by
Bror Jonsson
National Correspondent
Direktoratet for naturforvaltning
Tungasletta2 - N-7000 Trondheim
Fishery Biology and Management
Brown trout is the most common species of freshwater fish in Norway, and the life history of this species has been studied extensively during recent years. Many coastal stocks of this species are polymorphic, consisting of both freshwater residents, which mature sexually without any sea run, and migrants that smoltify and migrate yearly between freshwater and sea. Sex ratios are in favour of males among residents, and females among migrants. Most brown trout grow up in brooks or rivers, but move to lakes or pools in the river wherever possible, as they become older. Residents feed in lakes or rivers, and lake feeders live mainly in near-surface water. Young age-groups often feed close to the shore, while older age-groups are more roaming, and in deep lakes they are often found in the pelagic zone. Females are more pelagic than males. Migrants leave rivers during spring and move ca. 100 km in the sea before both immatures and matures return to their river during late summer and autumn. Mature individuals return before immatures. A regional study, investigating population variables like smolt age, growth-rates in freshwater and the sea, and age at sexual maturity of coastal brown trout is in progress in all parts of Norway at the present time.
Fishery management research at hydro-electric reservoirs has been continued. During the last 20 years more than 3 mill. brown trout parr have been tagged or marked and stocked in reservoirs, and two thirds of the releases have been made during the past decade. In later years recaptures have been controlled. Recapture-rates vary with age and size at release, time of release, stock origin, and environmental factors in the reservoir.
Studies in the regulated river, Sore Osa, have shown that a 60–90% reduction of the water flow has decreased the density of zoobenthos and fish (mainly brown trout) by 30–95%. Daily variations in water level in another regulated river Nidelva, decreased the survival-rate of young fish significantly.
From the 1960's onwards, low weirs have been built in many rivers influenced by water storage. The main purpose has been estethic, but small lakes and tarns formed above the weirs have increased the density of brown trout 10 times or more relative to pre-regulation densities.
Atlantic salmon live as freshwater residents in a few Norwegian rivers. In one of these rivers, the Namsen, part of the stock has been observed smoltifying and migrating to the sea, in spite of the fact that a waterfall prohibits their returning to reproduce. Most of these smolts leave the river in the beginning of August. In most rivers with anadromous Atlantic salmon, smolt migration occurs during May.
Water temperature affects the timing of smolting in Atlantic salmon, and in the River Imsa, temperature is also a key environmental variable influencing the timing of the seaward migration of smolts (R2=0.95). In another river the Orkla, the spring freshet in water discharge seems to regulate the timing of smolt migration. Current velocity seems to be important for smolts when orienting downstream. They experience difficulties in finding their way through lakes with low current velocity. Smolts move downstreams mainly at night. Illumination of the river at night delays the smolt descent. This indicates that nocturnal maximum activity levels among smolts are a response to light intensity.
Studies of predation on smolts in estuaries have been started, and cod seem to be an important smolt predator at the outlet of the River Surna.
Incubation time and mortality of Atlantic salmon eggs at experimental temperatures between 0-3°C have been studied. Results indicate that incubation time, and perhaps also a trigger such as rise in temperature, may be more important than the heat sum (degree-days) of the incubation period in determining the exact timing of the hatch.
Spawning acticity of Atlantic salmon in rivers has been studied by using airplanes. This method is very useful when counting number of spawning females in rivers. Redds of salmon and trout are distinguished by inspection of roes.
Control of ascending elvers and descending silver eels in the River Imsa was continued. In 1984 ca. 6 600 elvers ascended the river, whereas the number of ascending elvers in 1985 was very small. The number of ascending elvers is highly correlated with water temperature during summer (R2=0.89). The number of descending silver eels in 1984 was 3 169 with a weight of 1 221 kg. Corresponding figures for 1985 were 4 403 and 1 981 kg. The descent of silver eels in the River Imsa starts at the first autumn freshet in water discharge, and migration speed downstream is correlated with water discharge (R2=0.85). Silver eel migration is stopped by low water temperatures, and few eels descend the river at temperatures below 4°C. Median age of male silver eels in the river is 4 years, and mean length is 41 cm. Median age and length of corresponding females are 7 years and 62 cm.
Assessment of pelagic fish species has been monitored by use of a portable echosounder. The system used supports echointegration combined with a single fish echo-analyzing algorithm. Fish density, as well as relative fish size distribution can be calculated by a statistical method.
Few studies of age-0 whitefish, cisco and smelt have earlier been carried out in Norway. Recent studied in the River Gudbrandsdalslågen indicate that cisco and whitefish eleutheroembryos start drifting downstream into Lake Mjøsa concurrently with the beginning of the spring freshet. Annual duration of outdrift of fish larvae lasts from 2–5 weeks in late April and May. The maximum yearly outdrift is positively correlated with the rate of increase in water discharge, but not with water temperature during spring. Most larvae drift downstream at night. In Lake Mjøsa, both species began exogeneous feeding in littoral backwaters in late May and early June. Young whitefish dwell in littoral areas during summer and autumn, whereas young cisco abandon the littoral zone and become pelagic from August onwards. Cyclops and Bosmina are early food for both species. In addition, whitefish eat Chironomidae early instars. Later, age-0 cisco feed on zooplankton, whereas whitefish feed on a wide range of food items including zooplankton, surface insects, chironomid larvae and pupae, cyprinid larvae, and benthic caldocerans. Cisco grow from 1.0 to 11 cm during the first summer, while whitefish grow from 1.6 to 12 cm during the same period.
Smelt in Lake Mjøsa spawn in mid-May, and alevins hatch in early June. Pennate diatomes (Asterionella spp. and Tabellaria fenestra) are the most important food items during the first month of exogeneous feeding. Crustacean zooplankton are the most important food items later during summer. Some smelt larger than 7.4 cm start to feed om Mysis relicta during the first year, but few smelt attain that size during that time span.
Older whitefish in Lake Femunden have also been investigated. According to the number of gill rakers and body size, 3 phenotypes of whitefish occur in the lake. The smaller type (adult size 30–32 cm) has fewest rakers (≃ 29), spawns in deep waters, and is numerically dominant in all habitats. It also dominantes commercial catches. The second type (adult size 37–39 cm) has a meadium number of gill rakers (≃ 36), and is a river spawner. The largest type (adult size 35–45 cm) has the highest number of gill rakers (≃ 44), and spawns in shallow waters in the lake. The population in later years has been dominated by 2 strong year-classes: 1972 and 1973. Since 1982, however, these two year-classes have declined, and recruitment in the population is now good. During test sampling with benthic gill nets in 1984, ca. 45% of the catch consisted of 1–3 year old fish, 7–16 cm in length.
In the present intersessional period of EIFAC, a new project has been started to organize and coordinate recreational and commercial fisheries in a large watercource with diverse habitats (lakes, tarns, rivers, streams) and fish communities. We are looking forward with interest to the results of this work.
Fish Culture and Diseases
Salmon and rainbow trout food fish farming in floating pens in brackish water has increased substantially. In 1984 21 881 t of salmon and 3 569 t of rainbow trout were produced. Production of farmed Atlantic salmon is higher than the total world catch of “wild” Atlantic salmon.
Experiments with feeding of wild fish in pens have also been performed, but all attempts made thusfar have not been economically successfull.
Fluke, Gyrodactylus salaris, continue to eradicate salmon parr populations in infested rivers. We are now aware of 26 infested rivers, and so far no effective remedy against the parasite has been found. Strict measures have been taken to prevent the spreading of Gyrodactylus. Yearly losses of salmon due to the fluke are estimated to 250–350 t.
Ichtyoboda (“Costia”) necator infestations on the gills of Atlantic salmon smolts in sea-water are an increasing problem. IPN (Infectious Pancreas Necrosis) has been registered in increasing numbers of fingerlings and food-fish farms, but apparently the virus has not caused serious disease or mortality in the infected farms.
Vibriosis is a problem when rearing salmon and trout in brackish water. Five commercial vaccines against this disease are now tested, all of which are effective, and results are best when the vaccine is injected into the fish. As a result of this investigation, 3 vaccines against vibriosis are registered for sale in Norway.
The desease hemergous syndrom (the “Hitra disease”) causes high mortality in fish farms during winter. A project for finding measures against this serious disease has been started.
Fish and Polluted Water
Acid precipitation causes constant havoc among fish population, and the problem is surreyed continuously. In the two southernmost counties in Norway, 30% of lakes supporting brown trout and 17% of those supporting perch had lost fish stocks between 1976 and 1984. The fishery status of rivers in the country Rogaland was investigated in 1985. Damage caused by acid precipitation and aluminium increases continuously.
A major project on liming of lakes and rivers in southern Norway now exists.
A small acid lake, that had been treated with lime, was stocked with 5 000 salmon fry in 1983. The release resulted in more than 2 000 2-year-old smolts. Smolts, however, refused to leave the lake in spring, probably because of low water discharge. They had to be fished with a beach seine and released in a brook downstream.
Special studies on the toxicity of different fractions of aluminium and acidity for different fish species have been carried out. Brook trout have more resistance to acid water with labile aluminium than brown trout and Atlantic salmon. Brook trout have been released in 27 lakes in southernmost Norway where releases of brown trout have been unsuccessful due to the low pH in the water. In two of the lakes, successful spawning of brook trout has now been observed, and in 21 of the other lakes fish are still alive 3 years after releases.
Observation of Atlantic salmon indicate that 100–200 μgA1/1 in water at pH 5 reduce the enzyme activity of Na-K-ATPase and Carbonic acid anhydrase by 25–40% in the gills of Atlantic salmon and rainbow trout.
Bibliographic list
Andersen, T.F., Fjellheim, A., Karlsen, L.R. & Raddum, G.G. 1986. Variations in density of a population of brown trout and its relation to food resources in a regulated West Norwegian river. Symposium on Fish production in rivers, Lodz, Polan: in press.
Berg, O.K. 1984. Descent of relict Atlantic salmon, “småblanken”, in the River Namsen. Rep. Reguleringsundersøkelsene, DVF 15: 1–64 (In Norwegian, English summary).
Garnås, E. 1985. Effects of reduced water discharge of zoobenthos and fish in the River Søre Osa, Hedmark, 1982–1984. Rep. Reguleringsundersøkelsene, DVF 9: 1–84. (In Norwegian).
Garnås, E. & Gunnerød, T.B. 1983. Fishery biological studies 1980–1982 in 3 lakes in Sør-Trøndelag where Mysis relicta have been released. Rep. Reguleringsundersøkelsene, DVF 12: 1–56. (In Norwegian).
Garnås, E. & Hvidsten, N.A. 1984. Descent and production of smolts of Atlantic salmon and brown trout in the River Orkla, 1979–1983. Rep. Reguleringsundersøkelsene, DVF 7: 1–53. (In Norwegian).
Hvidsten, N.A. 1985. Mortality of young Atlantic salmon and brown trout due to water level fluctuations in the River Nidelva. Rep. Reguleringsundersøkelsene, DVF 4: 1–12. (In Norwegian).
Johnsen, B.O. 1983. Food consumption during the first period after release in a brook of hatchery reared brown trout. Rep. Reguleringsundersøkelsene, DVF 14: 1–23. (In Norwegian).
Johnsen, B.O. & Jensen, A. 1985. The parasite Gyrodactylus salaris on young salmon in Norwegian rivers. Rep. Reguleringsundersøkelsene, DVF 12: 1–145. (In Norwegian).
Melquist, P. (ed) 1984. Problems with supersaturation of water from electrical power plants. Komitéen for undersøkelser av gassovermetting, Vassdragsregulantenes forening. (In Norwegian).
Møkkelgjerd, P.I: & Gunnerød, T.B. 1985. Releases of brook trout (Salvelinus fontinalis) in regulated rivers in southernmost Norway. Rep. Reguleringsundersøkelsene, DVF 10: 1–53.
Sandlund, O.T. 1984. The fish community in the lake Femunden. Kvitfisken, SNV 3. (In Norwegian).
Sandlund, O.T. 1986. The traditional whitefish (Coregonus lavaretus) fisheries in the River Tufsinga, eastern Norway. Norsk Skogbruksmuseums Arbok. (In Norwegian, English summary): in press.
Sandlund, O.T., Kristensen, B., Rustad, O.H. & Vistad, N.B. 1985. Recreational fishing in Adal. Rep. SNV-Innlandsfiskeprosjektet, 37 pp. (In Norwegian).
