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Paraschiv, M., Økland, F., Lenhardt, M., Paterson, R.A., Hont, S., Cvijanovic, G., Havn, T.B., Iani, M., Smederevac-Lalic, M., Nikčević, M., Neacșu, N., Nikolić, D. & Thorstad, E.B. 2021. Restoration of fish migration in the Danube River at Iron Gate Dams in Romania and Serbia. Studies of fish behaviour in 2019 and 2021. NINA Report 2030: 1-36. The Iron Gate hydropower and navigation system is one of the largest river engineering projects in Europe, with the dams built to provide hydropower and facilitate navigation along the Danube. The Iron Gate Dam I was completed in 1972 and Iron Gate Dam II in 1984. The border between Romania and Serbia follows the Danube in this area, and the two countries share both dams. Before the construction of the dams, fish were able to migrate from the Black Sea to the upper parts of the Danube and its tributaries. The dams blocked upstream fish migration, and no fish passes have been constructed. Constructing fish passes at the Iron Gate Dams would open up 960 km of the Danube River to the Gabčíkovo Dam (Slovakia) to upstream migrating sturgeon and other fishes, and in addition access to numerous tributaries. Well-functioning fish passes require that migrating fish are able to find and use the entrance, and thereafter move through the entire fish pass. The entrances need to be located in areas where migrating fish seek a passable route past the dam. To establish functional fish passes is particularly challenging at large dams where there is a long distance between the river banks and large areas where the fish can migrate and aggregate. Results from a feasibility study of the behaviour of asp, barbel, common carp, common nase, Pontic shad and vimba bream in autumn 2019 and spring 2021 are presented in this report, as a step towards identifying suitable areas for fish pass entrances at the Iron Gate Dams. The specific aims were to examine behaviour and distribution of fish downstream of the Iron Gate Dam II and in the Gogosu branch, movements in the reservoir between the Iron Gate Dam I and Iron Gate Dam II, and behaviour and distribution of fish downstream of the Iron Gate Dam I, if tagged fish would reach this far. The study was performed by tagging fish with acoustic transmitters and releasing them downstream and upstream of Iron Gate Dam II. The movements of tagged fish were recorded by deploying receivers at strategic places in the river. Tagged fish were automatically recorded when they were within the detection range of the receivers. A total of 185 fish were tagged; 61 in autumn 2019 and 124 in spring 2021. The most common species among the tagged fish were vimba bream and common nase, followed by barbel and asp. Vimba bream, common nase, barbel and asp are known to be migratory species, which can perform long-distance movements. This was confirmed also for our study area, with many tagged individuals performing extensive movements, both below the Iron Gate II dam, in the Gogosu branch and in the reservoir. Several barbel, common nase and vimba bream moved upstream through the entire reservoir to the Iron Gate I dam, which was 76 km from the release site in the lower part of the reservoir. Barbel and vimba bream were the two species showing the most extensive movements. The study also confirmed that there is large individual variation in movement strategies and behaviour within these species. The detailed behaviour of the fish when approaching the Iron Gate II and Iron Gate I dams was studied by deploying several receivers with different detection ranges in the areas below the dams. The fish showed great individual variation in where they approached the dams, and most of the individuals were recorded by receivers on both sides of the river. Hence, a preference for moving along one particular side of the river was not identified for any of the species. Studies of detailed behaviour of tagged fish can only be as fine-scaled as the receiver deployment allows. In the present study, a relatively limited number of acoustic receivers was used, limiting the analysis of detailed behaviour below the dams. Further studies will be performed to increase the information of the detailed fish behaviour below the dams.

Havn, T.B., Økland, F., Heermann, L., Thorstad, E.B., Teichert, M.A.K., Sæther, S.A., Tam-bets, M. & Borcherding, J. 2018. Downstream migration of Atlantic salmon smolts at Unkelmühle power station and Buisdorf dam in 2016. NINA Report 1412. Norwegian institute for nature research. Background and study aim The aim of this study was to examine migration routes and losses of Atlantic salmon smolts past the Unkelmühle hydropower station in the Sieg. Results from the study in 2016 are the main focus of this report. The results from 2016 are also compared with results from similar studies at Unkelmühle in 2014 and 2015. Technical facilities at Unkelmühle are designed to facilitate safe passage of downstream migrating fish, including ten different bypass routes where fish can pass outside the turbines, and narrowly spaced racks installed in front of the turbine intakes to prevent fish from entering the turbines. The efficiency of these measures are evaluated. Downstream migration past Buisdorf dam was also examined in 2016. This enabled comparison of loss and migration speeds of smolts between the Unkelmühle hydropower station and a weir without a hydropower station at Buisdorf. Methods The study was performed by tagging 227 Atlantic salmon smolts with radio transmitters and recording their migration in the river and past Unkelmühle power station and Buisdorf dam. Their movements were recorded 1) on free-flowing reference stretches upstream of the power station and dam, 2) on impounded stretches upstream of the power station and dam, 3) when they passed the power station and dam, and 4) on downstream river stretches. Migration routes used by tagged fish when they passed the power station and dam were mapped in detail by using networks of automatic, stationary receivers. The loss of downstream migrating smolts due to impoundments and past the power station and dam was calculated by comparing losses in these areas with losses on the reference stretches. This is based on the assumption that the loss per km recorded on the reference stretches (termed “reference loss” in this report) is representative for the developed stretch (stretch affected by hydropower development) if it had been a free-flowing river instead of being impounded by a reservoir and having a power station or dam. To examine if the location of the reference stretches affected the estimated loss caused by hydropower development, we compared two estimates of loss due to Unkelmühle power station based on reference loss on two different reference stretches. Results and conclusions The loss of downstream migrating smolts due to Unkelmühle power station was minimum 2.9% during the study in 2016. This represents the percentage of smolts arriving at the power station area that were lost due to this being a power station area instead of a free-flowing river. The loss estimate represents direct loss at the power station and delayed mortality due to the power station on the stretches downstream (7.5 km). There was no difference in loss between fish using the headrace to pass the power station and those passing over the weir. Loss due to the Buisdorf dam was minimum 3.4 and 5.7% (two different estimates) and not significantly different from the loss at Unkelmühle. The loss estimates are minimum estimates, because fish injured when passing the power station or dam can experience delayed mortality at later stages than recorded in this study, and the total mortality might therefore have been higher. There was no turbine mortality at the power station, because none of the smolts passed through the bar racks in front of the turbines, as expected due to the narrow bar spacing (10 mm) of the racks. Hence, the extra loss of smolts passing the power station was likely related to physical injuries in bypass routes aimed at guiding smolts outside the turbines, and increased predation. Loss due to the hydropower station was lower in 2016 compared to the two previous study years (minimum 9.9% in 2014 and 12.8% in 2015). Although total loss due to the power station was highest in 2015, loss in the bypass route that leads smolts outside the turbines was higher in 2014 than in 2015 and 2016. This was likely caused by smolts becoming trapped in an area of the bypass route where debris and branches piled up in 2014, but not in 2015 and 2016. Water discharge was higher in 2016 compared to the previous study years, and the high water discharge was probably an important factor for reducing loss of smolts passing the power station in 2016. Results showed that the reservoir upstream of the power station can be an area of high mortality for downstream migrating smolts. Of all smolts entering the reservoir upstream of Unkelmühle, 7.2% in 2014, 17.1% in 2015 and 4.4% in 2016 were lost due to this being a reservoir instead of a free-flowing river. The reservoir upstream of Unkelmühle is 2.3 km long, with slow-flowing water, and more resembling a lake than a river. The main reason for the extra loss in the reservoir is likely presence of more fish predators in the slow-flowing reservoir compared to the free-flowing river stretches. These results show that reservoir mortality may vary among years, probably due to variation in the predator community. In contrast to at Unkelmühle, where fish were delayed at the power station, fish moved at the same speed past Buisdorf dam as on unimpounded stretches. This difference in migration speed was also found when comparing only fish that migrated through the spillway gate at Unkelmühle with fish that used the weir at Buisdorf. No fish spent time in the turbine intakes before swimming back upstream and using the spillway gate to pass the power station, so behaviour upstream of the power station did not seem to explain why fish moved slower past the power station compared to the dam. However, smolts seem to follow the main water flow when navigating past power stations, and the observed differences may be caused by a higher proportion of the total water discharge running over the weir at Buisdorf compared to the proportion running through the spillway gate at Unkelmühle, making navigation over the weir and exit of the tailrace faster at Buisdorf compared to Unkelmühle.

Økland, F., Teichert, M.A.K., Havn, T.B., Thorstad, E.B., Heermann, L., Sæther, S.A., Tambets, M. & Borcherding, J. 2017. Downstream migration of European eel at three German hydropower stations. NINA Report 1355: 53 pages, including appendix. Background and methods The aim of this study was to examine migration behaviour and losses of European silver eel when passing three run-of-the river hydropower stations in Germany. These were the Unkelmühle power station in the Sieg, the power station in Gengenbach in the Kinzig (both tributaries to the Rhine), and Kuhlemühle power station in the Diemel (tributary to the Weser). The Unkelmühle power station is designed with several bypass routes where fish can pass outside the turbines. At the power station in Gengenbach, the position of a movable turbine can be adjusted to let downstream migrating fish pass above or under the turbine. Narrowly spaced bar racks have been installed in front of the turbine intakes at both Unkelmühle, River Sieg (10 mm), and Gengenbach, River Kinzig (15 mm), to prevent fish from entering the turbines. At the Kuhlemühle power station, River Diemel, an Archimedes screw turbine is installed without a bar rack in front of its entrance. Archimedes screws are regarded as being fish-friendly turbines, but few studies have tested this assumption. The study was performed during 2014 to 2016 by tagging 542 European silver eels with radio transmitters. Their migration in the river and past the power stations was recorded. Results and conclusions Overall, we recorded low mortality for downstream migrating silver eels at these power stations. However, there are uncertainties linked to the survival estimates, particularly at Gengenbach and Kuhlemühle. The mortality of eels when they passed the Unkelmühle power station was 0-4% and 0-8% in the two consecutive study years. This shows that it is possible to obtain low mortalities for downstream migrating eels at run-of-the-river power stations with special protection measures to facilitate migration and reduce mortality. No direct turbine mortality occurred, as no eel slipped through the bar racks in front of the turbines, as expected due to the narrow bar spacing. The reason that we give mortality estimates as a range (0-4% and 0-8%), is that the fate of some tagged eels after passing the Unkelmühle power station is unknown, which makes it difficult to determine if they were alive or dead after passing. Further, three individuals showed movements indicating that they were taken by birds, but it is not known whether they were dead at the power station and taken by bird predators, injured by passing the power station and therefore taken by predators, or whether they were uninjured but taken by predators anyway. The estimates given as ranges take this uncertainty into account, and imply that the mortality at the Unkelmühle power station could have been zero in both study years, but the mortality could also have been up to 4% in the first study year and up to 8% in the second year. If there was some mortality linked to passing Unkelmühle power station, this must have been due to injuries occurring in the bypass routes, or increased predation at the power station area. Increased predation may occur if fish are injured and thereby easier prey. It is also possible that presence of injured fish of different species at power stations attracts predators, such that the likelihood of being taken by a predator increases also for uninjured fish. None of the tagged eels became stationary at the power station, indicative of being dead, neither at Gengenbach or Kuhlemühle. However, there are uncertainties for the survival estimates at these power stations, because eels may drift downstream after they are dead. Release of tagged dead eels showed that eels that potentially died when passing Gengenbach or Kuhlemühle could have drifted several kilometers and out of the monitored area below the power stations. Hence, mortality at the power station is in such cases not necessarily detected. The survival estimates at Unkelmühle were more certain, because the fish were tracked over a longer distance below the power station, and implicitly there were fewer individuals with an uncertain fate. Some eels became stationary on river stretches below Gengenbach and Kuhlemühle power stations, and might be dead (14% and 23% of the eels that passed the Gengenbach and Kuhlemühle, respectively). However, eels may cease migration and migrate downstream another year, so an eel becoming stationary may not necessarily be dead. Eels mainly used migration routes with a large proportion of the water flow when passing the power stations. At Unkelmühle, most of the downstream migrating eels used the spillway gate, or the bypass route leading fish from the bar racks in front of the turbines into the flushing channel and back to the river via a route outside the turbines. Only two eels used the custom-made side bypasses for eels, and only a small proportion of the eels (

Økland, F., Teichert, M.A.K., Thorstad, E.B., Havn, T.B., Heermann, L., Sæther, S.A., Diserud, O.H., Tambets, M., Hedger, R.D. & Borcherding, J. 2016. Downstream migration of Atlantic salmon smolt at three German hydropower stations. NINA Report 1203: 1-47. The aim of this study was to examine migration routes and losses of Atlantic salmon smolt past three hydropower stations, which were the Unkelmühle (River Sieg), Gengenbach (River Kinzig), and Kuhlemühle (River Diemel) power stations. These three power stations represent the use of different technologies to reduce negative impact on downstream migrating fish. The study was performed by tagging 525 Atlantic salmon smolt with radio transmitters and recording their migration when passing the power stations. In 2015, the loss of downstream migrating smolt due to the power station was 12.8% at Unkelmühle and 3.1-6.3% at the power station in Gengenbach. This represents the percentage of smolt entering the power station area that were lost due to this being a power station instead of a free-flowing river. Immediate mortality for smolt that passed through the Archimedes screw turbine at Kuhlemühle was estimated at 0-8%. Results showed that also reservoirs upstream of power stations can be areas of high mortality. Of smolt entering the reservoir upstream of Unkelmühle, 7.2% and 17.1% (two study years) were lost due to this being a reservoir instead of a free-flowing river. The main reason was likely presence of more fish predators (i.e., larger fishes eating smolt) in the slowflowing reservoir compared to the free-flowing river stretches. © Norwegian Institute for Nature Research. The publication may be freely cited where the source is acknowledged

External tagging of fish with electronic tags has been used for decades for a wide range of marine and freshwater species. In the early years of fish telemetry research, it was the most commonly used attachment method, but later internal implants became preferred. Recently, the number of telemetry studies using external tagging has increased, especially with the development of archival tags (data storage tags, DSTs), pop-up satellite archival tags (PSATs) and other environment-sensing tags. Scientific evaluations of the tagging method are rather scarce for most species. We identified 89 publications, reporting effects of external tagging for 80 different fish species, which constitute the main basis for this review. External attachment holds certain benefits compared to other tagging methods, for example, speed of application, and it may be the only option for fishes with a body shape unsuitable for surgical implantation, or when using tags with sensors recording the external environment. The most commonly reported problems with external tags are tissue damage, premature tag loss, and decreased swimming capacity, but the effects are highly context dependent and species specific. Reduced growth and survival have also been recorded, but direct mortality caused by external tagging seems rare. Most of the studies reviewed evaluate tag retention, survival, and tissue reactions. There is a general need for more research on the effects of external tagging of fish with electronic tags, but particularly there are few studies on predation risk, social interactions, and studies distinguishing capture and handling effects from tagging effects. For PSATs, especially those that are large relative to fish size, there are particular problems with a high proportion of premature tag losses, reduced swimming capacity, and likely increased predation, but there remains a paucity of tag effect studies related to the use of PSATs. Before embarking on a field study employing external tagging with electronic tags, we recommend the use of appropriate pilot studies, controlled where possible, to quantify potential impacts of tagging. Telemetry, Tag attachment, Archival tag, PSAT, Survival, Tissue damage, Tag retention, Growth, Swimming, Drag, Entanglement, Biofouling, Predation

Ulvan, E.M., Bremset, G., Aronsen, T., Thorstad, E.B., Solem, Ø., Økland, F., Havn, T. B. 2015. Oppvandring og fordeling av laks i Osenvassdraget i Molde kommune: Resultater fra undersøkelser i 2014. - NINA Rapport 1184. 34 s. Hovedmålet med prosjektet var å undersøke oppvandringstidspunkt, fordeling og vand-ringsmønster hos villaks og rømt oppdrettslaks i Osenvassdraget, som er et middels stort vassdrag med stryk og innsjøer. Delmålene med prosjektet var å: • Undersøke i hvilken grad og når rømt oppdrettslaks vandrer opp i Oselva • Sammenligne tidspunkt for oppvandring til rømt oppdrettslaks og villaks i elva og hvor de oppholder seg i gytetida • Sammenligne om mulige vandringshindre (stryk og laksetrapp) påvirker oppvand-ringen til rømt oppdrettslaks og villaks • Undersøke mulige effekter av innsjøer på fordeling av rømt oppdrettslaks og villaks • Beregne bestandsstørrelse av villaks og rømt oppdrettslaks i Oselva Undersøkelsen ble gjennomført ved å fange voksen laks med stang i nedre deler av Oselva og kastenot i sjøen ved munningen av vassdraget. Laksen ble deretter merket med radiosendere. Fem loggestasjoner ble installert i vassdraget for å registrere vandringen til radiomerket laks, fra like oppstrøms saltvannssonen og til det øverste vannet i den lakse-førende strekningen. Radiomerket laks ble automatisk registret når de passerte loggesta-sjonene. Det ble i 2014 fanget svært få rømte oppdrettslaks i Osenvassdraget. Andelene rømt opp-dretts laks var 3 % (2 av 59) i sportsfisket, 0 % (0 av 106) i lysfiske og 9 % (4 av 44) i overvåkingsfiske om høsten. Ved å slå sammen fangstene fra overvåkningsfisket og lysfis-ket blir andelen oppdrettslaks i Osenvassdraget om høsten 2,7 %. Også under fisket for å radiomerke laks ble det fanget få rømte oppdrettslaks (3 %, 3 av totalt 100). Dette til tross for en svært omfattende fiskeinnsats for å øke antallet merkede oppdrettslaks. Det var derfor ikke mulig å svare på spørsmålene relatert til rømt oppdrettslaks med hensyn på oppvand-ringstidspunkt, passering av vandringshindre og bruk av innsjøene. Det er likevel verd å bemerke at ingen av de tre radiomerkede rømte oppdrettslaksene vandret gjennom Osvat-net, men holdt seg i nedre deler av elva. Antallet radiomerkede oppdrettslaks var også for lavt til å beregne bestandsstørrelse av rømt oppdrettslaks i vassdraget. I denne rapporten har vi derfor fokusert på villaksen og dens vandringsmønster og bestandsstørrelse i vass-draget. Det ble funnet forskjell i vandringshastighet og andel av radiomerket villaks registrert i Oselva mellom individer fanget med stang og individer fanget med kastenot. Den radio-merkede villaksen fanget med stang brukte kortere tid (ca. 10 dager) til fra merking til første registrering ved nederste dataloggestasjon enn villaksen som ble fanget med kaste-not (ca. 20 dager). Det var også en større andel av den radiomerkede villaksen fanget med stang (88 %) som ble registrert i vassdraget, enn av villaksen fanget med kastenot (53 %). Radiomerket villaks fanget med kastenot brukte kortere tid fra merking til første registrering ved dataloggestasjonen nederst i vassdraget jo senere i sesongen de ble merket. Villaks som ble merket senere i sesongen, brukte imidlertid lengre tid fra nederste dataloggestasjon til dataloggestasjonen like nedstrøms Osvatnet, enn de som ble merket tidligere. Merketids-punkt påvirket dermed vandringshastigheten i nedre del av elva forskjellig fra vandringshas-tigheten fra sjø til elv. Osvatnet fungerte ikke som vandringshinder på den radiomerkede villaksen i Osenvassdra-get i 2014. Radiomerket villaks brukte i gjennomsnitt 29 timer (minimum 10 minutter og mak-simum 392 timer) på å vandre gjennom det 3,9 km lange Osvatnet. Den radiomerkede vill-aksen tilbrakte ikke lengre tid (gjennomsnitt 198 timer) ved laksetrappa enn ved de andre dataloggestasjonene. Villaksen som passerte laksetrappa passerte på vannføringer mellom 1,1 og 8,0 m3/s, noe som tyder på at laksetrappa kan fungere som et vandringshinder på høye vannføringer. Basert på merking og gjenfangst (Petersen metoden) ble størrelsen på gytebestanden av laks i Osenvassdraget i 2014 beregnet til å være rundt 1400. © Norsk institutt for naturforskning. Publikasjonen kan siteres fritt med kildeangivelse.

European whitefish is well described and renowned for its polymorphism and faculty to occupy a wide range of freshwater habitats. This includes observations of individuals in some populations that utilize brackish estuarine habitats as feeding areas, although information on these populations is generally scarce. By use of acoustic tracking technology and physiological sampling, the behaviour and salt water tolerance of European whitefish was therefore studied in the estuary and the adjacent fjords of the River Neiden in Northern Norway where individuals have earlier been captured in the estuary area. The results showed that European whitefish utilized the estuary extensively as a feeding habitat, with the capacity to stay for short periods in salinity concentration up to 30 ppt and with a mean water temperature of 15 ˚C. A correlation was found between their horizontal distribution along the estuary axis and the salt water influx following the tidal variations; fish were mainly using the lower estuary at lower salinity concentration following the tidal level, and retreat upstream with the salt water influx under high tide. This study therefore demonstrated that European whitefish at it northernmost distribution area can utilize the estuary extensively as a feeding area. There was also indications of that at least a few individuals could possibly cope with relatively high salinity concentration over some time. This is the first study that has mapped the individual behaviour of European whitefish in an estuarine area over time and therefore provides a better understanding on the ability of the species to cope with salt water habitat. Given that these populations are genetically unique, they should be given special management emphasis.