by
Bror Jonsson
National Correspondent
Directorate for Nature Management, Fish Research Division
Tungasletta 2, N-7004 Trondheim
I. Fishery Biology and Management
1. Crayfish
In Norway, crayfish (Astacus astacus) have earlier only been of minor interest to scientists. Therefore, investigations on yield, reproductive biology, food choice, spatial distribution, parasitism, genetics, and population regulations have been carried out in Steinsfjorden (surface area: 13.9 km2), one part of the Lake Tyrifjorden situated in south Norway.
Egg development time of freshwater crayfish is strongly influenced by temperature. By increasing water temperature the incubation period was decreased from 1800 to 1300 degree-days.
In the mesotrophic Steinsfjorden, the growth-curve of crayfish was linear: mean length increment per molt was 9 mm for males and 7.8 mm for females. Males and immature females had two molting periods per year, adult females molted only once a year. Female fecundity increased with size, and the number of pleopod eggs was 55–65% of the ovarian egg number. By reducing the crayfish density in the lake, female size at maturity decreased, and the percentage of adult females increased significantly.
Detritus, benthic macrophytes and benthic algae were the main food items, but the species fed also on small amounts of benthic crustaceans, insects, zooplankton and fish. The feeding intensity decreased during late autumn and during the moltings.
The yield of crayfish was estimated at 109,000–188,000 individuals per year, corresponding to an exploitation-rate of 0.75–0.91.
2. Brown trout
Brown trout (Salmo trutta) occur at higher densities, and grow at a higher rate in outlet than in inlet rivers of lakes. In the outlet streams drifting zooplankton and filter feeding zoobenthos are important food items. In the inlet streams, surface insects are important.
Sea-run migrant brown trout have been investigated in the small river Vardneselva, northern Norway. This diadromous fish feed in the sea within 80 km of the mouth of their nursery river. There is no significant difference in migratory distance between smolts and older migrants. The rate of growth in the sea is highest during the second part of June.
The food of age-0 hatchery reared brown trout released in streams in mid-Norway has been investigated. Most of the fish started to feed shortly after release. In the beginning, however, the amount of plant fragments in the food was high. This fraction, however, decreased quickly. One week after release, hatchery trout fed on wild prey items almost as well as did wild trout.
Several Norwegian rivers have altered water temperature regimes with higher winter and lower summer temperatures, due to regulations made for hydro-electric purposes. Based on samples from 11 Norwegian rivers, it has been made a matematical model which estimates the effect on growth of young trout by such temperature alterations.
The effects of transferring glacier-fed water to a clear-water river on production and food organisms of brown trout were investigated in a mountain river in southern Norway. Production in unregulated fluvial habitat was 271.5 g/100 m2yr compared with 103.1 g/100 m2yr in a glacier-fed reach of the river. This difference was due to low fish density and recruitment rate.
In a field experiment, it has been shown that tagged hatchery reared brown trout released in lakes can be used for estimation of the population size of the wild brown trout present.
3. Arctic charr
During a period of 6 years, heavy exploitation of a dense, resident, monomorphic population of Arctic charr (Salvelinus alpinus) was carried out in a small mountain lake in central Norway. At the beginning of the experiment, the density was 1,100 fish (71 kg) per hectare. The total annual yield decreased from 18.0 kg/hectage to 3.0 kg/hectare during the 6 years of study. Maximum sustainable yield was estimated to 7 kg/hectare. Fish size increased during the experiment.
Food and growth of Arctic charr under aquaculture conditions have been studied. Special emphasis has been placed on the investigation of the effects of abiotic and biotic factors on individual growth rates. The use of a X-radiographic method has made possible the monitoring of food intake by individual fish, allowing studies of daily and seasonal patterns of food consumption to be conducted. The method is being routinely incorporated into growth studies in order to make assessments of variations in food intake and how these may correlate with growth. Furthermore, much work has been concentrated on examination of patterns of gastric emptying in order to find suitable empirical models for use in field studies of food consumption rates and predator-prey interactions. Considerable differencies in emptying patterns have been revealed for different food/prey types and the consequences for absorptive and metabolic processes are under investigation.
The stomach content of Arctic charr in a lake was studied. Stomach contents weights were not normally distributed. The fish were sampled every 3 h throughout 24 h periods. The distributions of stomach contents weights were skewed to the right, and the variance was dependent on the sample mean, with the variance to mean ratio equaling 1. Although mean values of stomach contents weight showed diurnal variations, there were usually no significant differences between consecutive samples taken throughout each 24 h period, due to the large range of variation in stomach contents weight within each sample.
Polymorphic Arctic charr lakes of Bear Island have been investigated. This polar island is rich in rivers, lakes and ponds. Arctic charr is the only fish species present. The species probably immigrated in early post-glacial times. Today all stocks are landlocked. Most lakes have two charr morphs: “dwarf charr” with adult fork lengths usually shorter than 20 cm, and “normal charr” with adult fork lengths usually longer than 30 cm. Sympatric morphs have identical esterase distributions, but they differ in age and growth, spawning coloration and outer morphology, gill raker and pyloric caecum counts, sex ratio and fecundity.
In an inter-Nordic project carried out in Lake Thingvallavatn, Iceland, habitat use of polymorphic Arctic charr was investigated. In this lake, the species exhibits four morphs: small benthivorous charr living mainly within the stone matrix of the lava bottom in the littoral zone, large benthivorous charr feeding mainly epibenthicly in the littoral zone, and planktivorous and piscivorous charr feeding throughout the lake. The snail Lymnaea peregra is the main food item for both benthivorous morphs, crustacean zooplankton is the main food for planktivorous charr, and threespined stickleback (Gasterosteus aculeatus) and small planktivorous charr are main food items for piscivorous charr.
Genetic differentiation among local populations and sympatric morphs of Arctic charr have been investigated in 15 localities of southern Norway. The absolute values on Nei's genetic distance between morphs and stocks are small (typically in the order of 0.001), and morph changes may occur without accompanying changes in frequencies of esterase alleles. Differentiation among localities explains far more of the total gene diversity than differences between morphs in the four cases of naturally occurring sympatric morphs examined.
The geographical distribution of allele frequencies at the EST-2 (100) locus in Arctic charr was studied by examining literature data from 209 Arctic charr populations from most of the range of the species. A non-significant positive correlation was demonstrated bewteen the frequency of the EST-2 allele and latitude. Three observations suggest that genetic drift may override possibly existing temperature-dependent selection in determining esterase allele frequencies in the Arctic charr. Firstly, esterase allele frequencies show considerable variation within restricted geographical areas. Secondly, fixation for either of the esterase alleles is common in small populations. Thirdly, the proportion of populations fixed for one allele varies with the regional average frequency of that allele. On this background, selection appears to be of less importance in determining esterase allele frequencies in the Arctic charr than has hitherto been suggested.
The Norwegian studies on polymorphic Arctic charr indicate that the morph differentiation occur through trophic specialization. Polymorphism is observed in lakes where the species has the possibility to radiate into different stable food inches and can specialize on different food items.
4. Coregonids
Hatching of whitefish (Coregonus lavaretus) and vendace (C. albula) has been investigated in the Gudbrandsdalslågen, a river emptying into the Mjøsa, the largest lake in Norway (365 km2). The larvae started to drift downstream concurrently with the beginning of the spring freshet. The duration of the yearly outdrift lasted for 2–5 weeks in late April and May. The yearly maximum outdrift was positively correlated with the rate of increase in water discharge, but not with water temperature during spring. Most larvae drifted downstream at night. Experimentally, it has been demonstrated that movements of coregonid eggs caused by flowing water can induce hatching earlier than expected from the water temperature. Furthermore, equal number of eggs hatched during dark and light hours. The fact that most larvae drifted out during night indicates that the larvae are able to control their vertical position in the river, and the outdrift is not completely passive.
In Mjøsa, whitefish and vendace start feeding in the littoral zone. Whitefish are littoral during their first year. Vendace move into pelagic water in July/August. Crustecean zooplankton are important food items for both species. However, young whitefish have a more variable diet than vendace; they feed also on surface insects and zoobenthos.
In Lake Tyrifjorden, the habitat use of whitefish longer than 18 cm (2 years and older) was investigated. Whitefish shorter than 27 cm lived mainly in the littoral zone and in deep pelagic waters. Larger whitefish were most abundant in near-surface water of the pelagic zone. In the pelagic zone, all whitefish fed on zooplankton. In the littoral zone, small whitefish fed mainly on zooplankton, whereas large whitefish fed mainly on zoobenthos.
In Lake Femund, whitefish exhibited three morphs. The morphs were differentiated on basis of number of gill rakers, diet, and feeding and spawning habitats.
Limnocoral experiments with whitefish showed that the fish had great effects on the water quality. Enclosures with fish had higher levels of particulate phosphorus, phytoplankton biomass and production, sedimented carbon, and rotifer biomass, and lower water transarency, pH, and biomass of cladocerans and copepods, and decreased size of Daphnia galeata, than those without fish. A lake was treated with rotenone. A comparison of the physical, chemical and biotic conditions in the lake during the pre and post years, yielded similar conclusions as those drawn from the enclosure experiments.
Size selection of Bosmina longispina and Daphnia galeata by co-occurring whitefish, cisco and smelt (Osmerus eperlanus) were investigated. There was no significant correlation between gillraker spacing and size selection of cladoceran zooplankton. The results indicate that the feeding behaviour of the predator and the visual size of the prey are more important than gillraker spacing in the size selective feeding of planktivorous fish.
5. Smelt
The food of age-0 smelt was investigated in Mjøsa. Pennate diatoms, especially Asterionella spp. and Tabellaria fenestrata, were the most important food items during the first month of external feeding; rotifers and early stages of copepods were eaten occasionally. Later during summer and autumn, crustacean zooplankton (cladocerans and copepods) were the most important food items. Some smelt larger than 7.4 cm fed on Mysis relicta.
6. Pike
Habitat use, food and growth of pike (Esox lucius) in four Norwegian lakes have been studied. The pike were mainly littoral; the fraction of pelagic pike increased with increasing turbidity. The pike fed on fish; selected species were whitefish, smelt, and ruffe (Acerina cernua). The growth-rate was correlated with the size of the prey species exploited, but not with water temperature or the prey densities of the lakes.
7. European eel
European eel (Anguilla anguilla) has been studied extensively during recent years. The species feed in coastal rivers throughout Norway. Important food items are Lymnaea peregra, Gammarus lacustris, large insect larvae, and small fish, e.g. threespined stickleback. Negative food selection have been found for oligochaets, chironomid larvae, and Pisidium spp. In Norway, yellow eels feed extensively at temperatures well below 10°C, a temperature which many authors have claimed is the lower threshold temperature for feeding activity of the species.
The life history of the eels feeding in the River Imsa, south-western Norway has been investigated. Each summer between June and September, elvers ascend the river. In the river, mean annual length increment from the elver stage to age 8 years was 6.2 cm. Males grew more slowly than females. The male/female ratio was c. 1:20. Male and female yellow eels transformed into silver eels at lengths of c. 40 cm and 62 cm, respectively. Rapidly growing eels of both sexes transformed into silver eels at younger ages than more slow growing individuals. Length was more important than age for the onset of transformation.
In northern Norway, most silver eels migrate to the sea from mid-August to mid-September. In southern Norway, eel migration occurs mainly during October. In the River Imsa, connections between descent of silver eels and water discharge, water temperature, turbidity, photoperiod, light intensity, and moon phase have been studied, based on 10 years field data and field experiments. Mean water temperature for July-August and mean water discharge for August-October explained 91% of the total variation in the start of the eel run. Low water temperature during summer and high water discharge during autumn resulted in early start of the run, whereas high water temperature and low water discharge gave the opposite effect. Maximum silver eel descent was at a water temperature of 9°C. Few eels descended at temperatures below 4°C and above 18°C. The migration speed in the river was correlated with water discharge and temperature (R2 = 0.88). Water discharge alone accounted for 85% of the total variation in migration speed. Illumination of 20 lux upon the river reduced the descent of silver eels.
8. Miscellaneous
Selectivity curves of nylon gillnets for burbot (Lota lota) and Arctic charr have been found, and the efficiencies of a geometrical and an aritmetical series of mesh sizes have been calculated for salmonids, burbot and perch.
Mysis relicta have been released in some Norwegian lakes. The result of this has been investigated in Selbusjøen, central Norway. During the period 1974 to 1984, Mysis reduced the biomass of cladocerans with 87%, and the biomass of Arctic charr with between 61–87%. The biomass of burbot has increased between 4 and 30 times.
II. Fish Culture and Diseases
1. Rearing of salmonids
Atlantic salmon (Salmo salar) and rainbow trout (S. gairdneri) food fish farming in floating pens in brackish water has increased substantially. In 1986 42,500 t of salmon and 4,200 t of rainbow trout were produced.
2. Gyrodactylus salaris
The fluke, Gyrodactylus salaris, continue to eradicate salmon parr in infested rivers. We are now aware of 30 infested rivers, and so far no effective remedy against the parasite has been found. Strict measures have been taken to prevent the spreading of G. salaris, but still 2 or 3 new rivers are infested yearly. Yearly loss of salmon due to the fluke is estimated at 250–500 t.
3. Diseases
The burned spot desease of crayfish is caused by the fungus Ramularia astaci. The disease is reported from 4 localities in south-eastern Norway, and it is probably common in other Norwegian crayfish localities as well. Infection levels are probably low and have only minor impact on the Norwegian crayfish.
Strains of Aeromonas salmonicida were isolated from farmed Atlantic salmon suffering from furunculosis. Plasmid profiling was used to compare strains from 9 countries. Outer membrane proteins of various strans appear to be too much alike to be used as good markers in epidemiological work. Plasmid profiles appear, on the other hand, to vary too much to be used for this purpose.
Bacterial Kidney Desease (BKD) caused by Renibacterium salmoninarum was recorded in Norway in 1980. At present, BKD has been observed at 24 different locations in western and northern Norway. A research project on diagnosis and epdemiology of BKD has been initiated.
Since 1977, outbreakers of the “Hitra desease” (Haemorrhagic syndrome) have occurred in Norwegian salmon farms. Affected farms show up to 80% loss of their fish. Bacterial examination of kidneys of deseased or dead fish have resulted in isolation of the Vibrio, Vibrio salmonicida sp. nov. in almost all cases. All 186 V. salmonicida isolated had one common plasmid (21 megadalton). This plasmid was nick translated with radioactive nucleotides and used in colony hybridization experiments. The colony hybridization test was able to detect all V. salmonicida colonies tested mixed with a number of other marine and freshwater bacterial colonies.
There are severe and unexplained disease problems in some Norwegian sea farms which cause great losses. The following seems to characterize the problem: (1) The effected fish is Atlantic salmon during the first year in sea water, (2) the mortality is high, (3) pathological changes are dominated by anemia, a varying degree of heamorrhages, haemorrhagic intestines, a dark, fragile liver and ascites, (4) histopathological changes in the liver and digestive system, (5) blood analysis show a low PCV, (6) bacteria are not detected, and (7) IPN-virus is isolated.
III. Fish and Polluted Water
1. Acid water
Acid precipitation causes constant havoc among fish populations, and the problem is surveyed continuously. The problem is very severe in southernmost Norway, but is also evident in several areas of western and south-eastern Norway. For brown trout, which is the main fish species involved, the reductions are largely due to reproduction failure. Atlantic salmon has disappeared from more than 10 rivers, and is scarce in several others due to the acidification in southern and western Norway. Recordings of dead parr and presmolt during snow melt in spring indicate that mortality at these stage is important for the reduction of salmon stocks in acid softwater rivers.
Monitoring of behthic invertebrates in lakes and rivers verifies the data on fish community status and abundance, and gives early signs of shifting trends of acidification of the different watersheds.
In 1985 and 1986, the water quality and the fauna of benthic invertebrates were improved, and the abundance of salmon parr had increased in the studied rivers in south-western Norway, relative to the conditions in 1980–84. However, the rivers have still a poor water quality, and the fish stocks are easily hurt by occasional periods with acid rain.
Experiments to test effects of acidic and neutralized water on fish physiology, fish toxicity, and fish populations have been performed, with special emphasis on three chemical components: pH, calcium, and aluminium. Atlantic salmon smolts are more sensitive to acidic water than earlier life history stages of salmon and of brown trout and brook trout (Salvelinus fontinalis). A high sensitivity to acidic water of adult salmon entering rivers on their spawning migration, have also been documented. An increased resistance to acidic waters has been documented among brown trout strains living in acidified areas compared with those from non-acidified areas. The neutralization experiments have tested different bases to measure changes in aluminium chemistry and the biological response to stable and unstable chemical environments. By adding bases in high concentrations to pH above 7, aluminium ions can be formed. If the base consists of monovalent cations, stress or mortality of the fish can be induced by the aluminium ions. Presence of divalent cations like calcium is therefore important when neutralizing highly toxic water.
2. Mercury
Investigation of adipose fin and axial muscle mercury concentrations of brown trout showed that these were highly correlated, but adipose fin mercury concentrations were c. 80% lower. Thus, analysis of adipose fin mercury concentrations may be most helpful in documenting mercury contamination in salmonid fishes.
Muscle mercury concentrations in Arctic charr and brown trout in coastal watersheds in south-western Norway were between 0.05 and 0.31 ppm Hg, and increased significantly with fish age. The results indicate that the process of acidification may increase mercury accumulation in fish. No significant differences in mercury accumulation was found between sympatric stocks of brown trout and Arctic charr.
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