Fisketelling i Driva høsten 2011. Forekomst og fordeling av gytemoden sjøaure og laks før planlagt etablering av langtidssperre i Snøvasfossan

I slutten av september 2011 ble det gjennomført drivtelling av sjøaure og laks i hovedstrengen av Driva. Bakgrunnen for undersøkelsen er at det er planer om å etablere en langtidssperre i Snøvasfossan, som er om lag 26 km oppstrøms utløpet av vassdraget ved Sunndalsøra. Lang-tidssperra vil hindre sjøvandrende laksefisk i å vandre opp til midtre og øvre deler av Driva-vassdraget. Avsperringen er en del av en større tiltakspakke der målet er å gjenoppbygge livs-kraftige bestander av sjøaure og laks, etter at Gyrodactylus salaris er utryddet fra alle smittete vassdrag i Sunndalsfjorden. På den 85 km lange strekningen fra antatt vandringshinder i Magalaupet til utløp i sjø ved Sunndalsøra ble det observert til sammen 797 sjøaurer og 265 lakser. I områder med spesielt høy gradient var elvestrømmen så stri at det ikke var praktisk mulig eller sikkerhetsmessig for-svarlig å gjennomføre fisketellinger. Følgelig representerer registreringene bare en del av gyte-fisken som var i vassdraget på undersøkelsestidspunktet. Registreringene av gytefisk tilsvarer tettheter på ni sjøaurer og tre lakser per kilometer elvestrekning. De registrerte tetthetene av sjøaure er gjennomgående lavere enn det som er funnet i tilsvarende undersøkelser i nærlig-gende vassdrag som Eira, Surna og Toåa. Spesielt sammenlignet med Eira er de registrerte tetthetene av gytefisk i Driva svært lave. Underestimatet av gytebestandene som fisketellingene representerer skyldes flere metodiske begrensninger knyttet til fysiske, hydrologiske og biologiske forhold. Fisketellingene er likevel vurdert som representative for den relative forekomst av sjøaure og laks i ulike deler av vass-draget. Under forutsetning av at inntil 28 % av sjøaure og 35 % av laks ble registrert under fis-ketellingene høsten 2011, besto gytebestandene i Driva av i størrelsesorden 3800 sjøaurer og 1000 lakser. Medregnet usikkerhetsmarginer tilsier beregningene at gytebestandene besto av 2825-5550 sjøaurer og 750-1400 lakser. Ut fra habitatforhold er det egnete områder for gyting og oppvekst av sjøvandrende laksefisk i hele hovedstrengen av Driva. De midtre og øvre delene er vurdert bedre egnet for laksepro-duksjon enn aureproduksjon. Sammenliknet med mange andre, større vassdrag i Norge er Dri-va spesiell da det er god tilgang på egnede gyte- og oppvekstområder i de nederste delene. Det er jevnt over liten forekomst av sentflytende områder med fint bunnsubstrat, som er en om-rådetype som er lite egnet som gyte- og oppvekstområde for sjøvandrende laksefisk. Samlet sett har derfor Drivavassdraget en høy teoretisk produksjonsevne for både sjøaure og laks. De registrerte sjøaurene fordelte seg i 31 % små ( 3 kg) individer. Innslaget av store sjøaurer i Driva var vesentlig større enn det som er påvist i de senere år i nærliggende vassdrag som Eira, Surna og Toåa. Vassdraget sett under ett ble det observert sjøaure i alle størrelsesgrupper fra umoden fisk på 150-500 gram til stor-vokste individer på 8-10 kg. Trolig var det flere tusen umodne individer av sjøaure i vassdraget i undersøkelsesperioden.In late September 2011 potential spawners of sea trout and Atlantic salmon were recorded by drift diving in the main stem of River Driva, a large river system at the west coast of Central Norway. The main reason for this investigation is that a long-term fish barrier is scheduled to be located approximately 26 km upstream the outlet at sea. The fish barrier will prevent anadromous fish from migrating to the middle and upper reaches of the watercourse. This measure is a major part of a large-scale project of restoring vigorous populations of anadromous salmonids after eradicating the fatal salmon parasite Gyrodactylus salaris from the Sunndalsfjorden region. Along the 85 km long river stretch from the migration obstacle at Magalaupet to outlet at sea a total of 797 trout and 265 salmon were observed during the drift diving survey. In high-gradient river sections the water currents were too swift for drift diving, and those river sections were consequently not included in the study. The observed number of fish is thus an underestimate of the spawning stocks of salmon and trout. The drift survey yielded a density of nine trout and three salmon per kilometre river stretch. The observed densities are considerable lower than those found in corresponding studies in nearby rivers such as Eira, Surna and Toåa. The recorded densities in Driva are particularly low compared to Eira. The main reasons for the underestimate of spawners are methodological limitations caused by physical, hydrological and biological conditions. In spite of these shortcomings, the drift survey most likely gives an adequate overview of the relative abundance of sea trout and salmon along the main stem of Driva. Given the assumption that up to 28 % of all trout and up to 35 % of all salmon were recorded during the survey in September 2011, the spawning populations of trout and salmon were approximately 3800 and 1000 individuals, respectively. Included calculated margins of uncertainty in the population estimates, the true values most likely were within the range of 2825-5550 trout and 750-1400 salmon. Based on habitat suitability there are available spawning and nursery habitats for anadromous salmonids in all parts of the main stem of Driva. The upper and middle reaches are considered more suited for salmon production than trout production. Good availability of suited spawning and nursery habitats in the lowest part of Driva is in contrast to the situation in most of the major salmon rivers in Norway. There is a low occurrence of slowflowing habitats with fine river bed substrata, which are supposed to yield low production of riverine salmonids. Due to these physical characteristics, Driva is assumed to have a high theoretical production capacity for trout and salmon. The recorded trout consisted of 31 % small, 46 % medium-sized and 23 % large individuals. The proportion of large trout (individuals > 3 kg) in Driva was considerable higher than what is registered during drift diving surveys in nearby rivers such as Eira, Surna and Toåa. All combined, there was observed trout in all size groups from immature individuals of 150-500 grams to mature individuals up to 8-10 kg. Most likely there were several thousands of immature individuals in the river in the investigation period. The main occurrence of spawning trout was observed downstream of Gråura, which is a highgradient river section with large numbers of waterfalls. Particular high numbers were recorded between the Grensehølen pool and the bridge at Romfo. In this river section six pools held more than 20 sea trout, of which Teinøra and Kirkesteinshølen held 42 and 45 trout, respectively. There was low numbers of trout in river sections in Oppdal municipality, in spite of high availability of suited spawning and nursery habitats. Skeia and Purkeøya seemed to be important spawning areas in 2011, as 31 % of 108 trout in Oppdal (24 km river stretch) were recorded in these pools. The recorded salmon consisted of 13 % small ( 7 kg) individuals. The corresponding proportions in the reports from river fisheries were 22 %, 56 % and 22 %. The discrepancies in proportions between drift diving and river fisheries might partly or wholly be due to documented differences in catchability of difference size- and agegroups of salmon. However, this might also be partly or wholly a result of methodological shortcomings, such as a systematically error of assessing observed salmon a bit larger than true sizes. The latter is a common problem with underwater observations, as there is an optical augment of approximately 30 % in the air-water interface. A long-term fish barrier in the Snøvasfossan waterfall will make it impossible for sea trout to migrate into many of the most important spawning and nursery habitats in Driva. The length of the upstream area constitutes 71 % of the main stem, and 59-60 % of the production area for anadromous salmonids. In 2011 approximately 57 % of the 797 sea trout were observed upstream of the fish barrier site in Snøvasfossan. In lack of compensatory measures a maximum production loss of 55-75 % could be expected. On basis of the drift survey in 2011 this loss would have been up to 2800 trout. In order to avoid a considerable decline in trout production there ought to be compensatory measures to secure trout production upstream a future fish barrier. On the basis of high natural production capacity for anadromous salmonids there is a high potential for mitigation measures in the middle and upper reaches of Driva. The most effective compensatory measure is to transfer genetically tested sea trout and/or trout eggs to these areas. If at least 75 % of the ascending trout are transferred to the upstream areas during the function period, the overall production loss in sea trout is expected to be less than 25 % compared to the situation prior to the barrier. In periods of chemical treatment it is recommended to secure a sufficient number of immature and mature individuals caught in river sections downstream the fish barrier. Salmon should not be reintroduced to the upstream areas until the river system and the Driva region is documented free of Gyrodactylus salaris. A reintroduction of long term hosts (i.e. salmon and salmon x trout hybrids) during a period with a potential reservoir of parasites is considered to be a high risk strategy. A prolonged time horizon due to failure in eradication measures increases the risk of more or less irreversible changes in population genetics in trout and salmon.© Norsk institutt for naturforskning. Publikasjonen kan siteres fritt med kildeangivelse