Ferskvannsressurser og fiskeoppdrett

Fiskeoppdrett i storskala er blitt en vik­tig næring i Norge. Omfanget av små fiskeoppdrettsanlegg har mer lokal in­teresse selvom de anses å kunne være en viktig binæring i distriktene. Disse må imidlertid også behandles etter en rekke lovverk. Dagens forvaltnings­praksis tilsier at saksbehandlingen ikke er optimal. For at en søknad ikke skal ligge unødvendig lenge i forvalt­ningen, med dertil økt belastning for forvaltningen og søker, er det nødven­dig at vassdragsmyndigheten (NVE) blir trukket tidligere inn i søknadsfa­sen enn tilfellet er i dag. Vannkildens kapasitet og konsesjonsspørsmålet i forhold til vassdragsloven må derfor avklares i planleggingsfasen før søk­nad sendes. Fremtidens anlegg bør i størst mulig grad lokaliseres til vass­drag med tilstrekkelig vannkapasitet. Eksisterende verneplaner og villaks­utvalgets innstilling om vern av 50 el­ver og 9 fjorder/kyststrekninger vil få stor betydning for fremtidig etablering av oppdrettsnæring i tilknytning til vassdrag

Sammendrag av delprosjekter

Delprosjekt HYDRA F7, "Kvantifisering av usikkerhet i hydrologiske og meteorologiske prognoser", ble delvis initiert på bakgrunn av de store avvik mellom prognosene fra flomvarslingen på NVE og Glommens og Laagens Brukseierforening (GLB) for flommen i Glomma, Juni 1995. Uten verktøy for å tallfeste usikkerheten i prognosene ble det for publikum, og for hydrologene, vanskelig ata stilling til, og handle ut i fra to sterkt avvikende prognoser. I tillegg ble det klart at man manglet kunnskap om hva som skapte usikkerheten i prognosene. En vannføringsprognose har mange kilder til usikkerhet, feil i den hydrologiske modellen, feil i data for oppdatering av den hydrologiske modellen, feil i initialbetingelser for hydrologisk- og atmosfæremodell, ikkerepresentative meteorologiske data og prognoser og feil i de meteorologiske prognosene. Det er helt nødvendig å få kvantifisert og sammenlignet feilene fra de forskjellige feilkildene for å kunne forbedre metoder og rutiner for avløpsprognosering. En siste motivasjon for å igangsette et slikt prosjekt er at for fremtidens flomvarsling kan det være av større interesse for beslutningstakere i krisesituasjoner å få vite hva sannsynligheten for overskridelse av vannføringer med tilhørende kjent skadepotensial er, i stedet for bare å få presentert prognosert vannføring

Threshold values of river discharge and temperature for anglers' catch of Atlantic salmon, Salmo salar L

The catch by anglers of adult Atlantic salmon, Salmo salar L., was studied over a 5-year period in the River Gaula, Norway. Atlantic salmon were caught over a wide range (23–570 m3 s−1) of the observed extent of river discharge (13–950 m3 s−1) and throughout the range of temperature (4–23 °C), but both factors strongly affected catch rate. Significant correlations between the number of Atlantic salmon caught daily, and water temperature (r = 0.33) and river discharge (r = −0.42) were found in 1987 and 1989, respectively. The highest daily catch occurred between 50 and 150 m3 s−1, and at temperatures between 13 and 16 °C. Threshold values for water discharge and temperature were found to exist at 250 m3 s−1 and 8 °C, with the highest catches below and above these values, respectively

An experimental test of the genetic component of the ontogenetic habitat shift in Arctic charr (Salvelinus alpinus)

Fry of the Arctic charr, Salvelinus alpinus, were experimentally stocked into a small fish-free lake to test the hypothesis that the size-dependent habitat shift from the epibenthic to the pelagic habitat is genetically determined. The charr originated from a nearby lake inhabiting predatory brown trout Salmo trutta. The cohort of stocked charr was investigated for three years. The Arctic charr started to exploit the pelagic habitat in their first summer at a size of 7–9 cm in contrast to about 15 cm in the donor lake. In the next two summers, the pelagic fraction of the cohort increased. The main fraction lived in epibenthic areas, utilizing the same prey as pelagic charr. Water temperature moderated the habitat use of juveniles such that they avoided warm (>16°C) waters and resided in cool, deep areas. The result was consistent with the hypothesis of a tradeoff between feeding benefit and the predation risk producing spatial segregation of Arctic charr and demonstrated that the fish can facultatively respond to predation risk and adjust the size at which they migrate to the pelagic zone to feed on zooplankton

Estimating the mean areal snow water equivalent from satellite images and snow pillows

By modelling the snow accumulation process in time and space as sums of random gamma distributed variables, the mean areal snow water equivalent (SWE) can be estimated. In the methodology we make use of the fact that sums of gamma distributed variables with a certain set of parameters also are gamma distributed variables with parameters being functions of the original and the number of summations. The measured snow/SWE at a point at a certain time t, can thus be seen as the accumulation, or the sum of the snowfall process from the beginning of the snowfall season up to t. The integration of these points, which give an area, can be seen as another summation. From snow pillows and precipitation gauges the value of daily accumulated precipitation/snow has been found to be well represented by a two parameter gamma distribution. This distribution has been found to be representative for large areas. The number of events where the precipitation was accumulated can be estimated from snow pillows situated in the area. The mean snow coverage over an area, which represents the summation of the individual points over an area, can be derived from satellite images represented in a GIS. The methodology is tested for two nested catchments of size 4723 km' and 19832 km' in a mountainous area in Southern Norway for eight satellite scenes. The results are compared with simulated snow reservoirs using a rainfall-runoff model, and found to agree well. Large discrepancies in the snow reservoirs between the proposed method and the rainfall runoff model are found in late spring and are probably due to errors in the estimated mean snow coverage

Vulnerability of melanic Daphnia to brown trout predation

The coexistence of melanic Daphnia cf. longispina and facultatively planktivorous brown trout is reported from a clear-water, alpine lake. This co-occurrence is uncommon, presumably due to the vulnerability of pigmented Daphnia to fish predation. Lake Bjornesfjorden (Norway) provided an opportunity to test this assumption. About 20% of the fish caught in gill nets had fed on Daphnia. The trout exerted a marked selection for large-sized Daphnia prey, and a very strong selection for pigmented individuals relative to transparent ones. The persistence of a pigmented Daphnia population probably relies on limited recruitment and a low stock of the predator, and the availability of more favourable benthic prey organisms

Density of juvenile brown trout and Atlantic salmon in natural and man-natural riverine habitats

The density of juvenile brown trout (Salmo trutta L.) and Atlantic salmon (Salmon salar L.) was significantly higher along river bank areas protected against erosion than along natural river banks in the River Gaula, Central Norway. A habitat shift appeared in Atlantic salmon, and a behavioural shift was demonstrated by brown trout from August to October. The effect of habitat on densities of juvenile salmonids should be taken into account as mitigation measures on eroded river banks and when accessing fish production in rivers