Session C5: Experiments to Improve Passage Ways for Downstream Migrating Silver Eel

Abstract: A trial was conducted with passage ways for migrating silver eel at a low head weir in a tributary to River Mörrumsån in Sweden. Behavioral traits useful for maximizing the number of eel per volume of water were sought. Six openings were made in the weir and supplied with various set-ups. One of the openings, a plain hole, served as a control. A second opening had significantly more eel passing than the control (11 versus 2; p\u3c0.05). This setup (U-chute) had a raceway extending 7m upstream of the weir. Inflow of water (and eel) took place through vertical slots on the side. Further development of this model is advocated. Two other set-ups had about the same number of eel passing (4 and 2) as the control. The first was a nature like bypass. The other, the M-chute, had a submerged raceway extending 7m upstream. Inflow took place over the sides. The last two set-ups had no eel passing through or entering, and very few eel were observed near. The first was a dead end (no water flow) and the second had a flow of water coming out of the entrance. A separate trial, involving silver eel and modified water quality, was carried out at the aquarium experimental facility at Ar, Sweden. Eel, kept in 4m² experimental fresh water fish tanks, were offered two narrow escape ways. One opening had an inflow of brackish water, the other opening had an inflow of fresh water. Significantly more eel passed out through the opening with brackish water than the opening with fresh water (21 versus 8; p\u3c0.05). This effect was however not deemed strong enough for attracting eel to passages. It is argued there is a need for better knowledge of silver eel migration behavior during the descent to the sea

Feasibility and potential for farming and conditioning of wild fish fed with by-catches in Sweden

Small-scale fisheries face problems with declining fish stocks in poor condition, increasing interactions with seals and cormorants and partly non-efficient distribution systems, resulting in low profitability. One potential method to increase the value of their catch is rear the fish in farms until fish reach a size that render a higher price. This may not only provide a higher value of each animal but also a steadier supply of fish to consumers and retailers. In addition, by-catch of unwanted species may be used as feed ingredient to the farmed fish. This will not only cut the costs for the feed but is also a more sustainable alternative as it will both make use of by-catches that otherwise is discarded, and recirculate nutrients on a regional scale instead of importing new nutrients. Farming of wild caught fish and shellfish (grow-out or capture-based aquaculture) constitute a large part of aquaculture on a global scale. Except for eel is this type of aquaculture still limited in Sweden and Europe, with relatively little development. Here we make an overview of species that could be interesting for farming of wild caught fish, and identify benefits and challenges. The species we find most suitable for further development are cod, perch, whitefish, pike and pikeperch for which we can identify evident benefits of farming. In common for all these species is the need for an efficient feed system to ensure early and rapid weight gain and minimizing initial mortality. We speculate that a feed based on insect larvae could be one way to improve the feeding system for several species of wild caught fish. However, there are ethical and welfare issues related to farming wild born fish. As wild caught fish are not domesticated for life in captivity they can suffer from distress and increased susceptibility and transmissions of disease. Safeguarding the health and welfare of fish in capture-based aquaculture is a key to making it economically feasible, as an increased value for the end-consumers is necessary to compensate the fishermen for the additional costs associated with farming of wild caught fish. In addition, removal of wild fish may also impede natural stock size and recruitment of the natural stocks. Although our aim is to develop a farming system where local by-catches is used as a feed ingredient, local eutrophication effects and water pollutions (feed and fish residues) can cause degradation of local water quality. In conclusion, we find potential for farming of wild caught fish with local-by-catches as a feed ingredient. To be economically feasible there is a need for developing feeding systems, investigate stress responses and ethical and sustainability aspects important for marketing of such products

Förvaltning av signalkräfta i sjöar

Fisket efter signalkräfta har fått allt större ekonomisk och social betydelse i Sverige. Trots detta saknas väl underbyggda råd för hur ett hållbart fiske ska bedrivas.  Projektet ”Utveckling av fisket efter signalkräfta – hur ska man optimera fiske och förutsäga risken för populationskollapser?” är ett projekt som delfinansieras av Europeiska fiskerifonden 2009-2013. Som en inledande del i detta projekt gjordes en litteratursammanställning, och baserat på denna har planeringen av det framtida arbetet kunnat konkretiseras.  Målsättningen med litteraturgenomgången var att identifiera vilken information om signalkräftans biologi och ekologi som behövdes för att kunna ta fram bra fiskerimodeller för hur ett hållbart fiske bör bedrivas. Dessutom var det viktigt att förstå varför vissa bestånd av signalkräfta har kollapsat.  Fångsterna av signalkräfta varierar mellan sjöar. Denna variation kan, i sjöar som inte är försurade, till stor del förklaras med hur stor andel av sjöns botten som är täckt med sten. Finns det mycket sten i en sjö finns det också mycket signalkräftor. Det finns några få studier i Sverige på signalkräftan där populationer har följts under en längre tid (minst 15 år). Dessa visar att fångst per mjärde och uttag av konsumtionskräftor varierar mellan olika år inom en sjö. Dessa variationer kan till viss del förklaras med temperaturen under föregående år, men mekanismen bakom detta är inte känd. Studier av andra arter sötvattenskräftor och en del marina skaldjur (t.ex. hummer) tyder på att rekryteringen (reproduktionsframgången) till viss del kan förklara variationerna i fångstnivåer mellan olika år.  Denna litteraturgenomgång visar att det saknas väsentlig information om signalkräftans ekologi och biologi för att kunna ta fram teoretiska modeller som ska ligga till grund för rekommendationer om hur ett hållbart fiske ska bedrivas. De beståndsanalyser som bedömts vara intressanta för signalkräfta kräver vissa dataunderlag för att ge tillförlitliga resultat. De enskilt viktigaste faktorerna är rekryteringsframgång, tillväxt, naturlig dödlighet, och detaljerad fiskeristatistik (ansträngning, selektivitet, fångster etc.). Med anledning av resultaten från denna litteraturgenomgång bedömdes följande insatser som prioriterade:  • undersöka betydelsen av honans storlek för rekryteringsframgång • utveckla tekniken för märkning av kräftor i olika typer av bestånd för att sedan kunna använda återfångstdata för att bestämma individuell tillväxt, naturlig dödlighet och fiskeridödlighet  • uppskatta ytan tillgängligt kräfthabitat för olika kräftbestånd och bedöma i vilken mån det påverkar potentiellt fiskeuttag  • analysera ett flertal sjöar med och utan populationskollapser och undersöka vilka miljöfaktorer som kan förklara uppkomsten av kollapser  • analysera såväl pestfrekvens som infektionsgrad i enskilda kräftor och utvärdera om det finns en koppling mellan populationskollapser och ökade pestangrepp i sjöa

Mellanskarv

Denna rapport syftar till att belysa mellanskarvens inverkan på yrkesfiske, fiskodlingar och sportfiske i svenska insjövatten samt för det småskaliga yrkesfisket efter den svenska kusten. Materialet från insjöarna bygger på intervjuer av personer som är aktiva inom respektive sektor. För yrkesfiske längs kusten med passiva redskap har en frivillig journalföring över skador på fångst och redskap sammanställts. Den svenska mellanskarvpopulationen har ökat dramatiskt och därmed också oron inom speciellt yrkesfisket att skarven har en negativ inverkan på deras sysselsättning. Fiske med fasta redskap och nät får skador av skarv på både fångst och redskap. Skadad fångst är oftast osäljbar. En komplicerande faktor som är viktig att ta med i beräkningen är den dolda skadan, det vill säga den mängd fisk skarv plockar från redskapen utan att det märks, samt fisk, framförallt ål, som smiter ur redskapen där skarv har haft sönder maskor. För att översiktligt belysa påverkan av skarv på svenskt kust- och insjöfiske utfördes telefonintervjuer med 40 yrkesfiskare i svenska insjöar 2009, samt en sammanställning av fångst- och redskapsdata utifrån en frivillig journalföring av 44 yrkesfiskare längs kusten 2005 och 58 stycken 2006. Resultatet visade att majoriteten (85%) insjöfiskare fick skarvskador på fångst, framför allt på sik och siklöja. Majoriteten angav också negativa ekonomiska konsekvenser men omfattningen är oklar. Generellt var dock insjöfiskarena (62,5%) mer oroliga för konkurrens om resursen, det vill säga att skarven påverkade fiskförekomsten negativt. I kustfisket rapporterade var fjärde fiskare skarvskador, främst inom bottengarnfisket i egentliga Östersjön och ålryssjefisket på västkusten. Kommersiellt fiske efter sik och strömming uppvisade de största skadorna. För några få enskilda fiskare var skadorna stora men generellt var de klart mindre än vad beräkningar av sälskador visat. Aqua reports 2012:1 Insjöfiskarena uppskattade antalet bifångade skarvar till maximalt 30 per år och fiskare, med ett ungefärligt medelvärde på strax under 20 skarvar. I journalföringen rapporterades det i genomsnitt mindre än fyra bifångade skarvar per år och fiskare. Det är siffror som kraftigt understiger en tidigare uppskattning av bifångsterna via en telefonintervju med yrkesfiskare längs kusten under år 2002. För säkrare siffror på bifångster krävs direkta studier via observatörer eller kamerastudier. Majoriteten av fiskodlarna såg skarv regelbundet i odlingsområdet eller omgivande vatten men enbart var fjärde fiskodlare ansåg att skarven var ett problem. Fler uppgav istället att häger orsakade skador eller tog fisk. Representanter för fritidsfisket med handredskap såg skarven som en konkurrent om resursen och det fanns en oro för framtida fiskemöjligheter. Dock trodde en del att förändringar i miljön hade haft större bidragande orsaker till ett sämre fiske än skarven. En generell slutsats är att undersökningen visar på att det förekommer direkta skador på redskap och fångst men att detta inte är det allvarligaste problemet. Insjöfiskarna upplever generellt konkurrensen om fiskresursen som värre och längs kusten dominerar sälskador över skarvskador. Krav på förvaltningsåtgärder som minskar konflikten (jakt, äggprickning etc) är ett genomgående tema från de intervjuade personer och via kontakter med yrkesfiskare längs kusten. Det saknas tydliga data för att verifiera att skarven allvarligt påverkar fiskpopulationerna/ ekosystemen men det finns inte heller underlag som motsäger detta. Att så många fiskare och odlare anser sig drabbade av skarven är i sig ett tungt skäl till att de skall tas på allvar tills tydliga bevis om att skarven inte har en effekt tas fram

Evidence that grey seals (<i>Halichoerus grypus</i>) use above-water vision to locate baited buoys

Fishing gear in the Baltic is often raided by grey seals (Halichoerus grypus). The seals remove the fish and damage the nets, or entangle themselves and drown. In order to develop ways of mitigating the seals-fisheries conflict, it is important to know exactly how the seals locate the fishing gear. A field experiment was conducted in order to clarify whether seals use their vision above water to do this. Bait (herring; Clupea harengus) was attached to the anchor lines of buoys of the type that is commonly used to mark the position of fishing gear. In all, 643 buoys were set. Some of the buoys (210) were also fitted with camera traps. Weather data were collected from official weather stations nearby. Bait loss (mean 18%) was significantly correlated with buoy size (P = 0.002) and wind speed (P = 0.04). There was a significant association between bait loss and seal observations near the buoys (P = 0.05). Five photos of grey seals were obtained from the camera traps. No fish-eating birds, such as cormorants or mergansers, were ever observed near the buoys or caught on camera. It was concluded that a main cause of missing bait was scavenging by grey seals, and that they did use above-water vision to locate the buoys. It was also concluded that wind strength (i.e. wave action) contributed tothe bait loss. The camera trap buoys had a somewhat lower bait loss than the other buoys (P = 0.054), which was attributed to a scaring effect. Neither the number of seal observations nor the bait loss differed significantly between the 2 study areas in the experiment (P = 0.43 and P = 0.83, respectively). Bait loss was not affected by the buoy colour (red, white, or grey; P = 0.87). We suggest that the findings of this experiment could be put into practice in a seal-disturbed area by deploying a number of decoy buoys, or by hiding live buoys below the surface of the water. This would increase the cost of foraging for the seals, and hence discourage them from exploiting fishing gear as a feeding place

Grey seal induced catch losses in the herring gillnet fisheries in the northern Baltic

The interaction between grey seals (Halichoerus grypus) and the Baltic gillnet fishery for herring (Clupea harengus) during the period 2000-2005 was investigated, by comparing and contrasting 3 sources of information: data from the European Union (EU) official logbook system, data from a voluntary logbook system and data from field studies. While the EU official logbooks suggest that seal disturbance occurred in 30% of all herring gillnet fishing events, data from the voluntary logbook gavea figure of 60%. There was a pronounced seasonal variation in the frequency of seal-disturbed fishing efforts, with least interference in the early summer, and greatest at the end of the year. This variation is likely dependent on the life cycle of the seals and their main prey, the herring. Analysis of the EU logbook information also showed that catches were significantly higher on fishing days when there was no seal interference recorded, compared to days when there was such interference. Field experiments demonstrated that herring catches were reduced by 240 kg per fleet of net and fishing occasion when seals were present, which is much more than the observed seals in the area could possibly have consumed, and a very small number of fish remains were found in the nets after seals had been present. These observations suggest that the mere presence of seals does affect catch levels negatively. Nets baited with marked fish were used to estimate hidden losses, i.e. fish removed from the nets, leaving no trace. Seals were assumed to have visited the experimental nets in 14 of the 19 trials. In 11 of these, more than 95% of the marked fish went missing. The 3 different data sources altogether show that the herring gillnet fishery in the northern Baltic is severely disturbed by interaction with grey seals

Data from: Pontoon trap for salmon and trout equipped with a seal exclusion device catches larger salmons

The growing seal populations of the Baltic have led to more frequent interactions with coastal fisheries. The motivation for seals to interact with fishing gear is high. It provides high densities of fish. A successful means of mitigating the conflict is the pontoon trap. Seal visits here have been frequent. Seals have access to most parts of the trap system including the middle chamber, which is an overhead environment. Concerns have been raised about seals possible entanglement in this specific part of the trap. As a means of keeping seals from entering the middle chamber, two different Seal Exclusion Devices (SEDs) were tested. A diamond mesh SED and a square mesh SED, which was rotated 45°. The aim was to compare the functionality of the different SEDs with respect to seal deterrent abilities and catch composition. The hypothesis tested were (i) that seals would not be able to enter the middle chamber, (ii) that the catch would increase and (iii) that the SED would deter larger fish from swimming into the middle chamber. Catch data and underwater film were collected. Larger salmons were caught in traps equipped with SEDs. The SEDs did not affect the number of caught fish or the total catch per soak day

Data from: Pontoon trap for salmon and trout equipped with a seal exclusion device catches larger salmons

The growing seal populations of the Baltic have led to more frequent interactions with coastal fisheries. The motivation for seals to interact with fishing gear is high. It provides high densities of fish. A successful means of mitigating the conflict is the pontoon trap. Seal visits here have been frequent. Seals have access to most parts of the trap system including the middle chamber, which is an overhead environment. Concerns have been raised about seals possible entanglement in this specific part of the trap. As a means of keeping seals from entering the middle chamber, two different Seal Exclusion Devices (SEDs) were tested. A diamond mesh SED and a square mesh SED, which was rotated 45°. The aim was to compare the functionality of the different SEDs with respect to seal deterrent abilities and catch composition. The hypothesis tested were (i) that seals would not be able to enter the middle chamber, (ii) that the catch would increase and (iii) that the SED would deter larger fish from swimming into the middle chamber. Catch data and underwater film were collected. Larger salmons were caught in traps equipped with SEDs. The SEDs did not affect the number of caught fish or the total catch per soak day

A Trial of Natural Habitat Enclosure Traps as a Sampling Tool for Juvenile Crayfish

Recruitment is a vital factor in the assessment, management and population dynamics of decapods. Since the juvenile stages of crayfish often prefer heterogeneous habitats, sampling with quantitative and reproducible methods have so far been challenging. We evaluate a new quantitative sampling method for juvenile crayfish; the enclosure trap. A field test was carried out during two consecutive years on a population of signal crayfish, Pacifastacus leniusculus, in littoral areas of Swedish Lake Erken. The densities of 0+ crayfish varied substantially with year, sampling date and substrate type. As expected, juvenile crayfish densities decreased over time in both study years, indicating a high mortality rate during their first year of life. Juveniles preferred gravel and stone over soft and sand substrates. Mean growth rate varied from 0.15 to 0.22 mm day-1. We evaluate this method and present recommendations for how to design and optimize field studies using enclosure traps. We conclude that enclosure traps can be used to collect valuable data on density, growth and habitat preference of juvenile crayfish, thus providing useful information for studies on population dynamics and increasing the understanding of crayfish recruitment processes

Seal-induced losses and successful mitigation using Acoustic Harassment Devices in Estonian Baltic trap-net fisheries

During the last decades the number of grey seals Halichoerus grypus has increased more than tenfold in the Baltic Sea. having reached 38 000 in 2019, which has created increasing problems to the coastal fisheries. In the current study, 151 Estonian coastal fishermen were interviewed to sum all direct and indirect monetary losses. The total cost of the damage caused by seals to the Estonian fisheries was around 0.9 million euros in 2009. With the aim of mitigating seal impact, Acoustic Harassment Devices (AHDs, Lofitech) were tested. This study presents quantitative data on seal damage in Estonian trap-net fisheries and analyses the results of the attempts made to mitigate the damage by means of AHDs. It can be concluded that AHDs provided sufficient protection against the seal attacks