Long term trends of fish after liming of Swedish streams and lakes

AbstractThousands of Swedish acidified lakes and streams have been regularly limed for about 30 years. Standard sampling of fish assemblages in lakes and streams was an important part of monitoring the trends after liming, i.e. sampling with multi-mesh gillnets in lakes (EN 14757) and electrofishing in streams (EN 14011). Monitoring data are nationally managed, in the National Register of Survey test-fishing and the Swedish Electrofishing Register. We evaluated long-term data from 1029 electrofishing sites in limed streams and gillnet sampling in 750 limed lakes, along with reference data from 195 stream sites and 101 lakes with no upstream liming in their catchments. The median year of first liming was 1986 for both streams and lakes. The proportion of limed stream sites with no fish clearly decreased with time, mean species richness and proportion of sites with brown trout (Salmo trutta) recruits increased. There were no consistent trends in fish occurrence or species richness at non-limed sites, but occurrence of brown trout recruits also increased in acid as well as neutral reference streams. Abundance of brown trout, perch (Perca fluviatilis) and roach (Rutilus rutilus) increased significantly more at limed sites than at non-limed reference sites sampled before and after 1986. The mean species richness did not change consistently in limed lakes, but decreased in low alkalinity reference lakes, and fish abundance decreased significantly in limed as well as in non-limed lakes

Seasonal variation in thermal habitat volume for cold-water fish populations : implications for hydroacoustic survey design and stock assessment

For accurate stock assessment, survey design must consider fish behavior and ecology. Yearlings and older individuals of the commercially exploited cold-water species vendace (Coregonus albula) are found below the metalimnion through periods of thermal stratification. These stratification periods generally last for 3-4 months, from the middle of summer to early autumn. In lakes with heterogeneous distribution of depths, the habitat volume for vendace vary drastically within and across years, which affects the distribution and population densities. Variable thermal habitat volumes, with food and oxygen depletion in the hypolimnion through the period of stratification, may act as a population size-regulating factor.Using hydroacoustics in combination with trawl data and temperature profiles, we examined the distribution of vendace through annual periods of thermal stratification. We found that yearling and older vendace these periods were confined to cold-water habitat volumes representing less than 10 % of the total water volume of Lake Mälaren, the third largest lake in Sweden. By introducing stratification to the design of hydroacoustic surveys supported by midwater trawling, seasonal aggregations of fish in temporally restricted thermal habitat volumes can be used to lower survey effort and improve the precision in estimates of population size. Temporally restricted habitat volumes may induce risks for the populations to over-fishing and sensitivity to environmental changes that potentially may call for directed management

Ecological marginality and recruitment loss in the globally endangered freshwater pearl mussel

Aim Ecological marginality is the existence of species/populations in the margins of their ecological niche, where conditions are harsher, and the risk of extinction is more pronounced. In threatened long-lived species, the disparity between distribution and population demography may provide understanding of how environmental heterogeneity shapes ecological marginality, potential extinction patterns and range shifts. We set out to evaluate this by combining a species distribution model (SDM) with population-specific demography data. Location Sweden, 450,000 km(2). Major Taxa Studied Freshwater pearl mussel (FPM, Margaritifera margaritifera) and two salmonid fish species. Methods A SDM for the mussel was constructed with MaxEnt using salmonid host fish (Salmo trutta plus S. salar) density, extreme low and high temperatures, precipitation, altitude, and clay content as explanatory variables. The output was used to test the ecological marginality hypothesis by evaluating whether lowly predicted populations had higher loss of recruitment. Logistic regression was used to explicitly test the factors involved in recruitment loss. Results Host fish density contributed the most (50.3%) to the mussel distribution, followed by lowest temperature the coldest month (34.3%) and altitude (10.3%), while the remaining explanatory variables contributed minimally (<3.3%). Populations with lower SDM scores lacked recruitment to a significantly higher degree. Populations inhabiting areas at low altitude, with lower densities of host fish, and warmer winter temperatures have lost recruitment to a higher degree. Main Conclusions We found support for the ecological marginality hypothesis. The patterns indicate that FPM habitat niche may shift northwards over time. Salmonid host fish density seems to be a driving factor for both historical distribution and recent demographic performance. Finally, we emphasize the value of combining SDMs with independent data on population demography as it both lends rigidity to model validation and understanding of how ecological marginality affects species distribution and viability

Negative influence of a threatened species on ecological status classification: A case study of the influence of European eel within the Swedish fish index VIX

Biological indicators are important quality elements for classification of ecological status of water bodies ac-cording to the European Water Framework Directive. Multimetric indices are commonly regarded as robust and reliable indicators of human impact and are often used as quality elements. In fish-based indices, species are often grouped into guilds based on general tolerance to common anthropogenic pressures, with higher pro-portion of tolerant species being indicative of degraded systems. Within the Swedish electrofishing index VIX, the critically endangered European eel Anguilla anguilla (L.) is classified as a tolerant indicator species, and it therefore has a negative effect on classified ecological status. The scientific literature, however, suggests that eels are not generally tolerant and they benefit from similar environmental conditions as many insensitive species. VIX has been criticized for being too sensitive to the presence of eel in catch data, leading to low status clas-sifications when eels are caught in the monitoring surveys. In a case study using manipulations of historical electrofishing data, we assessed the influence of eel presence and abundance on the ecological status classifi-cation as determined by VIX. We demonstrate that reduction of eels in survey data have positive effects on the classified status, in many cases substantial effects. An increase of eels in the data had the reverse effect. Mere presence of eel had a strong negative effect, which is problematic if the aim is to increase the endangered eel population. Given the Swedish classification system where the quality element indicating the worst status is decisive, the classified ecological status of Swedish rivers can theoretically be improved by management actions disfavouring eel, unless the results from VIX are carefully evaluated by experts. Along the same lines, measures implemented with an aim to increase the endangered eel population will lead to a decrease in assessed ecological status of Swedish rivers. Our conclusion is that the usage of VIX within Swedish water management is prob-lematic and needs revision. From a broader perspective, the classification of species as generally tolerant need to be approached with great caution when developing new indices for assessing ecological status and integrity

Översikt, riskbedömning och förslag på åtgärder för puckellax (Oncorhynchus gorbuscha)

Puckellaxen (Oncorhynchus gorbuscha) tillhör ett släkte med tolv arter som brukar kallas stillahavslaxar. Flera arter inom släktet, däribland puckellaxen, är semelpara, d.v.s. de leker endast en gång och dör efter leken. Puckellaxen är den minsta av stillahavslaxarna och är i sitt ursprungsområde i Stilla havet den talrikaste. Flera av arterna i släktet, däribland puckellaxen, har introducerats på olika platser i världen – utanför arternas naturliga utbredningsområde. Det finns en befogad farhåga att invasiva främmande arter ska ha negativ påverkan på inhemska arter. En sammanställning som gjordes 2004 visade att invasiva arter är inblandade direkt eller indirekt i 6 % av alla utdöende eller kraftiga populationsminskningar av inhemska arter. I Sverige finns det idag ca 2000 introducerade arter av vilka 350 är klassificerade som invasiva, d.v.s. de hotar den biologiska mångfalden och ekosystemtjänster. Puckellaxen har på senare tid, framförallt under 2017, fångats eller observerats längs norska kusten, den svenska västkusten, Jylland i Danmark och även i Irland och Skottland. Flertalet av dessa puckellaxar härrör från ryska (sovjetiska) utsättningar runt Kolahalvön under perioden 1956 – 1998. Allt pekar på att arten har etablerat sig i området vid Norra ishavet och nu är inne i en snabb spridningsfas. Möjligheterna att utrota arten är förmodligen helt borta och det är osäkert om de ryska myndigheterna har önskan eller möjligheterna att vidta några åtgärder. Arten kan komma att påverka inhemska laxfiskar negativt och ur ett faunavårdsperspektiv vore det därför bra om puckellaxen kunde begränsas, både dess numerär och dess spridning. Puckellaxen har en strikt två-årscykel och i ursprungsområdena runt Stilla havet förekommer både udda- och jämna-års-bestånd. Eftersom det är udda-års-bestånden som lyckats bäst i nordvästra Ryssland så kommer förmodligen färre puckellaxar att observeras 2018, men eventuellt kan det bli än fler under 2019, färre 2020, fler 2021, och så vidare

Jämför- och referensvärden från Svenskt Elfiskeregister

Rapporten utgörs av en mängd tabellerade jämför- och referensvärden, från elfiskeundersökningar spridda över landet åren 2008-2015. Materialet har delats in efter avrinningsområdets storlek, geografisk region samt typ av öringpopulation (strömlevande, insjövandrande, havsvandrande) eller laxpopulation (insjövandrande (Vänern) eller havsvandrande). Förslag ges (avsnitt 4) på hur man kan använda detta referensmaterial för att jämföra med sina egna elfiskeundersökningar. De tätheter som anges utgör beräknade tätheter från elfisken utförda i de olika regionerna, det vill säga fältdata korrigerade för fångsteffektivitet genom antingen upprepade utfisken eller skattade fångsteffektiviteter. Tätheterna visar inte på beräknad potential/produktion vid opåverkade förhållanden. Jämförvärdena och referensvärdena ger en jämförelse med motsvarande vatten idag, med den status de för tillfället har

Environmental Flow Scenarios for a Regulated River System: Projecting Catchment-Wide Ecosystem Benefits and Consequences for Hydroelectric Production

To enable prioritization among measures for ecological restoration, knowing the expected benefits and consequences of implementation is imperative but rarely explicitly quantified. We developed a novel method to prioritize among environmental flow measures to rehabilitate ecosystems in the Ume River catchment in northern Sweden, a river system heavily regulated for hydropower production. Our strategy was to identify measures with minimal impact on hydropower production while providing substantial environmental benefits. Based on field surveys of remaining natural values and potential for ecological rehabilitation, we quantified the projected gain in habitat area of implementing environmental flows for target organism groups, for example, lotic fish species and riparian vegetation, along the whole river length. We quantified the consequences for hydropower production by identifying a set of hydropower operational rules reflecting the constraints added by environmental flows. We then used production optimization software to calculate changes in hydropower production and revenues. Implementing restrictions on zero-flow events by mandating minimum discharge at all run-of-river hydropower stations and allocating 1%-12% of mean annual discharge to bypassed reaches in the entire catchment would result in a 2.1% loss of annual electricity production. Adding flow to fishways would increase the loss to 3.1% per year. With implementation of more natural water-level fluctuations in run-of-river impoundments, the loss increases to 3.8%. These actions would increase the habitat for lotic species like the grayling Thymallus more than threefold and increase the area of riparian vegetation by about 66%. Our method forms a basis for ongoing implementation of nationwide environmental rehabilitation schemes