Summary1. The effects of high spring floods on survival and growth of juvenile Atlantic Salmon, Salmo salar, and Brown Trout, Salmo trutta, are explored, using data from a long-term study in the River Saltdalselv, northern Norway. The flow regime in this river is typical for northern rivers. 2. There was considerable variation in year class strength of both species. 3. Mortality of Atlantic Salmon increased significantly in years with high discharge during the alevin stage as well as the first week after emergence. High discharge during the egg stage and more than 1 week after emergence seemed to be of minor importance. Water temperature at emergence was rather high (average 10·5°C) and did not significantly affect year class strength. 4. Brown Trout emerged earlier than Atlantic Salmon at an average water temperature of 8·2°C. Highest mortality was observed in years with low water temperatures at emergence as well as high discharge during the alevin stage. 5. For 1-year-old fish or older, the size of the spring peak flood did not influence mortality significantly. 6. Growth of Atlantic Salmon parr was diminished in years with a high peak spring flood. A similar effect on Brown Trout was not detected. 779Floods and survival of juvenile salmonids stages, but little is known about the relative susceptibility of different life stages and the functional relation between discharges and mortality. A washout effect from abrupt increases in stream discharge has been observed for young fish with limited swimming ability (Heggenes & Traaen 1988). However, the susceptibility to washout may decrease rapidly with increasing fish size (Heggenes & Traaen 1988). Obviously, moderate flows at a particularly sensitive stage may cause higher mortality than considerably higher flows at less sensitive stages.In the present study we quantify the effects of spring peak floods on the same populations over many years, using data from a long-term study of Atlantic Salmon and anadromous Brown Trout from a river in northern Norway. In the River Saltdalselv, we estimated densities of Atlantic Salmon and Brown Trout parr in a systematic way for 22 years . For each year, we also estimated the time when eggs hatched, and the swim-up time of fry. From these data, we analysed the effects of maximum discharge on survival and growth of fish, elucidating the stages that are most sensitive to high discharge. Materials and methodsThe River Saltdalselv in northern Norway (67°N, 15°E) has a catchment area of 1550 km 2 (Fig. 1). The mean annual water discharge is 12·1 m 3 s -1 at our sampling station (Station D), increasing to 55·4 m 3 s -1at the outlet to the sea. The flow regime is typical for northern rivers (Fig. 2). Water quality, including pH, is favourable for salmonids (Koksvik 1977). Atlantic Salmon and anadromous Brown Trout are the dominant fish species in the river. There are no predatory fish present (except a few eels and cannibalistic trout and salmon). Also, predatory birds are few. Both species typically stay 4 or 5 years (range 3-7 yea...

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Over the 6 years 1980-85, 212 Norwegian rivers have been examined for occurrence of Gwoducrylus sularis: it was found in 26 rivers and six salmon hatcheries scattered throughout the country from Troms county in the north to Sogn og Fjordane in western Norway. The distribution of G. salaris is connected with the stocking of fish from infected salmon hatcheries. The populations of salmon parr have been drastically reduced in the infected rivers. In later years catches of ascending salmon in these rivers have also sharply declined: in 1984 salmon fishery losses were estimated at 250-500 t.Gyrodactylus suluris is most probably a recent introduction to Norwegian rivers. A primary aim is to exterminate this parasite from all infected rivers and hatcheries: so far this has been accomplished in one river and one hatchery.

1.A new laboratory-based growth model for brown trout (Salmo trutta) was used to explore latitudinal variation in growth among natural populations. The model included the eects of dierences in ambient temperatures and ®sh size among populations. Annual growth rates of anadromous brown trout parr from 22 Norwegian populations at 61±70 N were compared with predictions from the growth model. Published ®eld data from one Spanish, 15 British and four Danish populations at 44±58 N were included in the analysis to increase the latitudinal range. 2. Among the Norwegian populations, the ratio between observed and predicted growth rates was not signi®cantly dierent from 1Á00 in eight rivers, but was signi®cantly higher in eight, and was signi®cantly lower in six. Observed growth was highest, relative to predicted growth, in the coldest rivers. In Spanish, British and Danish rivers, observed growth did not exceed predicted growth. 3. The ratio between observed and predicted annual growth rate decreased signi®cantly with increasing annual mean temperature. Observed annual growth was higher than predicted growth only in rivers with an annual mean temperature lower than 5Á1 C, and this indicates that some kind of thermal adaptation may occur in trout populations in the coldest rivers. 4. Regression analyses showed that besides the direct eects of temperature and body size predicted by the growth model, annual growth rates were signi®cantly related to annual mean temperature, densities of juvenile salmonids, duration of twilight (average for May±August) and latitude. Adding these variables to the original model increased the explanatory power from 73Á9 to 80Á6%.

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Daily ascents of adult Atlantic salmon (Salmo salar) in the Laksforsen waterfall in the River Vefsna were recorded over 3 years. Ascent was correlated to 12 physical and meteorological parameters by multiple regression analysis. Change in water temperature was included in the regression model in all 3 years, and change in river flow was also included in 2 of the 3 years. No other variable was included in the regression model at the 0.05 significance level.

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Abstract. Hatchery‐reared brown trout, Salmo trutta L., yearlings were captured shortly (3h to one week) after their release in a Norwegian stream. The feeding of recaptured hatchery fish was compared with that of wild brown trout. The investigations were carried out during three different periods (May, July and October). Investigations of drift fauna indicated that food availability was best in May. Most hatchery‐reared brown trout started feeding shortly after their release in all three periods. Hatchery fish went through a learning process with respect to feeding. This was most clearly demonstrated by the amounts of plant fragments in their stomachs, which were always greater in hatchery fish than in wild fish but which decreased with time after release in hatchery fish stomachs in all three periods. By about a week after release, hatchery trout appeared to be feeding on wild prey nearly as well as did wild fish, but they achieved this better in May than in October.