Large individuals in dense brown trout (Salmo trutta) populations: not always a paradox

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Auren i Øvre Heimdalsvatn 2020

Som ein lekk i å oppretthalda den lange tidsserien med innsamling av bestandsdynamiske data på aure i det subalpine Øvre Heimdalsvatn, i Øystre Slidre, har det kvart år i dei siste seks åra, inkludert 2020, vore gjennomført eit garnfiske med ein såkalla pilotserie av settegarn med maskeviddene 24, 26, 2 x 29, 31, 2 x 35, og 39 mm. Denne serien er tilnærma same pilotserien som vart brukt ved bestandsstudiane på sekstitalet, der også data på fangst per innsats og estimert storleik på kvar aldersklasse inngjekk. Desse data kunne difor brukast til å estimera antal fisk i kvar av aldersklassane 4 – 8 vintrar, og ≥9 vintrar, basert på fangst per pilotserienatt i åra 2015 – 2020. Aldersbestemminga av auren er gjort med bruk av otolittar som før avlesing vert knekt gjennom sentrum og brende. Otolittane frå aure i Øvre Heimdalsvatnet ser ut til å gje svært presis aldersbestemming. Det er dessutan fullt samsvar mellom skjellalder og otolittalder fram til auren stagnerer i vekst. Det er ein klar årleg auke i lengdevekst til auren er minst 8-9 vintrar. Nokre fisk har ekstra god årleg tilvekst, og det kan vera eit resultat av fiskeeting, men dette er ikkje førebels avklart. Rekrutteringa til bestanden, målt som talet på fireåringar, har halde seg låg i dei siste seks åra, på same måte som i åra 1993 – 2009, bortsett frå 2018 då rekrutteringa ser ut til å ha skote i veret. Talet på eldre fisk har gått ned i løpet av dei siste fem åra, medan yngre fisk (aldersklassane 4 – 6 vintrar) har gått litt opp. Regresjonen mellom årstilveksten for seksåringar i ein rekke år i perioden 1992 – 2019 og junitemperaturen i Hinøgla er signifikant (Lineær regresjon, p <0.01), slik det også vart funne under studiane på sekstitalet, sjølv om det den gongen vart nytta lufttemperatur på Vågåmo som forklaringsparameter. Det vart då funne at temperaturen i juni og bestandstettleik kvar for seg forklarte om lag 50 % av variasjonen i årleg individuell vekst. Førebels er ikkje samanhengen mellom bestandsstorleik, temperatur og årleg tilvekst testa for perioden etter 1993. Trass i ein mykje større biomasse av eldre fisk i dei siste åra samanlikna med på sekstitalet, er veksten tilsynelatande bortimot den same som for rundt 50 år sidan då bestanden var utsett for eit langt hardare fiske enn i dag. Dette har mest sannsynleg samanheng med den reduserte rekrutteringa til bestanden. Det er noe sportsfiske i Øvre Heimdalsvatn, men det er dokumentert at dette har lite omfang. Auka beskatning ville høgst sannsynleg gje ein endå meir attraktiv bestand å fiska på.With the aim to continue the long time series regarding the population dynamics of brown trout, Salmo trutta, in the subalpine lake, Øvre Heimdalsvatn, a gillnet fishery with a pilot fleet consisting of gillnets with the following mesh sizes: 24, 26, 2 x 29, 31, 2 x 35, og 39 mm, has been conducted annually since 2015. This pilot gillnet fleet is approximately the same as used in the extensive brown trout studies in the lake during the 1960ies, which also included data on catch per unit effort and estimated number of each age-class. These data have been used as a basis in the present study to estimate the number of fish in the ageclasses 4 -8 winters, as well as age-class ≥9 winters, by catch per pilot fleet per night in end of August – beginning of September during the years 2015 - 2020. Otoliths have been used for age determination of the whole catch, with the following procedure: The otoliths were cut through the center, and one or both halves were burnt before reading the age under a stereo microscope. This is a method which seems to give precise age of brown trout from this lake. There is a distinct annual growth in length until at least the age 8 – 9 winters for most fish. Both scales and otoliths give the same age until stagnation in annual growth takes place. Some trout have an exceptional fast growth which may be connected to piscivory, although this has not yet been studied. The recruitment to the brown trout population in Øvre Heimdalsvatn, estimated as the number of four winter old fish, seems to have been reduced by around 50 % from the 1960ies to the period 1993 – 2009. This decline seems to have continued in the years 2015 – 2020, except in one year, 2018. The number of fish ≥10 winters seems to have declined during the period 2015 – 2020, but at the same time the number of fish in age-classes 4 – 6 winters has increased. The linear regression between the annual length increment of six winter old fish and the mean June temperature in the outlet, Hinøgla, within the time lag 1960 – 2019, was significant (Linear regression, p <0.01). This is comparable to the results obtained by the studies in the 1960ies, although at that time the air temperature at Vågåmo was used as an explanatory parameter, and June temperature and population density each explained about 50 % of the variation in annual individual growth. Preliminary, the interconnection between population density, water temperature and annual growth has not been modeled, but this will be performed when the annual data series is extended. Despite a much larger biomass of older fish today compared to the biomass in the 1960ies, the annual growth and size of older fish are nearly the same as around 50 years ago when the population was heavily exploited. Most probably, this is connected to the reduced annual recruitment to the population. Øvre Heimdalsvatn is open for recreational fishery, however, the sport fishery has a documented low effort. Increased exploitation will probably give a population with increased quality of individual fish, and thus a more attractive fishery

Betydninga av lange tidsseriar i forvaltningsrelatert havbruksforsking: Sjøaure i Hardangerfjorden - Årlege svingingar i antal og vekst hos parr og vekst hos postsmolt

The report includes studies on numbers, growth and survival of seatrout and salmon par in River Øyre, Kvinnherad from 1988 to 1998. The abundance o

Mysis segerstralei, an unexpected but important prey for resident Arctic charr (Salvelinus alpinus) in a Svalbard lake

Mysis segerstralei is distributed over a wide geographic area and in habitats with a variety of salinity conditions, including marine and brackish waters around Svalbard. The species has seldom been found in freshwater lakes, and the discovery of M. segerstralei in Lake Pulmankijärvi at the border between Northeastern Norway and Finland, may represent the single known freshwater occurrences in western Europe. Svalbard lake systems are characterized by very low water temperatures, long-term ice cover, and low levels of nutrients. Food is thus limited, and chironomids generally dominate the stomach contents in Arctic charr, the only freshwater fish species on Svalbard. Based on several surveys in more than 30 of Svalbard lakes over many decades, M. segerstralei has only been found as food for Arctic charr in Lake Vårfluesjøen. In a later fishery survey, we studied the diet of Arctic charr in this lake. The stomach contents from Arctic charr sampled in the profundal habitats were dominated by M. segerstralei, but the species was also among the most frequent prey items in the littoral and pelagic habitats. This unexpected occurrence of M. segerstralei demonstrates the high importance of mysids even in a low-productive, High Arctic lake. Glacial relicts · Mysis segerstralei · Arctic charr · Salvelinus alpinus · Svalbard freshwater lakes · High ArcticMysis segerstralei, an unexpected but important prey for resident Arctic charr (Salvelinus alpinus) in a Svalbard lakepublishedVersio

Arctic charr (Salvelinus alpinus) squeezed in a complex fish community dominated by perch (Perca fluviatilis)

In the complex fish community of Lake Skasen, southeastern Norway, the relative population density, habitat use and diet of Arctic charr, perch, roach and burbot was studied by a gill net survey during June-September 2010. A marked segregation in habitat use was observed, with Arctic charr and burbot captured in the profundal and deepest part of the pelagic habitat, and perch and roach captured in the littoral and upper part of the pelagic. Perch dominated the total catches, followed by roach. Arctic charr occurred in low numbers in the catches, and also had a low annual growth rate. Even in June, at low water temperatures, Arctic charr were confined to the profundal. Both Arctic charr, roach and perch fed on the same cladocerans, but all size groups of perch had fish as an important part of the diet. Analysis of stable carbon and nitrogen isotopes revealed a narrow trophic niche of Arctic charr, positioned at the extreme pelagic end of the carbon gradient relative to the other fish species. These had a wider span of δ13C signatures, but more positioned towards the littoral end of the carbon gradient. The low growth rate of Arctic charr, despite a low population density, indicates that food is a limiting resource for charr in this lake, probably due to a confinement to the profundal habitat as a result of competition and predator avoidance. Since all age-classes of Arctic charr seem to be enclosed in the profundal habitat, intraspecific competition and predation may be supplementary stressors resulting in low annual recruitment and low population density, as well as low individual growth rate, i.e. the population is squeezed. The narrow trophic niche of Arctic charr compared to perch, roach and burbot, revealed by stable isostope analysis, supports this conclusion

Gillnet catchability of brown trout Salmo trutta is highly dependent on fish size and capture site

Use of experimental gillnet fleets is common both in scientific studies of fish populations and in fish sampling for management purposes. Fish catchability may vary considerably with fish and gillnet mesh size, and catches obtained by gillnet fleets composed of nets with different mesh sizes may give length and age distributions that deviate considerably from the length and age structure of the population. We have estimated the absolute catchability of allopatric brown trout (Salmo trutta) in the littoral and pelagic habitat of a small lake based on a mark-recapture experiment. The brown trout catchability varied considerably both with fish size and habitat type, probably due to a size-related variation in swimming distance per time unit and a size-related use of the different lentic habitats. The sampling bias in experimental gillnet fishing may be reduced by operating the gillnet fleets in all possible lentic habitats and most fundamentally, by use of catchability data obtained from populations with ‘known’ length and age structures. By reducing this sampling bias, more realistic estimations of the age and length distribution for a given population will be possible

Expected Climate Change in the High Arctic—Good or Bad for Arctic Charr?

Lakes in the High Arctic are characterized by their low water temperature, long-term ice cover, low levels of nutrients, and low biodiversity. These conditions mean that minor climatic changes may be of great importance to Arctic freshwater organisms, including fish, by influencing vital life history parameters such as individual growth rates. In this study, Arctic charr sampled from two Svalbard lakes (78–79◦ N) over the period 1960–2008 provided back-calculated length-at age information extending over six decades, covering both warm and cold spells. The estimated annual growth in young-of-the-year (YOY) Arctic charr correlated positively with an increasing air temperature in summer. This increase is likely due to the higher water temperature during the ice-free period, and also to some extent, due to the winter air temperature; this is probably due to thinner ice being formed in mild winters and the subsequent earlier ice break-up. However, years with higher snow accumulation correlated with slower growth rates, which may be due to delayed ice break-up and thus a shorter summer growing season. More than 30% of the growth in YOY charr could be explained specifically by air temperature and snow accumulation in the two Arctic charr populations. This indicated that juvenile Svalbard Arctic charr may experience increased growth rates in a future warmer climate, although future increases in precipitation may contradict the positive effects of higher temperatures to some extent. In the longer term, a warmer climate may lead to the complete loss of many glaciers in western Svalbard; therefore, rivers may dry out, thus hindering migration between salt water and fresh water for migratory fish. In the worst-case scenario, the highly valuable and attractive anadromous Arctic charr populations could eventually disappear from the Svalbard lake systems. High Arctic; Svalbard lake systems; climate impact; Arctic charr; growth rate; anadromypublishedVersio

The Hardanger Fiord - salmonid aquaculture and effects on wild salmonid populations

Wild populations of salmon and seatrout in the Hardanger Fjord show significant declines in recent years.The report also describes the hydrographical conditions and the aquaculture production of salmonids in the fjord. The official numbers of adult salmon lice predating on cage reared salmon are used to estimate the production of salmon lice during spring in this fjord. It is assumed that salmon lice are the most acute threat to wild salmonids in the fjord, but the genetic impact of farmed escapees is also of great concern. A lowering of the action level for salmon lice treatment and initiation of a environmental survaillance programme is proposed. NORSK SAMMENDRAG: Bestandsutviklingen for ville bestander av laks og sjøørret i Hardangerfjorden er faretruende. Hydrografiske forhold og produksjonen av laksefisk i soner i fjorden blir beskrevet. Tellinger av lakselus på fisk i merdene er hentet inn fra 1997 og blir korrelert med fiskemengden i merdene for å kunne estimere produksjonen av lakseluslarver om våren. Basert på kunnskap om lakselusens biologi antas det at lakselus er den største akutte trussel mot ville laksefisk i fjorden, men at rømming og oppvandring i elvene er en høyst potensiell genetisk trussel. Det foreslås at tiltaksgrensen for behandling mot lakselus senkes og at det initieres et overvåkingsprogram