Laks- og sjøørretbestanden i regulerte Bævra, Møre og Romsdal. Undersøkelser i 2005 og 2006

Abstract

Lund, R.A. & Johnsen, B.O. 2007. Laks- og sjøørretbestanden i regulerte Bævra, Møre og Romsdal. Undersøkelser i 2005 og 2006. - NINA Rapport 267. 98 s. Bævra er et sterkt regulert vassdrag og 43 % av nedslagsfeltet ble ved reguleringen i 1963 overført til kraftverket som ligger ca 4 km ovenfor vassdragets utløp i sjøen. De øvre 16 km av den lakseførende delen av hovedelva fikk svært liten vannføring ved reguleringen. I 2005 ble det påbegynt fiskebiologiske undersøkelser med formål å kartlegge bestandsstatus for laks og sjøørret i vassdraget. Prosjektet har videre som formål å vurdere effekter av reguleringen på fiskebestandene, tilrå aktuelle kompensasjonstiltak som kan øke den naturlige rekrutteringen av ungfisk i vassdraget og vurdere virkningen av utsetting av en-somrige laksunger og smolt i vassdraget. Laksefangstene i vassdraget har i senere år vært på et historisk lavmål, mens de rapporterte fangstene av sjøørret har økt. Som følge av den lave vannføringen i området ovenfor kraftverket, fanges lite fisk i dette området. Det er også mulig at de mange tersklene i et parti av elva ovenfor kraftverket er vandringshindre på de vannføringer som forekommer i fiskesesongen. Ut fra vurderinger av vannføringsforhold og fangstregistreringer i 2005 og 2006 så det ut til at få laks og sjøørret var oppvandringsklare i Bævra i juni. Etter at fiskeoppvandringen var begynt, gav økende vannføringer som følge av nedbør, et bedre fiske selv om kraftverket ble driftet ved tilnærmet full slukeevne. Fangstene økte også på dager med økte driftsvannføringer i perioder der kraftverket ble driftet med varierende vannføringer i et år med lite nedbør (vist for ørret i 2006). Det settes ut laksesmolt og en-somrige laksunger i Bævra etter et pålegg om årlige utsettinger på 10 000 smolt og 30 000 en-somrige. Gjenfangster av slik fisk utgjorde en vesentlig andel av sportsfiskefangstene i 2006 (20-30 %) og en mindre andel i 2005 (7 %). Gjenfangstraten for utsettinger av laksesmolt i 2004 og 2005 ble vurdert til å være henholdsvis god og ekstremt dårlig. Det er vanskelig å finne en rimelig forklaring på den uvanlig dårlige overlevelsen av laksesmolt som vandret ut av Bævra i 2005, da det ikke var spesielt uheldige vannføringsforhold i utvandringstiden for smolten. Gjenfangstene av en-somrige laksunger utsatt i 2003 ble vurdert som relativt god. Den svært lave ungfisktettheten i elva nedenfor kraftverket i 2006 kan ha sammenheng med raske vannstandsreduksjoner ved driften av kraftverket og tap av fiskunger ved stranding. I 2005 og 2006 ble det registrert mange driftstans ved kraftverket. I de fleste av disse situasjonene ble kraftverket avstengt fra vannføringsnivåer på 4-6 m3/s over et to-timers intervall. Det var moderate tettheter av ville laks- og ørretunger i områdene ovenfor kraftverket. I en slik situasjon anser vi det for å være mulig at det konsesjonspålagte antallet en-somrige laksunger er så høyt at denne fisken vil konkurrere med villfisk om næring og skjulplasser da produksjonsarealene i området er begrenset som følge av lav vannføring i store deler av året. Bævra, laks, sjøørret, vannkraftregulering, fisketetthet, vekst, produksjon, gytebestand, fiskeutsettinger, tiltak KEY WORDS The river Bævra, salmon, sea trout, hydro power development, parr density, growth, production, spawning stock, stocking of fish, mitigating measures

Lund, R.A. & Johnsen, B.O. 2007. The salmon and sea trout population in the regulated river Bævra, Møre and Romsdal county. Surveys in the years 2005 and 2006. - NINA Report 267. 98 pp. The river Bævra i significantly regulated for hydro power purpose. In 1963, 43 % of the catchment area was transferred to the hydro power station being situated 4 km above the river outlet in the sea. The water discharge of the anadromous stretch above the hydro power station (16 km) thereby was heavily reduced. In 2005 fish biological studies were commenced to improve the knowledge of status of the salmon and sea trout populations in the river. The aims of the project are to consider effects of the regulation on the populations, recommend mitigating measures to improve the natural recruitment of the populations and to consider the effect of annual releases of one summer old, marked salmon parr (30 000 ind.) and smolts (10 000 ind.) in the river. The annual salmon catches in the river have in recent years been at historical low level, while sea trout catches have been increasing. Due to the low discharge above the hydro power station, few fish are caught in this area. The many weirs just above the power station may also be obstructive to fish migration at the water discharges available during the fishing season. Based on considerations of the water discharge and catch records, it seemed that few salmon and sea trout were ready to ascend the river Bævra in June. After the commencement of the upstream migration, increasing water discharge as a consequence of precipitation, resulted in increased catches even if the power station was drifted at nearly full capacity. The catches were increasing in periods having increasing discharge through the power station in a fishing season generally having low precipitation (as shown for sea trout in 2006). Recaptures of smolts and one-summer parr constituted a substantial part of the angling catches in 2006 (20-30 %) and a lesser proportion in 2005 (7 %). The recapture rates of the smolt releases in 2004 and 2005 were considered to be respectively good and extremely low. It is difficult to find a reasonable explanation to the extraordinary low survival of salmon smolts outmigrating in 2005 as the water discharge during the suspected time of migration was not extraordinary low. The recapture rate of one-summer old parr realesed in 2003 was considered to be relatively good. The very low parr density in the river below the power station may have coherence to rapid decreases of the water level by the running of the power station and loss of parr by stranding. In 2005 and 2006 numerous drift cessations of the power station were recorded. In most of these situations the drift was shut down from water discharges levelling 4-6 m3/sec during two ours intervals. Above the power station there were moderate densities of both salmon and trout parr. In that situation we assume that the imposed amount of releases of one-summer old parr, can amount levels where these fish will compete with wild parr for food and shelter. This, because the production area is limited by low water discharge in large parts of the year. Absence of or low densities of salmon fry in substantial parts of the river above the power station in spite of presence of spawners in the same areas in the previous autumn, may indicate limited areas of suitable spawning grounds for salmon. In case the same areas show up with low densities of salmon fry in repeated years, will strengthen this assumption. In such areas fish releases will be a proper measure. Depositing spawning gravel will be an alternative measure. In the unregulated part of the water course above the outlet of the tributary Lille Bævra, neither parr nor adult salmon were observed in 2005 and 2006. Adult sea trout were sporadic observed in this area while the density of parr was moderate. Compared to studies in other rivers, the egg density estimated from spawners of wild and released salmon was near to low (1.4-1.8 egg pr m2 i 2006) to provide optimal production presupposed there is suitable spawning grounds in all parts of the river. Of the total amount of eggs estimated for the salmon population i Bævra in 2006, the contribution from wild, recaptures of one-summer old releases, recaptures of smolts released/farmed salmon escaped at the smolt stage (the two groups not possible to separate by the available method), farmed salmon and undetermined salmon was respectively 29 %, 7 %, 45 %, 13 % and 6 %. Future estimates will be improved significantly by being able to identify recaptures of smolts released, as this group constitute a high proportion of the egg amount. This can be achieved by marking the smolts before release. The exploitation rates estimated for salmon (35 % and 22 % in 2005 and 2006, total rates for wild, released and farmed salmon) are significantly lower than recorded in many other rivers in Norway. However, when considering the egg density provided from wild and released salmon (originating from wild parents), the exploitation rates seem to be too high. The exploitation rate of sea trout was high in 2005 (58 %) and low in 2006 (8 %) while the estimated egg density was low in both years (respectively 0.7-0.8 and 1.1-1.2 egg pr m2), being lowest in the year having the highest exploitation rate. The low egg densities pay attention towards attempts to reduce the exploitation level in future years. The low age of the sea trout (on average 3.0 summers in sea both in 2005 and 2006) also indicate high level of exploitation. In 2006 the river above the power station was the main area of presmolt production both for salmon and trout (respectively 87 % and 86 % of total production). According to commonly used ratios of sea survival (5-10 %), the estimated numbers of smolts outmigrating in the spring of 2007 (3200 smolts), will provide 160-320 salmon available for exploitation in the sea and river fisheries. The regulation seems to have changed the competitive state between salmon and trout and also to have reduced the fish production. This can be seen both from the incidence of parr of salmon and trout and from catch statistics.