Spöprovfiske efter gädda i Vänern

För en hållbar förvaltning av svenska gäddbestånd behövs kunskap om beståndens status. Dataunderlag för att bedöma status kommer huvudsakligen från standardiserade övervakningsprogram. De metoder som i dag används för fiskövervakning lämpar sig dock inte för gädda varför riktade spöprovfisken har kommit att bli en viktig metod för uppföljning av gäddbestånd. Under våren 2022 genomfördes för första gången ett standardiserat spöprovfiske i sex vikar i Vänern, fördelade på tre områden. Fisket följde den metodik som i stor utsträckning har använts på ostkusten, vilket möjliggjorde en jämförelse av täthet (fångst per ansträngning) och storleksstruktur mellan Vänern och ostkusten. Resultaten visar att gäddbestånden i Vänern är glesare, med färre antal fångade gäddor per spöfisketimme (0.24), jämfört med ostkusten, där två tidigare studier fångat 0.75 respektive 0.61 gäddor per spöfisketimme. Däremot är gäddorna större i Vänern. Medianstorleken var 74 cm, vilket är ca 13 cm mer än på ostkusten. Indikatorn L90, vilken anger längden som 10 % av gäddorna är längre än, var 103 cm i Vänern jämfört med omkring 75 cm på ostkusten. Därtill redovisas information om lekstatus, könskvot samt kroknings- och blödningsskador i rapporten. Insamlat data är användbart för bedömningar av gäddans status i Vänern och för jämförelser med andra svenska gäddbestånd. Metodiken som har använts bedöms utgöra en grund för framtida uppföljning av Vänerns gäddbestånd

Estimating recreational trap-fishing effort for crayfish from zig-zag line transects, drone surveys and enforcement surveys

Estimates of recreational fishing effort and catch are needed to inform fisheries management, but such estimates can be challenging to obtain. We combined a zig-zag line transect survey of buoys, a fisheries enforcement survey and a drone survey to estimate recreational effort in the open-access trap fishery for signal crayfish (Pacifastacus leniusculus) in the sixth largest lake in Europe, Lake V & auml;ttern, Sweden. Using supporting variables such as size of nearby ports and bathymetry, our model-assisted estimate of total effort was 6400 (+/- 4300 CI) buoys during the three-weekend season. Using the frequency of buoys to traps as a constant, the total number of crayfish traps was estimated to be 25,500 (+/- 17,000 CI). The sampling design and methods presented are useful for ecologists and resource managers to design future multi-source surveys of recreational trap fishing effort

Predation and spatial connectivity interact to shape ecosystem resilience to an ongoing regime shift

Ecosystem regime shifts can have severe ecological and economic consequences, making it a top priority to understand how to make systems more resilient. Theory predicts that spatial connectivity and the local environment interact to shape resilience, but empirical studies are scarce. Here, we use >7000 fish samplings from the Baltic Sea coast to test this prediction in an ongoing, spatially propagating shift in dominance from predatory fish to an opportunistic mesopredator, with cascading effects throughout the food web. After controlling for the influence of other drivers (including increasing mesopredator densities), we find that predatory fish habitat connectivity increases resilience to the shift, but only when densities of fish-eating top predators (seals, cormorants) are low. Resilience also increases with temperature, likely through boosted predatory fish growth and recruitment. These findings confirm theoretical predictions that spatial connectivity and the local environment can together shape resilience to regime shifts

Strong positive relationships between eDNA concentrations and biomass in juvenile and adult pike (Esox lucius) under controlled conditions: Implications for monitoring

Reliable abundance information is the foundation for managing aquatic resources. Species with low catchability are, however, often overlooked in monitoring programmes. Thus, governing bodies lack the data necessary to make well-informed management decisions. Environmental DNA (eDNA) can produce quantitative estimates of fish abundances, but the precision varies greatly depending on the species and system. It is, therefore, necessary to evaluate its performance and investigate how fish biomass and density affects eDNA dynamics on a case-by-case basis before eDNA-based monitoring can be a viable option. Here, we evaluate how biomass and density of an ecologically and socioeconomically important top predator, the Northern pike (Esox lucius), relate to eDNA concentrations in controlled aquarium and mesocosm experiments. We carried out experiments using both juvenile and adult individuals and evaluated eDNA, biomass and density relationships at three different time points using a previously developed TaqMan assay, targeting the cytochrome oxidase I gene. We also evaluated the performance of multiple extraction methods (DNeasy Blood & Tissue kit, DNeasy PowerWater kit, and Chelex 100), and filtering systems (single- vs. double-membrane filters). The results from both pike experiments showed a strong positive linear relationship between eDNA concentration and pike biomass (R2 = 0.74 – 0.87). Levels of eDNA dropped drastically within the initial 24 h of juvenile pike being removed from the aquaria, and low levels were detectable for up to 308 h. Of the extraction methods, Chelex 100 yielded the highest DNA concentration, offering a quick and cost-effective alternative compared with existing widely used extraction methods. Using double membrane filters of different material showed no increase in DNA yield regardless of the extraction method but it allowed more water to be processed. Although several challenges remain, our results show that eDNA holds promise to become a useful tool for monitoring fish biomass in natural environments

Developing management goals and associated assessment methods for Sweden’s nationally managed fish stocks : a project synthesis

This report summarizes and synthesizes results from the Swedish Agency of Marine and Water Management (SwAM, or HaV) funded project “Förvaltningsmål för nationella arter (Management goals for nationally managed species)”. The objectives of the project have been to promote the development of management goals and associated status assessment methods and indicators, as well as reference points, for some nationally managed fish stocks both in coastal as well as freshwater areas. The report focusses largely on species and stocks that can be defined as data-poor. Such stocks are characterised by marked limitations in data availability and/or resources allocated to detailed analytical stock projections. Data-poor stocks also often lack carefully formulated management goals and associated methods and indicators for assessing stock status. In this report, we provide an overview of potential assessment methods and indicators and try to synthesise how they work and what the strengths and weaknesses are by applying them to selected data poor stocks such as pikeperch, pike, whitefish, and vendace. We also discuss how they relate to different potential management goals and provide recommendations for their application. We grouped the indicators and assessment methods by the three categories that are now used in the yearly status assessment framework provided by SLU Aqua (Resursöversikten/Fiskbarometern) – i) mortality, ii) abundance/biomass and iii) size/age structure. The results are also described for these three main categories of assessment indicators. Included is also a status report from a size- and age-based population dynamics model (Stock Synthesis 3) that is being developed for pikeperch in Lake Hjälmaren.An important experience from the project is that to improve the assessment methods for Swedish national fish stocks, it is important that managers develop both general as well as more detailed quantitative goals for the individual stocks. This should ideally be conducted in various forms of collaboration with the main stakeholders and scientists involved with assessment as participatory processes foster legitimacy. Carefully articulated management goals, which are possible to translate into quantitative targets, will facilitate the development of various approaches and methods to monitor stock statuses. Given the strong and complex interactions of fish and their environments it is also important to consider other pressures than fisheries when developing indicators and assessment methods.Our synthesis highlights a number of areas where the assessment of data-poor stocks can be improved:1. Apply precautionary principles for data-limited stocks, particularly ones that are known to be vulnerable to exploitation.2. Tailor approaches to how fisheries are managed in Sweden. Swedish nationally managed fish stocks are not managed by quotas (with one exception, vendace in the Bothnian Bay) and do not aim for maximum sustainable yield. Instead, the coastal and inland fisheries are managed by regulating the effort in the small-scale commercial fisheries (number of fishers/licenses and amount of gear). Regulation of recreational and subsistence fisheries effort, in terms of licenses or number of fishers) is not applied, nor possible since the fisheries is lacking obligatory notification and reporting systems. All national fisheries, however, are regulated by various technical measures (closed areas, size-limits, bag-limits, gear restrictions etc). Thus, goals and assessment methods that result in harvest limits or quota recommendations expressed in e.g. biomass/numbers are difficult to use as basis for management. Instead, there is a need for alternative management goals and associated assessment methods.3. Use best practice methods and indicators and adapt as scientific knowledge is developed. Data-limited methods are developing rapidly, and new methods/approaches are proposed in the scientific literature every year. It is thus important to be updated on the most recent developments. 4. Clearly describe limitations/assumptions of methods used. It is important to be aware of and critically evaluate the assumptions underlying the analyses, and to carefully communicate uncertainty together with the stock status assessment.5. Be particularly careful with low sample numbers. Many indicators and methods can be applied also on small sample sizes, however, the accuracy and precision of the estimates risk being low in such cases.6. Accept that there is no "gold standard" for fisheries assessment. Each case study is unique and needs to be balanced against data availability, local needs and other important factors. This also means that analysts need to be careful when using generic reference levels or “borrowing” data from other stocks.7. If possible, use several different methods/indicators. Although several indicators aim to measure similar aspects of the stock, small methodological differences can support the overall interpretation of individual indicator values. It is particularly important to incorporate many aspects and indicators (size/age/abundance/mortality) in order to produce a balanced assessment.8. Develop means of communication. Indicators and goals should be easy to understand. However, interpretation of results from multi-indicator frameworks can be challenging. There is thus a need for finding ways of communication that can convey complicated results in a simple-to-understand manner.9. For details on additional improvements, we refer the reader to the sub-header “recommendations for the future” found under each chapter.The implementation of Stock Synthesis for pikeperch in Lake Hjälmaren showed that it is possible to develop a more ambitious and detailed stock assessment model for a relatively data-poor stock. The model results partly support earlier interpretations of the development of the stock and the importance of the changes in regulations in 2001 (increased minimum size, increased mesh size and reduced mortality of undersized pikeperch). Before the model can be implemented and used for practical management, a number of actions for improvement are needed, which are highlighted in the relevant chapter. The most important next step is establishing management goals and reference levels for this stock. We recommend that such a dialogue is initiated by managers. The fisheries management goals should consider both biomass, fisheries mortality and size-based targets.To conclude, we stress the importance of improving all ongoing aspects related to the assessments of data-poor Swedish stocks. Strong local stocks and sustainable fisheries are vital for a variety of fisheries-related businesses and practices, particularly in rural areas, providing economical and societal value. Fishes also have important roles in aquatic food-webs and it is important that ecological values are managed wisely in order to reach targets for water quality, ecosystem structure and diversity. Given the strong and complex interactions of fish and their environments it is also important to consider other pressures than fisheries when developing indicators and assessment methods

Long-term decline in northern pike (Esox lucius L.) populations in the Baltic Sea revealed by recreational angling data

In the Baltic Sea, the large predatory fish northern pike (Esox lucius L.) is important for both recreational fisheries and ecosystem functioning. As existing fishery-independent surveys do not adequately monitor pike populations, a general lack of knowledge on population status and trends poses challenges for management. Here we use recreational angling data as an alternative method to describe pike population development along the Swedish Baltic Sea coast from 1938 onwards and assess the change over time in potential mortality factors by estimating harvest by fisheries and consumption by large predators. Data from a Swedish national register on trophy-sized pike (>12 kg) showed that numbers and maximum sizes peaked in 1990-1995, after which declines in both metrics are evident. In logbooks from a sport fishing club in the Stockholm archipelago, a simultaneous decrease in maximum sizes of pike was observed, together with a decrease in the total number of pike caught per fishing day. Jointly, these data indicate a decline in the abundance of large pike since around 1990. While commercial pike fisheries in the Baltic Sea have decreased continuously since the 1950s, recreational fishing increased after 1985, when Swedish coastal waters were made open access to anglers. The declines in large pike starting in the 1990s could, thus, have been driven by an increase in mortality from recreational fisheries, which at the time primarily practiced catch and kill. Since the 2000s, bag and slot length limits, spawning closures, and a general increase in catch-and-release fishing has reduced the landings of pike in recreational fisheries. Despite these fisheries regulations and higher release rates the decline in catches of large pike has continued, indicating an effect of other mortality factors. The strong growth of grey seal (Halichoerus grypus) and great cormorant (Phalacrocorax carbo sinensis) populations suggest that predation pressure on pike has increased over time. In the Stockholm archipelago these two predators were estimated to remove 5-18 times (range based on different diet composition estimates) more pike biomass than total fisheries landings in 2014-2017. To improve the situation for northern pike in the Baltic Sea managers need to consider both fisheries restrictions and measures to decrease predation pressure and increase recruitment. Catch data from recreational fisheries may be used to evaluate such management efforts by providing information on the population development of this keystone species

Fritidsfiskets omfattning i Vättern 2017

Fritidsfiske är en populär aktivitet med positiva ekonomiska, sociala och hälsofrämjande effekter för både den enskilde individen och för samhället. För flera av Sveriges fiskbestånd är idag fångsterna större i fritidsfisket än i yrkesfisket. På samma sätt som yrkesfisket är därför fritidsfisket en nationellt viktig resurs, men utgör samtidigt en verksamhet som belastar ekosystemen. Det åligger Sverige att bedöma fritidsfiskets ekologiska effekter och dess sociala och ekonomiska betydelse för samhälle och människor. Idag saknas dock god kunskap och robusta underlag om fritidsfiskets omfattning, värde och påverkan. Det finns därför ett behov av att samla data från fritidsfisket både för att möjliggöra en kunskapsbaserad rådgivning till fiskförvaltningen men också för att belysa fritidsfiskets omfattning och betydelse. I denna rapport undersöktes olika metoder för datainsamling inom fritidsfisket efter röding, lax och öring i Vättern under juli och november 2017, med fokus på resultat till nytta för fiskförvaltningen och bedömning av beståndens status. Två huvudsakliga undersökningar genomfördes: inventeringar av ansträngningen (antal fiskande) och en enkätundersökning för att beskriva fisket och fångsterna. Antal fiskande och deras fiskemetoder inventerades från flygplan, båt och via fritidsbåthamnar. Via enkäter samlades information om de svarandes fiskeerfarenheter och om fångsterna. Genom att kombinera de två undersökningarna estimerades sedan totalfångster av röding, öring och lax. Beräknat antal fiskande från de tre inventeringsmetoderna korrelerade väl och hade en liknande precision. Flyg- och båtinventeringen gav en bättre rumslig täckning och en säkrare bedömning att fiske faktiskt bedrivits, medan hamninventeringar av naturliga skäl är begränsade till vissa platser. Med hjälp av lokala hamnvärdar kunde dock fler dagar och flera tillfällen per dag undersökas än vad flyg- och båtinventeringar normalt medger. Från flyget, som täckte hela Vättern, estimerades antalet trollingfiskande båtar (± 1 standardfel) i juli respektive november till 1 260 (±450) och 370 (±140). För juli beräknades antalet båtar som bedrev vertikalfiske till 420 (±170) och i november fiskade 130 (±60) personer från land eller landnära med flytring. I kombination med enkätundersökningen gav detta att det i juli i trollingfisket respektive vertikalfisket totalt fångades (behållen och återutsatt fisk) 5 840 (±2 220) och 1 430 (±630) rödingar. Motsvarande fångster av lax var 940 (±520) och 15 (±15). I trollingfisket i november beräknades fångsterna av röding till 910 (±390) samt lax till 390 (±160). Fångsterna av öring var i paritet med lax i både juli och november. För alla tre arter i både trolling- och vertikalfisket återutsattes en betydande andel av de fångade fiskarna (ca 40–70 %). Kunskap om fångster av röding, öring och lax utgör ett viktigt underlag för fiskförvaltningen i Vättern. Samtidigt visar studien på tre goda exempel hur fritidsfiskets omfattning och fångster kan samlas in, där val av metod i huvudsak styrs av ett givet fritidsfiskes rumsliga och temporala utbrednin