Smelts are crucial forage prey for many piscivorous fish species and have, as such, been frequently introduced outside their indigenous ranges, a management measure causing controversy, as smelts are also efficient predators and competitors. In Lake Vättern, relict Arctic char, fourhorn sculpin, smelt and seven species of crustaceans form a deep-water food-web, and interact closely as competitors, predators and prey. Here, we analyzed new and archived data back to the late 1960s in order to understand and relate observed life-history shifts in the smelt population, to concurrent adverse life-history changes observed in the endangered Arctic char population. Smelt growth was sigmoidal and the size frequency distribution dominated by fish up to 110 mm and age 0–4, although smelt occasionally reached 250 mm. Smelt diet shifts as the fish grow, from zooplankton, to Mysis relicta, to piscivory for individuals larger than 150 mm. Recruitment commonly demonstrates regular inter-annual pulses of YOY abundance although strong year classes produced during years with higher spring temperature cause post-annual irregularity of year class strength. Young smelt reside in surface water and older fish in deeper water. This ontogenetic shift in distribution depends on temperature occupied, prey availability and risk of inter- and intra-specific predation. Interestingly, even though very small Arctic char feed on smelt, most small Arctic char are found in deep water together with large smelt, and most large Arctic char are found in shallow water with small smelt.

Eels (Anguilla spp.) are in decline worldwide, and the signs of reduced recruitment have been observed in continental Europe since the early 1970s. To aid recovery of the European eel, stocking is used by many European countries as a management option. In this study, tagging experiments were conducted to follow eel migration from Lake Malaren and four sites along the Swedish east coast in the Baltic Sea. The recaptured tagged eels were retrieved from fishermen, allowing for the opportunity to investigate their origin (brackish water, stocked in freshwater or a mix in between) by otolith microchemistry and to assess for morphological differences after tagging. Several changes took place; for example, eye index increased while weight and condition decreased with migrated distance and time until recapture. In Lake Malaren, the majority of tagged eels did not migrate out of the outlets, irrespective of their origin. Most of them were caught in the opposite direction and continued to be caught in the lake 1-3years after tagging, with significant weight losses. Overall, overwintering is suggested to be an inferior option, but it is uncertain whether this is a natural behaviour or a result of translocation and restocking. For coastal eels, origin had no effect on migratory behaviour; a majority of the tagged eels migrated towards the outlet of the Baltic Sea. Interestingly, a minority of the recaptured eels originated from stocked fish. Instead, recaptures were dominated by natural immigrants that had spent most of their lives in brackish waters.

Remote sensing techniques may provide a higher temporal and spatial resolution than traditional water monitoring methods. We tested if this auxiliary information can be used to (i) explain patterns in fish assemblage composition and (ii) test candidate metrics to assess ecological status in large lake water bodies. We used MERIS-derived layers describing chlorophyll a, total suspended matter, and colored dissolved organic matter (CDOM) overlaid on all available fish monitoring data from the four largest Swedish lakes (Vänern, Vättern, Mälaren, and Hjälmaren). We assessed the influence of remote sensing-derived parameters in the pelagic, offshore benthic, and the inshore benthic habitats. Our results demonstrated that chlorophyll a and CDOM together with depth at the sampling site explained a significant part of the variation in the distribution of fish assemblages. These predictors were particularly important not only in pelagic, but also in inshore benthic areas. Furthermore, we identified three potential candidate metrics to assess pressure from eutrophication in large lakes: density of pelagic fishes, biomass of planktivorous species, and the proportion of cyprinids when roach was excluded. Remote sensing was considered a useful tool to support analyses of fish community composition and dynamics.

In response to the drastic decline of the European eel (Anguilla anguilla (L.)) fisheries have been reduced and elvers are stocked in areas where natural abundances are low. Are these measures adequate? To answer different aspects of this question, we have analysed more than a century of eel tagging, using both traditional and more novel capture – recapture analyses. Based on these long-term data, we have evaluated the impact of the Swedish eel coastal fisheries using Survival analysis. Our analysis indicates that the fishing mortality just prior the 2009 fishing restrictions were in the order of 10%.More recent tagging programs have focused on issues related to the fate of stocked fish. If and how they migrate out of the Baltic Sea and further on towards the Atlantic Ocean. Both earlier and our new studies reveal that all eels recaptured on the Swedish East Coast, no matter of their origin, migrate at a reasonable speed and direction towards the outlets of the Baltic Sea. Even though it is sometimes difficult to determine their origin, our analyses indicate that stocked fish were scarce among the recaptures. In an experiment on the Swedish West Coast, we knew the individuals’ origin (stocked or wild) and they had similar migration patterns.In contrast, silver eel in Lake Malaren – assumed to have been stocked as elvers or bootlace eels – seemed to have difficulties in finding the outlets. Instead they overwintered and lost weight. However, weight losses are also significant among non-stocked individuals in the Baltic Sea, both if they overwinter and if they appear to be on their way out from the area. It remains an open question whether eels from the Baltic region in general, and whether the overwintered fish in particular, manage to reach the spawning area in the Atlantic Ocean.Based on current knowledge, I advocate invoking the precautionary approach and to concentrate Swedish eel stockings to the West Coast and allow the young fish to spread out on their own.

Invasive species often have instable population dynamics and are known to collapse or oscillate heavily after passing through the initial lag/growth phases. Long-term data-series documenting these fluctuations are however rare. We use long-term (starting in the early 1960s), semi-quantitative data on the invasive signal crayfish (Pacifastacus leniusculus), capturing its population development after introduction in 44 Swedish lakes. In total 18 (41 %) of these populations had experienced a collapse. A stepwise discriminant function analysis including 20 different ecological or physicochemical characteristics identified three variables explaining collapses in the following order: stocking year, population age and mean air temperature. Populations stocked in the 1980s were more likely to collapse than populations stocked in the 1970s. Lakes with collapses were located in areas with 0.4 °C higher yearly mean air temperatures than the still viable populations. Collapses also depended on the time phase of the population and started to occur 12 years after stocking and were most frequent in the interval 16–20 years after stocking and after 11–15 years duration of the established phase with harvestable densities. An analysis of prevalence and pathogen load of Aphanomyces astaci was conducted in eight of the studied populations. A. astaci was present in all populations but neither the level of prevalence nor the pathogen load in infested specimens differed significantly between lakes with collapses and lakes without. Our results highlight the potential sensitivity and instability of introduced crayfish. The importance of density-dependence and temperature suggest that both climate variability and/or fisheries can influence these processes.

Fisket efter signalkrafta har fatt allt storre ekonomisk och social betydelse i Sverige. Trots detta saknas val underbyggda rad for hur ett hallbart fiske ska bedrivas. Projektet ”Utveckling av fisket efter signalkrafta – hur ska man optimera fiske och forutsaga risken for populationskollapser?” ar ett projekt som delfinansieras av Europeiska fiskerifonden 2009-2013. Som en inledande del i detta projekt gjordes en litteratursammanstallning, och baserat pa denna har planeringen av det framtida arbetet kunnat konkretiseras. Malsattningen med litteraturgenomgangen var att identifiera vilken information om signalkraftans biologi och ekologi som behovdes for att kunna ta fram bra fiskerimodeller for hur ett hallbart fiske bor bedrivas. Dessutom var det viktigt att forsta varfor vissa bestand av signalkrafta har kollapsat. Fangsterna av signalkrafta varierar mellan sjoar. Denna variation kan, i sjoar som inte ar forsurade, till stor del forklaras med hur stor andel av sjons botten som ar tackt med sten. Finns det mycket sten i en sjo finns det ocksa mycket signalkraftor. Det finns nagra fa studier i Sverige pa signalkraftan dar populationer har foljts under en langre tid (minst 15 ar). Dessa visar att fangst per mjarde och uttag av konsumtionskraftor varierar mellan olika ar inom en sjo. Dessa variationer kan till viss del forklaras med temperaturen under foregaende ar, men mekanismen bakom detta ar inte kand. Studier av andra arter sotvattenskraftor och en del marina skaldjur (t.ex. hummer) tyder pa att rekryteringen (reproduktionsframgangen) till viss del kan forklara variationerna i fangstnivaer mellan olika ar. Denna litteraturgenomgang visar att det saknas vasentlig information om signalkraftans ekologi och biologi for att kunna ta fram teoretiska modeller som ska ligga till grund for rekommendationer om hur ett hallbart fiske ska bedrivas. De bestandsanalyser som bedomts vara intressanta for signalkrafta kraver vissa dataunderlag for att ge tillforlitliga resultat. De enskilt viktigaste faktorerna ar rekryteringsframgang, tillvaxt, naturlig dodlighet, och detaljerad fiskeristatistik (anstrangning, selektivitet, fangster etc.). Med anledning av resultaten fran denna litteraturgenomgang bedomdes foljande insatser som prioriterade: • undersoka betydelsen av honans storlek for rekryteringsframgang • utveckla tekniken for markning av kraftor i olika typer av bestand for att sedan kunna anvanda aterfangstdata for att bestamma individuell tillvaxt, naturlig dodlighet och fiskeridodlighet • uppskatta ytan tillgangligt krafthabitat for olika kraftbestand och bedoma i vilken man det paverkar potentiellt fiskeuttag • analysera ett flertal sjoar med och utan populationskollapser och undersoka vilka miljofaktorer som kan forklara uppkomsten av kollapser • analysera saval pestfrekvens som infektionsgrad i enskilda kraftor och utvardera om det finns en koppling mellan populationskollapser och okade pestangrepp i sjoar