A long-term study of the impact of the invasive species Eurasian minnow Phoxinus phoxinus on brown trout Salmo trutta production in a high mountain lake, Southern Norway

The shallow Lake Skjerja was originally one of the most outstanding brown trout lakes on the Hardangervidda mountain plateau. During the period 1973–1985, the annual yield was on average 3.35 kg ha-1. In the mid-1980s, Eurasian minnow established a dense population in the lake. Since 1994, 5304 kg of Eurasian minnow have been removed with baited traps, which correspond to an annual mean catch of 177 kg year-1 or 1.12 kg ha-1. The competition for food, in addition to longer food chains, has had a strong negative impact on the brown trout production, with an annual yield reduced to 1.07 kg ha-1, or 32% of the historical catches (1973-1985). The total removed biomass of the two fish species correspond to 65% of the brown trout yield before the invasion of the Eurasian minnow. In Lake Skjerja, the two crustacean species Gammarus lacustris and Lepidurus arcticus, have historically been staple food items for brown trout. Fishing with baited traps on the minnows, and the presence of a large fraction of predatory brown trout, may have reduced the predation pressure on the two crustacean species, and they are still a part of the brown trout diet. The availability of fish as food has resulted in a significant increase in the presence of large individuals of brown trout with maximum weights above four kg. Despite reduced annual yield of brown trout, Lake Skjerja is still a popular fish destination with nearly 200 big-sized brown trout (mean annual weight 1065 grams) landed annually. On Hardangervidda, an increase in air temperature has been observed since the 1980s, with a corresponding change in water temperature which may benefit Eurasian minnow. In warm summers, water temperature approaches 14°C which has proved to be near to the upper thermal threshold for Lepidurus arcticus. Hardangervidda, climate change, brown trout yield, Eurasian minnow removalpublishedVersio

Merkingsforsøk i fisketrappa ved Løpet i Renavassdraget i perioden 1985-2000

I perioden 1985-2000 ble 3159 harr og 1110 ørret registrert og merket i fisketrappa ved Løpet. Oppgangen av begge arter varierte mye fra år til år. Det var en negativ utvikling i oppgangen av ørret i løpet av undersøkelsesperioden, mens det var ingen tilsvarende trend i den årlige oppgangen av harr. Det vil si at det relative innslaget av harr i forhold til ørret i trappa har økt i perioden. Med ett unntak, en sik på 39 cm, var det kun harr og ørret som ble registrert i trappa. Gjennomsnittslengden til harr og ørret som ble merket i trappa var på henholdsvis 29.7 og 28 cm, og det var hovedsakelig individer > 20 cm av begge arter som ble registrert. Lengdefordelingen til harr eller ørret som gikk i trappa så ikke ut til å ha endret seg i løpet av undersøkelsesperioden. Innrapporterte gjenfangster av harr og ørret fra fiskerne viste at henholdsvis 88 % og 90 % av gjenfangstene ble foretatt innenfor en avstand på 20 km fra merkestedet. En stor andel av gjenfangstene av harr (87 %) og ørret (61 %) ble gjenfanget oppstrøms merkestedet, og områdene ved Rødsbrua (9 km fra Løpet) og Deset (17 km fra Løpet) peker seg ut med spesielt mange gjenfangster. Merkingene viser at både harr og ørret kan tilbakelegge store avstander i vassdraget. Ørret merket ved Løpet ble gjenfanget oppstrøms så langt opp som ved Flena’s utløp i Storsjøen (60 km fra Løpet) og nedstrøms så langt ned som til Strandfossen i Glomma (32 km fra Løpet). Gjenfangstene av harr ble innrapportert så langt opp som ved Andrå i Storsjøen (50 km fra Løpet) og nedstrøms så langt ned som til Strandfossen. Selv om enkelte gjenfangster av både harr og ørret viser at de kan forflytte seg over store avstander, er trolig omfanget av fiskevandringer i vassdraget mindre nå enn tidligere. Merkings-forsøk av harr i Løpet på slutten av 1960-tallet viste blant annet en større spredning av gjenfangstene enn det som ble registrert i denne undersøkelsen. Det ble ikke registrert massiv tilbakevandring av harr på høsten til overvintringsområder i Storsjøen slik som beskrevet tidligere (bl.a. Sømme’s beskrivelse av ”gangharren”). Gjennomsnittlig gjenfangstprosent (av fiskere) av harr og ørret i løpet av undersøkelsesperioden var på henholdsvis 13.6 og 12.3 %, men det var en tydelig nedgang i gjenfangstprosenten i løpet av undersøkelsesperioden for begge arter. Dette kan nok delvis skyldes lavere beskatning, men skyldes sannsynligvis også at andelen fiskere som sendte inn merke sank i løpet av perioden.English: A total of 3159 grayling Thymallus thymallus and 1110 brown trout Salmo trutta were tagged with Floy anchor tags in the fish ladder at Løpet in the regulated river Rena during the period 1985-2000. The annual number of observations of brown trout declined during the study period, whereas no significant trend in the annual number of observations of grayling was found. The mean lengths of tagged grayling and brown trout were 29.7 and 28 cm, respectively, and few individuals < 20 cm were recorded in the fish ladder. Most recaptures of both grayling (87 %) and brown trout (61 %) by anglers were reported from the upstream face of the dam. The mean frequencies of recaptures of grayling and brown trout reported by anglers were 13.6 % and 12.3 %, but there was a decreasing trend in the frequency of individuals recaptured during the study period. This might partly be explained by a lower rate of fishing mortality. The frequency of anglers actually reporting recaptures, however, was probably declining towards the end of the study period

A long-term study of the impact of the invasive species Eurasian minnow Phoxinus phoxinus on brown trout Salmo trutta production in a high mountain lake, Southern Norway

The shallow Lake Skjerja was originally one of the most outstanding brown trout lakes on the Hardangervidda mountain plateau. During the period 1973–1985, the annual yield was on average 3.35 kg ha-1. In the mid-1980s, Eurasian minnow established a dense population in the lake. Since 1994, 5304 kg of Eurasian minnow have been removed with baited traps, which correspond to an annual mean catch of 177 kg year-1 or 1.12 kg ha-1. The competition for food, in addition to longer food chains, has had a strong negative impact on the brown trout production, with an annual yield reduced to 1.07 kg ha-1, or 32% of the historical catches (1973-1985). The total removed biomass of the two fish species correspond to 65% of the brown trout yield before the invasion of the Eurasian minnow. In Lake Skjerja, the two crustacean species Gammarus lacustris and Lepidurus arcticus, have historically been staple food items for brown trout. Fishing with baited traps on the minnows, and the presence of a large fraction of predatory brown trout, may have reduced the predation pressure on the two crustacean species, and they are still a part of the brown trout diet. The availability of fish as food has resulted in a significant increase in the presence of large individuals of brown trout with maximum weights above four kg. Despite reduced annual yield of brown trout, Lake Skjerja is still a popular fish destination with nearly 200 big-sized brown trout (mean annual weight 1065 grams) landed annually. On Hardangervidda, an increase in air temperature has been observed since the 1980s, with a corresponding change in water temperature which may benefit Eurasian minnow. In warm summers, water temperature approaches 14°C which has proved to be near to the upper thermal threshold for Lepidurus arcticus

An automatic image-based system for detecting wild and stocked fish

Fish stocking is the method of raising fish in a hatchery and releasing them into a river or lake to sustain or increase an existing population or to create a population. This has been practised in many countries, including Norway. Before the fish are released, the adipose fin is commonly removed in order to identify that it is a stocked fish. Cameras have been mounted in several Norwegian rivers in order to monitor fish populations. Classification of fish from these cameras is today a manual task carried out by people. In this paper we propose an automatic classification method to separate wild fish from stocked fish using machine learning. Experiments on an image set of trouts (Salmo Trutta) show a very high accuracy of the proposed method.publishedVersionThis paper was presented at the NIK-2019 conference; see http://www.nik.no

Hva om Hunderørret var laks

Storørret defineres som en selvreproduserende stamme med regulær forekomst av store fiskespisende individer, ofte med et nisjeskift og markert vekstomslag ved overgangen til fiskediett. Den mest kjente storørretstammen her i landet er den storvokste Hunderørreten som gyter og vokser opp i nedre del av Gudbrandsdalslågen og bruker Mjøsa som matfat. Til tross for likhetstrekkene med laks, er forvaltningen av Hunderørreten svært forskjellig fra moderne lakseforvaltning. Vi foreslår derfor at prinsippet med forvaltning etter gytebestandsmål også brukes for Hunderørreten. Det vandrer inntil 700 storørret forbi fisketrappa i Hunderfossen kraftverk årlig. Strekningen ovenfor Hunderfossen kraftverk og opp til Harpefossen representerer et stort potensial for naturlig rekruttering. Ved å øke oppvandringen forbi Hunderfossen ved hjelp av fangst og transport av gytefisk, er det mulig å øke bestanden betydelig basert på naturlig rekruttering. Vi mener det er realistisk å øke oppvandringen til 2000 fisk årlig, noe som vil sikre bevaring av bestanden og øke avkastningen i fremtiden.publishedVersio

Pasvikelva – Habitatkartlegging og ungfiskundersøkelser med fokus på storørret

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Fiskebiologiske undersøkelser i Selbusjøen og Nea med sideelver i 2016

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Changes in ecosystem services from wetland loss and restoration: An ecosystem assessment of the Danube Delta (1960–2010)

Deltaic flood plains provide critically important ecosystem services, including food production, fresh water, flood control, nutrient cycling, spiritual values and opportunities for recreation. Despite growing recognition of their societal and ecological importance, deltaic flood plains are declining worldwide at alarming rates. Loss of wetland ecosystem services bears socio-environmental costs overlooked in land-use planning. Conversely, wetland restoration can deliver important long-term benefits. This paper examines effects of different land use policies on ecosystem services provided by the Danube Delta, one of Europe’s largest and most outstanding wetlands. First, we identify, characterize and measure the most important ecosystem services provided by the Danube Delta. Second, we assess trends between 1960 and 2010, contrasting periods of economic development (1960–1989) and ecological restoration (1990–2010). Our results indicate that i) the Danube Delta provides important services with benefits accrue from local communities to humanity at large, ii) that two thirds of the Delta’s ecosystem services have declined over the studied period and iii) that ongoing restoration efforts have so far been unable to reverse trends in ecosystem service decline. Benefits from ecological restoration policies are already becoming apparent, but at a scale not yet comparable to the costs from ecosystem decline incurred over previous decades. Ecosystem assessment Socio-environmental costs Wetlands Restoration Danube Delta RomaniaacceptedVersio

Mismatch between fishway operation and timing of fish movements: a risk for cascading effects in partial migration systems

Habitat fragmentation is a growing problem worldwide. Particularly in river systems, numerous dams and weirs hamper the movement of a wide variety of species. With the aim to preserve connectivity for fish, many barriers in river systems are equipped with fishways (also called fish passages or fish ladders). However, few fishways provide full connectivity. Here we hypothesized that restricted seasonal opening times of fishways can importantly reduce their effectiveness by interfering with the timing of fish migration, for both spring- and autumn-spawning species. We empirically tested our hypothesis, and discuss the possible eco-evolutionary consequences of affected migration timing. We analyzed movements of two salmonid fishes, spring-spawning European grayling (Thymallus thymallus) and autumn-spawning brown trout (Salmo trutta), in Norway's two largest river systems. We compared their timing of upstream passage through four fishways collected over 28 years with the timing of fish movements in unfragmented river sections as monitored by radiotelemetry. Confirming our hypothesis, late opening of fishways delayed the migration of European grayling in spring, and early closure of fishways blocked migration for brown trout on their way to spawning locations during late autumn. We show in a theoretical framework how restricted opening times of fishways can induce shifts from migratory to resident behavior in potamodromous partial migration systems, and propose that this can induce density-dependent effects among fish accumulating in lower regions of rivers. Hence, fragmentation may not only directly affect the migratory individuals in the population, but may also have effects that cascade downstream and alter circumstances for resident fish. Fishway functionality is inadequate if there is a mismatch between natural fish movements and fishway opening times in the same river system, with ecological and possibly evolutionary consequences for fish populations

Efficiency of a fishway on brown Trout (Salmo trutta) spawning populations

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