Population differences in the survival rates of nymphs were also found. The growth (G) measured by the body length of nymphs in relation to age (D) could be given by a simple linear regression equation G = a + bD. For the three populations studied growth was temperature dependent and significant differences in growth rates were noted at certain temperatures. Based on the results presented the following hypothesis can be given. During the post-glacial period, natural selection has led to the development of specific growth rates of different populations of L. hippopus that are adapted to the local environmental conditions so that emergence occurs at the correct time of the year for optimal adult survival and reproduction.

AbstractIn Fennoscandia the highest number of Nemoura species (6) occurs in the north. Three species, Nemoura arctica, N. sahlbergi and N. viki, are restricted to the north. Three other species occur throughout much of Fennoscandia, while in the south there are four and in the costal areas in the west only a single species. Nymphs of the seven species of Nemoura occurring in Fennoscandia are described. The first instar nymphs of five of the species have also been examined. A key for species identification of nymphs is given and the nymphs of Nemoura viki Lillehammer and N. sahlbergi Morton are included in a key for the first time.

Field studies of Nemoura arctica and N. viki showed that the two species preferred different biotopes and indicated differences in their life‐cycles. N. arctica seemed to have a semivoltine life‐cycle, at least in some years, this was not the case for N. viki. Laboratory studies showed that the temperature tolerance of the eggs of N. arctica was wider than for those of N. viki. The length of the egg incubation period of both species was influenced by the ambient temperature, but no significant interspecific difference existed between the regression lines of the relationship between the temperature (T°C) and egg incubation period (Y days), as given by the regression equation Y = aT−b for the log values. Reared at constant food supply, nymphal growth occurred in two periods. Firstly a rapid growth to about 4 mm, followed by a period of slow growth until emergence. During the first period growth (G)at (D) days was linear, according to the simple linear regression equation G = a+bD . The temperature tolerance of the nymphs of N. arctica was wider than for those of N. viki and significant interspecific differences between the species in growth were recorded at relatively high rearing temperatures, such as 12° and 16°C, but not at low temperatures. At constant food supply, nymphal growth was greatly influenced by the rearing temperature. At a mean temperature of 16°C N. arctica nymphs grew rapidly and emerged after 120 days; whereas at a mean temperature of 4°C growt h was slow and the nymphs did not even manage to reach the emergence stage after 700 days.

The composition of the stonefly fauna, the growth phases of all the species and the food composition of the predatory species (Fam. Perlodidae) were studies in a lake outlet biotope at Valdresflya in the Jotunheimen mountains, in central southern Norway. The biotope, in a region with a subarctic or tundra macro-climate, is ice-covered from early October to late June. Five differen stonefly species were studied: Arcynopteryx compacta, o-Diura nanseni, hoperla obscura, Capnia atra and Amphinemura standfussi. The two herbivorous species, C. atra and A. standfussi have completely separate life-cycles. In contrast, the nymph stages of the three predatory species all occurred at the same time of the year, although separated by size during the growth period and with little real overlap in their food habits. All the three predatory species were omnivorous, eating plant detritus and algae at certain periods of the year.

Forty species of stoneflies, belonging to seven different families, occur in Fennoscandia. In Norway all these families are represented in the inner fjord areas and in the continental lowland. The highest number of species are found in continental areas subject to a wide annual amplitude in temperature and the lowest number in coastal areas with a small temperature amplitude. In Fennoscandia the highest number of different stonefly species have been recorded from areas north of the polar circle and in the continental parts of the boreal and subalpine vegetation belts. The lowest number have been recorded from the coastal areas and on the islands. The former is due to the strong represen- tation of north-eastern species, as exemplified by the genus Nemoura, and the latter phenomenon is also due to special ecological factors. Predatory species are sparsely distributed in the coastal areas in southern Fennoscandia and are absent from islands such as Gotland, Oland and Bornholm. They are commonest at high altitudes and latitudes, being especially abundant in the northernmost parts of Norway, such as the areas close to the North Cape. The herbivorous species of the genus Amphinemura are especially abundant in south-western Norway, also to some extent in the east. Capnia species are most abundant at high altitudes and latitudes and is sparsely distributed in southern Fennoscandia. In northern Norway, Nemoura species are often the most abun- dant herbivores, while N. cinerea is the most successful species in the coastal areas and on islands such as Oland and Gotland. One or two Leuctra species seem to be abundant in most parts of Fennoscandia. The individual stonefly species show local differences in distribution, largely due to differences in dura- tion of their egg incubation periods and in the growth rates of the nymphs.

The river Storelva, situated in Sauda, Rogaland in western Norway has been regulated for hydroelectric power since 1914 and about 80% of the water-flow is transferred through tunnels to the various power stations and then to the fjord. The river held a uniform benthic invertebrate fauna which was dominated numerically by chironomids and a few other groups, viz. the trichopteran Polycentropus flavomaculatus and the stoneflies Amphinemura sulcicollis and Leuctra fusca. The feeding groups collectors, both filterers and gatherers, and predators were strongly represented in this regulated river. The population of salmon was negligible and only low numbers of trout occurred. The young trout mainly fed on the chironomid larvae.

Studies of the egg incubation period of Leuctra fusca (Linnaeus 1 7 58 ) and L. digitata Kempny 1 899 were made experimentally at water temperatures of 2 °, 4 0, 8 °, 1 2 °, 1 6 0, 20 ° and 24 °C . Both the egg survival rates and the differences in the duration of the incubation period showed that the eggs of both species are greatly influenced by ambient water temperature [T] . Development was extremely slow at low T, but no diapause was recorded . Differences in egg survival were noted at different T, but no specific difference in the length of the incubation period was recorded. For both species a combined regres­ sion analysis of the relationship between temperature (T °C) and the length of the incuba­ tion period (Y days) yielded the following equation : y = 3 1 6 T-0.857 , r = 0 .976 (p < 0 .00 1 ) . The growth (G) of nymphs at (D) days can be expressed by the simple linear regression G = a + bD, (a and b are constants) . The growth-rates (b) of nymphs of both species were T dependent . Where both species occurred in the same biotope, significant differences in the growth-rate were found at a T of 2 °C, but not at 4 °c and 1 2 °C.

Twenty stations were sampled monthly during the Ice free period (June‐September) in 1972. Emergence traps, emptied daily, were also employed. The major macroinvertebrates in the exposed zone were Ephemeroptera, Gammarus lacustris, Trichoptera, Chironomidae, Plecoptera and Coleoptera, and these constituted over 90% of total numbers. Densities of G. lacustris, Ephemeroptera, Tipulidae and Plecoptera showed a positive correlation with detritus. No significant relationship was found for the other taxa. Nearly 70% of the fauna emerged, and did so during the period from June to September. Chironomidae, Ephemeroptera, Plecoptera, Trichoptera and Tipulidae accounted for nearly all emergence and their total average annual emergence was 372 cal m−2. Of this total, Chironomidae constituted 28%, Ephemeroptera 28%, Plecoptera and Trichoptera 15% each and Tipulidae 4%.

For the purpose of benthic studies Øvre Heimdalsvatn was divided into three depth zones. These were the exposed zone (from the shore down to 1–2 m depth), the macrophytic zone (1–2 m to 5–5.5 m depth) and the non‐macrophytic zone (5–5.5 m to 13 m). The reasons for this division and the main characteristics of each zone, including the substrata present, are given.

The lotic communities in Øvre Heimdalen were completely dominated by insects. The species distribution of stoneflies (Plecoptera), mayflies (Ephemeroptera) and caddisflies (Trichoptera) is given together with autecological notes on the Plecoptera and certain other major species. The species distribution of these groups, especially the Plecoptera, was clearly related to the deciduous vegetation along the streams. There was a reduction in species parallel to a reduction in Salix vegetation.The main inlet stream to the lake, Øvre Heimdalsvatn, the outlet and the outlet stream had different faunal compositions. The inlet fauna consisted to a large extent of winter growing species, mainly detritus feeders, while the outlet and outlet stream had a fauna dominated by summer growing species, several of which were filter feeders.

Trichopteran larvae were sampled on soft substrata at all depths, on stony substrata in the exposed zone, and the adults in emergence traps, placed along the lake shore. Fourteen species were taken as larvae in the lake. On stony substrata and in the exposed zone Polycentropus flavomaculatus (Pictet) was dominant, followed by Limnephilus nigriceps (Zetterstedt) in terms of numbers. In terms of biomass L. nigriceps dominated followed by Potamophylax cingulatus (Stephens).Species which usually inhabit running water made up for a larger part of the trichopteran fauna of the exposed zone.The two most numerous species showed different habit preferences. P. flavomaculatus was most common on stable stony bottom, while L. nigriceps was most common on unstable stony bottom. The other species showed no significant preference. On unstable soft bottom at 3 m and below, Mystacides azureus (L.) dominated, accounting for about 90% of the total trichopteran fauna both in terms of weight and numbers.The major trichopteran species in the lake were either detritus feeders, such as M. azureus, L. nigriceps and Potamophylax spp. or omnivores such as P. flavomaculatus and Molanna albicans (Zetterstedt).

The Norwegian subalpine lake. Øvre Heimdalsvatn, has a surface area of 0.78 km 2 and a maximum depth of 13 m. It is ice‐covered for 7.5–8 months, has a marked spring spate and a mean annual renewal period of about two months. The water is poor in electrolytes. Intensive studies have been made by an interdisciplinary team of the lake's physical and chemical properties, primary production and secondary production under the auspices of IBP/PF from 1969 to 1973, Allochthonous material accounted for 1/3 of utilized plant input. The major lake predator, the brown trout, fed largely on benthic organisms and did not exploit the zooplankton biomass. On account of the long period of ice cover and the rapid rise in temperature after ice break, many organisms (both planktonic and benthic) showed synchronous development. Abiotic conditions, such as the nature of the spring spate and the temperature rise, strongly affect species and community development.