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P. Aass
Directorate for Wildlife and Freshwater Fish
As, Norway


The brown trout populations of Norway, especially in the high-lying forest and mountain regions are mainly the result of introductions which go back to the Stone Age and have continued to the present day. As a consequence of the rediscovery of artificial propagation, the Government Inspectorate of Salmon and Inland Fisheries was established in 1855, and introductions and stocking became more organized.

About 3 million brown trout are now released yearly, of which fry and autumn fingerlings comprise the major part. About two thirds of the total number are stocked into hydro-electric reservoirs. The released fish are meant to supplement the natural recruitment, and the stocking density accordingly varies with the local conditions. Ten fingerlings to one hectare is a density often used.

To explore the effects of stocking, experiments now in progress include 2.5 million fin-clipped or tagged young. The work has mainly taken place in hydro-electric reservoirs with severe living conditions.

After some years of stocking with fingerlings, the proportion of released fish in the catches normally varies between 30 and 50 percent but extremes of 5 and 75 percent are recorded. On the basis of total catches and assuming a total rate of exploitation of 75 percent, it is estimated that in most cases less than 10 percent of stocked juveniles survive to catchable size. The number may be as high as 60 percent in lakes where low natural recruitment and high food production are combined. Increasing the size of the stocked fish will also increase efficiency. Depending on local conditions, one two-summer old juvenile is equivalent to 4–12 autumn fingerlings. In sparsely populated lakes this ratio may be smaller, in reservoirs with predators higher. Releases of fish of catchable sizes will normally give a return of 40–60 percent, depending on fishing intensity.

Stocking in spring is more efficient than in autumn and so is stocking directly into the reservoirs compared to release in tributaries. The use of fish-eating trout strains in reservoirs crowded with small pelagic fish may give rise to fast-growing individuals, attaining a maximum weight of 15 kg. Such strains are often used in the control of dwarfed char populations. Good returns are also achieved by releasing two-year old predatory trout into fjord systems without prior adjustment of the fish to brackish water.

At present prices, the value of fish flesh recaptured rarely exceeds the cost of stocking. Only the use of big predatory trout or fry and fingerlings in sparsely populated lakes may be directly profitable. However, good fishing is in many places a basis for the tourist industry. Income by hiring out fishing rights or sale of licences may surpass the purchase sum of the stocked fish. Furthermore, the accomodation of anglers will contribute to making stocking profitable.


En Norvège, les populations de truites (Salmo trutta) notamment dans les régions montagneuses et dans les zones forestières d'altitude, résultent essentiellement d'introductions. Celles-ci remontent à l'âge de pierre. En 1855, une fois redécouverte la propagation artificielle, la Norvège a décidé de créer une Inspection des pêches continentales et du saumon et on a commencé à organiser de façon plus systématique les introductions et le repeuplement.

On déverse chaque année dans les eaux intérieurs quelque 3 millions de truites; il s'agit surtout de fretin et d'alevins d'automne. Les deux tiers à peu près de ces truites servent à repeupler des réservoirs hydro-électriques. Le repeuplement est destiné à compléter le recrutement naturel et sa densité varie donc en fonction des conditions locales. Elle est souvent de dix alevins à l'hectare.

On procède actuellement à des expériences avec 2,5 millions de juvéniles marqués (coupe des nageoires ou autre méthode) pour étudier les effets du repeuplement, principalement dans les réservoirs hydro-électriques où les conditions de vie sont difficiles.

Après quelques années de repeuplement avec des alevins, les poissons qui en sont issus représentent normalement de 30 à 50 pour cent des captures mais on a également observé des minimums de 5 pour cent et des maximums de 75 pour cent. Compte tenu du total des captures et en évaluant à 75 pour cent le taux d'exploitation, on estime que, dans la plupart des cas, moins de 10 pour cent des juvéniles utilisés pour le repueplement atteignent une taille exploitable. Toutefois, cette proportion peut atteindre 60 pour cent dans les lacs où le recrutement naturel est faible et la production alimentaire élevée.

Le repeuplement donne de meilleurs résultats si l'on utilise des poissons de plus grande taille. En fonction des conditions locales, un juvénile de deux étés équivant à 4–12 alevins d'automne. Ce rapport sera sans doute plus faible dans les lacs peu peuplés alors qu'il sera plus élevé dans les réservoirs où il y a des prédateurs. En lâchant des poissons de taille exploitable, on obtiendra normalement un rendement de 40–60 pour cent, en fonction de l'intensité de la pêche.

Il est préférable de mettre les poissons à l'eau au printemps plutôt qu'en automne et de repeupler directement les réservoirs plutôt que leurs affluents. L'utilisation de races de truites piscivores dans des réservoirs très densément peuplés en petits poissons pélagiques permet d'obtenir des individus à croissance rapide, pouvant atteindre un poids maximum de 15 kg. Ces races sont souvent utilisées pour empêcher le développement de populations d'ombres nains. On obtient également de bons résultats en lâchant des truites prédatrices de deux ans dans les fjords sans les adapter au préalable aux eaux saumâtres.

Aux prix actuels, la valeur du poisson recapturé est rarement supérieure au coût du repeuplement. Seule l'utilisation de grandes truites prédatrices ou de frai et d'alevins dans des lacs peu peuplés peut être directement profitable. Toutefois, la pêche est souvent un attrait pour le touriste. Les recettes tirées de l'allocation de droits de pêche ou de la vente de permis peuvent être supérieures aux dépenses faites pour le repeuplement. Le logement des pêcheurs contribuera aussi à rendre le repeuplement rentable.


The brown trout (Salmo trutta L.) is the most widespread and important freshwater fish species in Norway. The populations, especially in the high-lying forest and mountain regions are mainly results of introductions which date from as far back as the Stone Age and have continued to the present day. As a consequence of the rediscovery of the artificial propagation, the Government Inspectorate of Salmon and Inland Fisheries was established in 1855, and introductions and stocking became more organized. Recently about 3 million brown trout are released yearly, fry and autumn fingerlings comprising the major part. About two thirds of the total number are stocked into hydro-electric reservoirs. The released fish now mostly supplement the natural recruitment.

To explore the effects, experiments including 2.5 million tagged and fin-cut young have been in progress in southern Norway for the last 20 years. The studies have mainly taken place in hydro-electric reservoirs with severe living conditions. Few results have been published (Aass, 1971, 1973, 1981) and the present paper tries to sum up the knowledge so far.


The returns depend both on local conditions, such as existing fish populations and food production, and experimental conditions, e.g., time of stocking and fish size and strain. Traditionally, stockings take place in autumn with fingerlings averaging 5 cm and 1.5 g. The stocking density mostly used is 5–10 fingerlings/ha, the reservoir areas measured at maximum water level.

After some years of stocking, the reared fish may constitute 30–50 percent of the trout catch in number. But great deviations from these figures occur with extremes of 5 and 75 percent having been recorded. The variations are due to different ratios of food production to recruitment. Stocking is only imposed when an impoundment is expected to interfere with recruitment. But experience shows that it is difficult to assess the rate of damage in advance. The figures mentioned include several year-classes of reared fish. The return from a single stocking mostly lies between 3 and 7 percent. Generally, the costs of stocking greatly exceed the value of fish flesh produced or harvested. However, the plantings imposed on the hydro-electric companies are not expected to be profitable but are intended to exploit the remaining fish food production. Stocking success may also be achieved. Two successive releases in a mountain lake situated 1 350 m above sea level yielded returns of 55 and 50 percent, respectively. In this case recruitment was reduced proportionally more than the production of bottom organisms. By continued stocking the return dropped to 25 percent. An even bigger decline may occur in the most oligotrophic reservoirs, e.g., Limingen, where the return decreased from 15 to 3 percent after few years of stocking. As frequently demonstrated, the rate of survival is density dependent. Thus, an increase of number planted rarely gives the results expected. By doubling the number of fingerlings released in Lake Tunhovdfjord from 5 to 10 per hectare, the return only rose by 40 percent.

Some of the bigger lakes harbour fast-growing trout strains, feeding entirely on prey fish. To stock such lakes with autumn fingerlings would be in vain, and two-year old trout are used. The return depends on whether the young are planted in the lower sections of adjacent rivers or in the lake itself. River stocking gives a high return in number because the groups are subject to exploitation before moving into the lake. Lake release gives a higher return of adult fish Accordingly, the return in number varies strongly, perhaps between 10 and 50 percent. Plantings of large young contribute 50 percent toward the maintenance of the most famous trout strain in Norway, the Hunder trout of Lake Mjosa.

Recently, fast-growing strains of fish-eating trout have been stocked into fjord systems, mainly the Oslofjord, without prior acclimatization of the young to brackish or salt water. The recorded returns average about 20 percent, but the fishermen are reluctant to report recoveries, and the real number is considerably higher. The fjord plantings are perhaps the most promising stocking project for the time being.

River angling for trout is the most popular recreational fishery in Norway. In many rivers, the demand for sport cannot be met by the local fish production. To enhance the resident river populations, plantings of 0+ and 1+ young seem to be of little use. Stocking with sizable fish is more rewarding both for economic and recreational reasons. Depending on exploitation, a return of about 50 percent may be expected, but the results are also influenced by fish size and time of planting. Put-and-take fishing is not yet common practice in Norway.

All the return figures have been given in percentage of number planted. Returns expressed as a percentage of total weight released may give a better indication of the success of stocking with larger young. The best return from Lake Mjosa is estimated to be about 600 percent and from the Oslofjord to 400 percent of the total weight released.


For introductions and plantings into tarns and lakes either empty of fish or with small trout stocks, fry is the natural choice. Yet only one experiment with fry has been carried out in Norway. The little Lake Ljosevatn, 1 100 m above sea level, harbouring a small resident stock was stocked every second year with 100 fry/ha. After 17 years, Dahl (1933) estimated the return to be 4.7 percent. The yields of the strengthened year-classes were doubled. When conditions become more complicated, especially in connexion with the development of the water courses for hydro-electric power production, a change to bigger fish is necessary. In Norway, this transition mainly took place in the fifties. The return figures already mentioned show that even fingerlings may be inadequate in places. Consequently 1+ fish have been compared to fingerlings in a series of reservoirs where trout is living sympatrically with char (Salvelinus alpinus L.), whitefish (Coregonus lavaretus L.) or grayling (Thymallus thymallus L.). The two-summer old fish gave 3–12 times better return than fish one-summer old stocked at the same time. Where trout is co-existing with perch or other predatory species the ratio is even more favourable. The higher production and transportation expenses make a return ratio of 1:4 necessary for a change to two-summer old fish to be profitable.

The change from one to two-summer old fish means an increase in average stocking size from 5–6 to 13–16 cm. The significance of size has also been confirmed in experiments with groups of 0+ young differing 2–3 cm in average length. Two-year old fish are used when stocking lakes containing fish species serving as prey for the trout. The minimum size mainly imposed on the hydro-electric companies is 20 cm, and a bonus is given for young exceeding 24 cm. The figures are based on return results from Lake Mjosa where the trout co-exists with about 20 species but mainly feeds on smelt (Osmerus eperlanus L.) and cisco (Coregonus albula L.). The return of adult fish doubled by increasing the size of the young from 17 to 20 cm and doubled again by a further increase to 23 cm. Stocked as untagged fish, the differences might have been less striking but it is assumed that tag shedding and tagging mortality are not the main reasons for the superiority of the bigger young. The greater part of the loss is ascribed to predation and competition. A speeding up of growth in the hatcheries will, however, accelerate maturity and consequently decrease the spawning size.

Also time of release seems to be a factor of major importance to rate of survival and return. Of comparable releases in autumn and spring, the spring-stocked fish always give the highest return. In the mountain lake Skurdalsvann, spring yearlings gave four times better result than autumn fingerlings. Part of the result may be ascribed to a size difference of 3 cm, the average lengths being 6 and 9 cm. As always, strain and hatch were kept the same. But even when the fingerlings and yearlings are of the same size, the spring-stocked fish seem to be superior (Strange and Kennedy, 1979). However, differences in rate of growth make comparisons between Scandinavian and Continental/British experiments difficult.

In Norway hatchery wintering nearly doubles the per-unit production cost of young fish. Therefore, many experiments with stocking fish-eating 1+ young have been made in the autumn. But releases of the same brood as two years old in spring have always given a higher return, in the case of Lake Mjosa 3–6 times higher. The same ratio is achieved if the young are released into the fjords. The results are perhaps not surprising because most Norwegian lakes are ice-covered 6 to 9 months a year. Fish released in autumn may start their free life with a fast lasting half a year. But spring stocked fish also give a higher return when climate is less severe (Kennedy et al., 1982). During a stocking experiment in Lake Mjosa, batches of two year-old young were released every fortnight throughout most of the year. The returns increased from winter to spring and decreased from summer to autumn. The batches giving the highest returns were released in early June. At this time of the year, the behaviour of the fast-growing young very much resembles that of smolts.

Sizable trout, mean lengths of batches 25–35 cm, are mostly planted in the heavily fished rivers of eastern Norway. Releases late in autumn have given a return of 20–25 percent in the next season, whereas spring stocked fish have given a return of 40–60 percent. Most recoveries are made within a few weeks, but early planting may give a more even distribution of returns throughout the season. The winter mortality of the remaining stocked fish exceeds that of the resident trout by 3–4 times. The migrations of the stocked trout are in general very short (Aass, 1978, 1981a).


If bottom food exists in abundance, the strain used in fingerling stocking is of little importance, but if typical trout food is scarce, the survival of different strains may vary considerably. In the oligotrophic Lake Tunhovdfjord domesticated foreign and Norwegian lowland trout have proved inferior to the indigenous strain. When Danish trout were used, their proportion in the catch varied between 0.5 and 3.6 percent in different years. After changing to the local strain stocked fish made up 34–37 percent of the catches. Mountain strains should be used for stocking of high-altitude reservoirs or lakes, but not all mountain strains are fit for stocking. Slow-growing strains mostly give a low recapture (Aass, 1973).

In lakes where smelt, coregonids and cyprinids co-exist with fast-growing trout, the diet of the adult trout consists entirely of prey fish. This situation may have lasted for centuries. But because of damming and impoundments fish-eating trout strains have become more usual in recent years. To compensate for the loss of bottom food, the trout in some reservoirs invade the pelagic zone for their subsistence, predating on small or stunted, plankton-feeding fish species. The development in lake Tunhovdfjord is most illustrative. Twenty-thirty years after the impoundment, char had become the staple food of the adult trout. The change in diet takes place when the trout become about 25 cm long and is accompanied by a change in growth rate. The maximum weight now reaches 13–15 kg against 2–3 kg before impoundment (Aass, 1973). But in some reservoirs the local trout strain is unable to utilize the char resources. Therefore, Tunhovdfjord trout are now stocked in many reservoirs with stunted char populations. The best results are obtained when the length of the mature char do not exceed 20–22 cm. The conditions being favourable, the planted trout may grow to 3–5 kg in 5–6 years. But when stocked into lowland lakes, the Tunhovdfjord strain do not prey on coregonids or cyprinids. On the other hand, the Hunder trout of Lake Mjosa do not forage on char if stocked in char reservoirs. The strains thus seem to prefer certain prey species. But the fish-eating lowland trout strains, accustomed to prey on a multitude of species, have given the best returns in the fjord experiments. The transfer of small wild trout from coastal lakes to the fjords has not been successful. Most re-enter fresh water quickly and even if they stay put their rate of growth is slow.


Aass, P., 1971 Norske erfaringer med settefisk av orret, regnbueorret, og relikt laks. Inf.Inst.Freshwat.Res.,Drottningholm, (12):35 p. (in Norwegian)

Aass, P., 1973 Some effects of lake impoundments on salmonids in Norwegian hydroelectric reservoirs. Acta Univ.Upsaliensis,Abstr.Uppsala Diss.Sci., (234):1–14

Aass, P., 1978 Age, growth, stocking and yield of brown trout in River Hallingdalselv at Gol, East Norway. Inf.Terskelprosj., 7.33 p. (in Norwegian, with English summary)

Aass, P., 1981 Kontrollerte utsettinger av merkete orretunger i innlandsvassdrag. As, 45 p. (in Norwegian)

Aass, P., 1981a The brown trout fishery of River Hemsil, East Norway 1979. Inf.Terskelprosj., (18):50 p. (in Norwegian, with English summary)

Dahl, K., 1933 Forsok over lonnsomheten av a utslippe orretyngel i fiskevann. N.J. and F.F. Tidskrift, 62:361–70 (in Norwegian)

Kennedy, G.J.A., C.D. Strange and G.O. O'Neill, 1982 Tagging studies of various age classes of brown trout (Salmo trutta L.). Fish.Manage., 13(1):33–41

Strange, C.D. and G.J.A. Kennedy, 1979 Yield to anglers of spring and autumn stocked, hatchery reared and wild brown trout (Salmo trutta L.). Fish.Manage., 10(2):45–52

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